Nouveau - Nov 2019 - [PATCH v2 02/15] mm/mmu_notifier: add an interval tree notifier

On 10/28/19 1:10 PM, Jason Gunthorpe wrote:
...
>  include/linux/mmu_notifier.h |  98 +++++++
>  mm/Kconfig                   |   1 +
>  mm/mmu_notifier.c            | 533 +++++++++++++++++++++++++++++++++--
>  3 files changed, 607 insertions(+), 25 deletions(-)
> 
> diff --git a/include/linux/mmu_notifier.h b/include/linux/mmu_notifier.h
> index 12bd603d318ce7..51b92ba013ddce 100644
> --- a/include/linux/mmu_notifier.h
> +++ b/include/linux/mmu_notifier.h
> @@ -6,10 +6,12 @@
>  #include <linux/spinlock.h>
>  #include <linux/mm_types.h>
>  #include <linux/srcu.h>
> +#include <linux/interval_tree.h>
>  
>  struct mmu_notifier_mm;
>  struct mmu_notifier;
>  struct mmu_notifier_range;
> +struct mmu_range_notifier;
Hi Jason,

Nice design, I love the seq foundation! So far, I'm not able to spot
anything
actually wrong with the implementation, sorry about that. 

Generally my reaction is: given that the design is complex, try to mitigate 
that with documentation and naming. So the comments are in these areas:

1. There is a rather severe naming overlap (not technically a naming conflict,
but still) with existing mmn work, which already has, for example:

    struct mmu_notifier_range

...and you're adding:

    struct mmu_range_notifier

...so I'll try to help sort that out.

2. I'm also seeing a couple of things that are really hard for the reader
verify are correct (abuse and battery of the low bit in .invalidate_seq, 
for example, haha), so I have some recommendations there.

3. Documentation improvements, which easy to apply, with perhaps one exception.
(Here, because this a complicated area, documentation does make a difference,
so it's worth a little extra fuss.)

4. Other nits that don't matter too much, but just help polish things up
as usual.
>  
>  /**
>   * enum mmu_notifier_event - reason for the mmu notifier callback
> @@ -32,6 +34,9 @@ struct mmu_notifier_range;
>   * access flags). User should soft dirty the page in the end callback to
make
>   * sure that anyone relying on soft dirtyness catch pages that might be
written
>   * through non CPU mappings.
> + *
> + * @MMU_NOTIFY_RELEASE: used during mmu_range_notifier invalidate to
signal that
> + * the mm refcount is zero and the range is no longer accessible.
>   */
>  enum mmu_notifier_event {
>  	MMU_NOTIFY_UNMAP = 0,
> @@ -39,6 +44,7 @@ enum mmu_notifier_event {
>  	MMU_NOTIFY_PROTECTION_VMA,
>  	MMU_NOTIFY_PROTECTION_PAGE,
>  	MMU_NOTIFY_SOFT_DIRTY,
> +	MMU_NOTIFY_RELEASE,
>  };

OK, let the naming debates begin! ha. Anyway, after careful study of the overall
patch, and some browsing of the larger patchset, it's clear that:

* The new "MMU range notifier" that you've created is,
approximately, a new
object. It uses classic mmu notifiers inside, as an implementation detail, and
it does *similar* things (notifications) as mmn's. But it's certainly
not the same
as mmn's, as shown later when you say the need to an entirely new ops
struct, and
data struct too.

Therefore, you need a separate events enum as well. This is important. MMN's
won't be issuing MMN_NOTIFY_RELEASE events, nor will MNR's be issuing
the first
four prexisting MMU_NOTIFY_* items. So it would be a design mistake to glom them
together, unless you ultimately decided to merge these MMN and MNR objects
(which
I don't really see any intention of, and that's fine).

So this should read:

enum mmu_range_notifier_event {
	MMU_NOTIFY_RELEASE,
};

...assuming that we stay with "mmu_range_notifier" as a core name for
this
whole thing.

Also, it is best moved down to be next to the new MNR structs, so that all the
MNR stuff is in one group.

Extra credit: IMHO, this clearly deserves to all be in a new
mmu_range_notifier.h
header file, but I know that's extra work. Maybe later as a follow-up patch,
if anyone has the time.
>  
>  #define MMU_NOTIFIER_RANGE_BLOCKABLE (1 << 0)
> @@ -222,6 +228,26 @@ struct mmu_notifier {
>  	unsigned int users;
>  };
>  
That should also be moved down, next to the new structs.



A little bit above these next items, just above "struct mmu_notifier"
(not shown here,
it's outside the diff area), there is some documentation about classic MMNs.
It would
be nice if it were clearer that that documentation is not relevant to MNRs.
Actually,
this is another reason that a separate header file would be nice.
> +/**
> + * struct mmu_range_notifier_ops
> + * @invalidate: Upon return the caller must stop using any SPTEs within
this
> + *              range, this function can sleep. Return false if blocking
was
> + *              required but range is non-blocking
> + */
How about this (I'm not sure I fully understand the return value, though):

/**
 * struct mmu_range_notifier_ops
 * @invalidate: Upon return the caller must stop using any SPTEs within this
 * 		range.
 *
 * 		This function is permitted to sleep.
 *
 *      	@Return: false if blocking was required, but @range is
 *			non-blocking.
 *
 */

> +struct mmu_range_notifier_ops {
> +	bool (*invalidate)(struct mmu_range_notifier *mrn,
> +			   const struct mmu_notifier_range *range,
> +			   unsigned long cur_seq);
> +};
> +
> +struct mmu_range_notifier {
> +	struct interval_tree_node interval_tree;
> +	const struct mmu_range_notifier_ops *ops;
> +	struct hlist_node deferred_item;
> +	unsigned long invalidate_seq;
> +	struct mm_struct *mm;
> +};
> +
Again, now we have the new struct mmu_range_notifier, and the old 
struct mmu_notifier_range, and it's not good.

Ideas:

a) Live with it.

b) (Discarded, too many callers): rename old one. Nope.

c) Rename new one. Ideas:

    struct mmu_interval_notifier
    struct mmu_range_intersection
    ...other ideas?

>  #ifdef CONFIG_MMU_NOTIFIER
>  
>  #ifdef CONFIG_LOCKDEP
> @@ -263,6 +289,78 @@ extern int __mmu_notifier_register(struct mmu_notifier
*mn,
>  				   struct mm_struct *mm);
>  extern void mmu_notifier_unregister(struct mmu_notifier *mn,
>  				    struct mm_struct *mm);
> +
> +unsigned long mmu_range_read_begin(struct mmu_range_notifier *mrn);
> +int mmu_range_notifier_insert(struct mmu_range_notifier *mrn,
> +			      unsigned long start, unsigned long length,
> +			      struct mm_struct *mm);
> +int mmu_range_notifier_insert_locked(struct mmu_range_notifier *mrn,
> +				     unsigned long start, unsigned long length,
> +				     struct mm_struct *mm);
> +void mmu_range_notifier_remove(struct mmu_range_notifier *mrn);
> +
> +/**
> + * mmu_range_set_seq - Save the invalidation sequence
How about:

 * mmu_range_set_seq - Set the .invalidate_seq to a new value.

> + * @mrn - The mrn passed to invalidate
> + * @cur_seq - The cur_seq passed to invalidate
> + *
> + * This must be called unconditionally from the invalidate callback of a
> + * struct mmu_range_notifier_ops under the same lock that is used to call
> + * mmu_range_read_retry(). It updates the sequence number for later use by
> + * mmu_range_read_retry().
> + *
> + * If the user does not call mmu_range_read_begin() or
mmu_range_read_retry()
nit: "caller" is better than "user", when referring
to...well, callers. "user"
most often refers to user space, whereas a call stack and function calling is 
clearly what you're referring to here (and in other places, especially
"user lock").
> + * then this call is not required.
> + */
> +static inline void mmu_range_set_seq(struct mmu_range_notifier *mrn,
> +				     unsigned long cur_seq)
> +{
> +	WRITE_ONCE(mrn->invalidate_seq, cur_seq);
> +}
> +
> +/**
> + * mmu_range_read_retry - End a read side critical section against a VA
range
> + * mrn: The range under lock
> + * seq: The return of the paired mmu_range_read_begin()
> + *
> + * This MUST be called under a user provided lock that is also held
> + * unconditionally by op->invalidate() when it calls
mmu_range_set_seq().
> + *
> + * Each call should be paired with a single mmu_range_read_begin() and
> + * should be used to conclude the read side.
> + *
> + * Returns true if an invalidation collided with this critical section,
and
> + * the caller should retry.
> + */
> +static inline bool mmu_range_read_retry(struct mmu_range_notifier *mrn,
> +					unsigned long seq)
> +{
> +	return mrn->invalidate_seq != seq;
> +}
> +
> +/**
> + * mmu_range_check_retry - Test if a collision has occurred
> + * mrn: The range under lock
> + * seq: The return of the matching mmu_range_read_begin()
> + *
> + * This can be used in the critical section between mmu_range_read_begin()
and
> + * mmu_range_read_retry().  A return of true indicates an invalidation has
> + * collided with this lock and a future mmu_range_read_retry() will return
> + * true.
> + *
> + * False is not reliable and only suggests a collision has not happened.
It
let's say "suggests that a collision *may* not have occurred."  

...
> diff --git a/mm/mmu_notifier.c b/mm/mmu_notifier.c
> index 367670cfd02b7b..d02d3c8c223eb7 100644
> --- a/mm/mmu_notifier.c
> +++ b/mm/mmu_notifier.c
> @@ -12,6 +12,7 @@
>  #include <linux/export.h>
>  #include <linux/mm.h>
>  #include <linux/err.h>
> +#include <linux/interval_tree.h>
>  #include <linux/srcu.h>
>  #include <linux/rcupdate.h>
>  #include <linux/sched.h>
> @@ -36,10 +37,243 @@ struct lockdep_map
__mmu_notifier_invalidate_range_start_map = {
>  struct mmu_notifier_mm {
>  	/* all mmu notifiers registered in this mm are queued in this list */
>  	struct hlist_head list;
> +	bool has_interval;
>  	/* to serialize the list modifications and hlist_unhashed */
>  	spinlock_t lock;
> +	unsigned long invalidate_seq;
> +	unsigned long active_invalidate_ranges;
> +	struct rb_root_cached itree;
> +	wait_queue_head_t wq;
> +	struct hlist_head deferred_list;
>  };
>  
> +/*
> + * This is a collision-retry read-side/write-side 'lock', a lot
like a
> + * seqcount, however this allows multiple write-sides to hold it at
> + * once. Conceptually the write side is protecting the values of the PTEs
in
> + * this mm, such that PTES cannot be read into SPTEs while any writer
exists.
Just to be kind, can we say "SPTEs (shadow PTEs)", just this once? :)
> + *
> + * Note that the core mm creates nested invalidate_range_start()/end()
regions
> + * within the same thread, and runs invalidate_range_start()/end() in
parallel
> + * on multiple CPUs. This is designed to not reduce concurrency or block
> + * progress on the mm side.
> + *
> + * As a secondary function, holding the full write side also serves to
prevent
> + * writers for the itree, this is an optimization to avoid extra locking
> + * during invalidate_range_start/end notifiers.
> + *
> + * The write side has two states, fully excluded:
> + *  - mm->active_invalidate_ranges != 0
> + *  - mnn->invalidate_seq & 1 == True
> + *  - some range on the mm_struct is being invalidated
> + *  - the itree is not allowed to change
> + *
> + * And partially excluded:
> + *  - mm->active_invalidate_ranges != 0
I assume this implies mnn->invalidate_seq & 1 == False in this case? If
so,
let's say so. I'm probably getting that wrong, too.
> + *  - some range on the mm_struct is being invalidated
> + *  - the itree is allowed to change
> + *
> + * The later state avoids some expensive work on inv_end in the common
case of
> + * no mrn monitoring the VA.
> + */
> +static bool mn_itree_is_invalidating(struct mmu_notifier_mm *mmn_mm)
> +{
> +	lockdep_assert_held(&mmn_mm->lock);
> +	return mmn_mm->invalidate_seq & 1;
> +}
> +
> +static struct mmu_range_notifier *
> +mn_itree_inv_start_range(struct mmu_notifier_mm *mmn_mm,
> +			 const struct mmu_notifier_range *range,
> +			 unsigned long *seq)
> +{
> +	struct interval_tree_node *node;
> +	struct mmu_range_notifier *res = NULL;
> +
> +	spin_lock(&mmn_mm->lock);
> +	mmn_mm->active_invalidate_ranges++;
> +	node = interval_tree_iter_first(&mmn_mm->itree, range->start,
> +					range->end - 1);
> +	if (node) {
> +		mmn_mm->invalidate_seq |= 1;

OK, this either needs more documentation and assertions, or a different
approach. Because I see addition, subtraction, AND, OR and booleans
all being applied to this field, and it's darn near hopeless to figure
out whether or not it really is even or odd at the right times.

Different approach: why not just add a mmn_mm->is_invalidating 
member variable? It's not like you're short of space in that struct.

> +		res = container_of(node, struct mmu_range_notifier,
> +				   interval_tree);
> +	}
> +
> +	*seq = mmn_mm->invalidate_seq;
> +	spin_unlock(&mmn_mm->lock);
> +	return res;
> +}
> +
> +static struct mmu_range_notifier *
> +mn_itree_inv_next(struct mmu_range_notifier *mrn,
> +		  const struct mmu_notifier_range *range)
> +{
> +	struct interval_tree_node *node;
> +
> +	node = interval_tree_iter_next(&mrn->interval_tree,
range->start,
> +				       range->end - 1);
> +	if (!node)
> +		return NULL;
> +	return container_of(node, struct mmu_range_notifier, interval_tree);
> +}
> +
> +static void mn_itree_inv_end(struct mmu_notifier_mm *mmn_mm)
> +{
> +	struct mmu_range_notifier *mrn;
> +	struct hlist_node *next;
> +	bool need_wake = false;
> +
> +	spin_lock(&mmn_mm->lock);
> +	if (--mmn_mm->active_invalidate_ranges ||
> +	    !mn_itree_is_invalidating(mmn_mm)) {
> +		spin_unlock(&mmn_mm->lock);
> +		return;
> +	}
> +
> +	mmn_mm->invalidate_seq++;
Is this the right place for an assertion that this is now an even value?
> +	need_wake = true;
> +
> +	/*
> +	 * The inv_end incorporates a deferred mechanism like
> +	 * rtnl_lock(). Adds and removes are queued until the final inv_end
Let me point out that rtnl_lock() itself is a one-liner that calls mutex_lock().
But I suppose if one studies that file closely there is more. :)

...
> +unsigned long mmu_range_read_begin(struct mmu_range_notifier *mrn)
> +{
> +	struct mmu_notifier_mm *mmn_mm = mrn->mm->mmu_notifier_mm;
> +	unsigned long seq;
> +	bool is_invalidating;
> +
> +	/*
> +	 * If the mrn has a different seq value under the user_lock than we
> +	 * started with then it has collided.
> +	 *
> +	 * If the mrn currently has the same seq value as the mmn_mm seq, then
> +	 * it is currently between invalidate_start/end and is colliding.
> +	 *
> +	 * The locking looks broadly like this:
> +	 *   mn_tree_invalidate_start():          mmu_range_read_begin():
> +	 *                                         spin_lock
> +	 *                                          seq =
READ_ONCE(mrn->invalidate_seq);
> +	 *                                          seq ==
mmn_mm->invalidate_seq
> +	 *                                         spin_unlock
> +	 *    spin_lock
> +	 *     seq = ++mmn_mm->invalidate_seq
> +	 *    spin_unlock
> +	 *     op->invalidate_range():
> +	 *       user_lock
> +	 *        mmu_range_set_seq()
> +	 *         mrn->invalidate_seq = seq
> +	 *       user_unlock
> +	 *
> +	 *                          [Required: mmu_range_read_retry() == true]
> +	 *
> +	 *   mn_itree_inv_end():
> +	 *    spin_lock
> +	 *     seq = ++mmn_mm->invalidate_seq
> +	 *    spin_unlock
> +	 *
> +	 *                                        user_lock
> +	 *                                         mmu_range_read_retry():
> +	 *                                          mrn->invalidate_seq != seq
> +	 *                                        user_unlock
> +	 *
> +	 * Barriers are not needed here as any races here are closed by an
> +	 * eventual mmu_range_read_retry(), which provides a barrier via the
> +	 * user_lock.
> +	 */
> +	spin_lock(&mmn_mm->lock);
> +	/* Pairs with the WRITE_ONCE in mmu_range_set_seq() */
> +	seq = READ_ONCE(mrn->invalidate_seq);
> +	is_invalidating = seq == mmn_mm->invalidate_seq;
> +	spin_unlock(&mmn_mm->lock);
> +
> +	/*
> +	 * mrn->invalidate_seq is always set to an odd value. This ensures
This claim just looks wrong the first N times one reads the code, given that
there is mmu_range_set_seq() to set it to an arbitrary value!  Maybe you mean

"is always set to an odd value when invalidating"??
> +	 * that if seq does wrap we will always clear the below sleep in some
> +	 * reasonable time as mmn_mm->invalidate_seq is even in the idle
> +	 * state.
> +	 */
Let's move that comment higher up. The code that follows it has nothing to
do with it, so it's confusing here.

...
> @@ -529,6 +852,166 @@ void mmu_notifier_put(struct mmu_notifier *mn)
>  }
>  EXPORT_SYMBOL_GPL(mmu_notifier_put);
>  
> +static int __mmu_range_notifier_insert(struct mmu_range_notifier *mrn,
> +				       unsigned long start,
> +				       unsigned long length,
> +				       struct mmu_notifier_mm *mmn_mm,
> +				       struct mm_struct *mm)
> +{
> +	mrn->mm = mm;
> +	RB_CLEAR_NODE(&mrn->interval_tree.rb);
> +	mrn->interval_tree.start = start;
> +	/*
> +	 * Note that the representation of the intervals in the interval tree
> +	 * considers the ending point as contained in the interval.
Thanks for that comment!
> +	 */
> +	if (length == 0 ||
> +	    check_add_overflow(start, length - 1,
&mrn->interval_tree.last))
> +		return -EOVERFLOW;
> +
> +	/* pairs with mmdrop in mmu_range_notifier_remove() */
> +	mmgrab(mm);
> +
> +	/*
> +	 * If some invalidate_range_start/end region is going on in parallel
> +	 * we don't know what VA ranges are affected, so we must assume this
> +	 * new range is included.
> +	 *
> +	 * If the itree is invalidating then we are not allowed to change
> +	 * it. Retrying until invalidation is done is tricky due to the
> +	 * possibility for live lock, instead defer the add to the unlock so
> +	 * this algorithm is deterministic.
> +	 *
> +	 * In all cases the value for the mrn->mr_invalidate_seq should be
> +	 * odd, see mmu_range_read_begin()
> +	 */
> +	spin_lock(&mmn_mm->lock);
> +	if (mmn_mm->active_invalidate_ranges) {
> +		if (mn_itree_is_invalidating(mmn_mm))
> +			hlist_add_head(&mrn->deferred_item,
> +				       &mmn_mm->deferred_list);
> +		else {
> +			mmn_mm->invalidate_seq |= 1;
> +			interval_tree_insert(&mrn->interval_tree,
> +					     &mmn_mm->itree);
> +		}
> +		mrn->invalidate_seq = mmn_mm->invalidate_seq;
> +	} else {
> +		WARN_ON(mn_itree_is_invalidating(mmn_mm));
> +		mrn->invalidate_seq = mmn_mm->invalidate_seq - 1;
Ohhh, checkmate. I lose. Why is *subtracting* the right thing to do
for seq numbers here?  I'm acutely unhappy trying to figure this out.
I suspect it's another unfortunate side effect of trying to use the
lower bit of the seq number (even/odd) for something else.
> +		interval_tree_insert(&mrn->interval_tree, &mmn_mm->itree);
> +	}
> +	spin_unlock(&mmn_mm->lock);
> +	return 0;
> +}
> +
> +/**
> + * mmu_range_notifier_insert - Insert a range notifier
> + * @mrn: Range notifier to register
> + * @start: Starting virtual address to monitor
> + * @length: Length of the range to monitor
> + * @mm : mm_struct to attach to
> + *
> + * This function subscribes the range notifier for notifications from the
mm.
> + * Upon return the ops related to mmu_range_notifier will be called
whenever
> + * an event that intersects with the given range occurs.
> + *
> + * Upon return the range_notifier may not be present in the interval tree
yet.
> + * The caller must use the normal range notifier locking flow via
> + * mmu_range_read_begin() to establish SPTEs for this range.
> + */
> +int mmu_range_notifier_insert(struct mmu_range_notifier *mrn,
> +			      unsigned long start, unsigned long length,
> +			      struct mm_struct *mm)
> +{
> +	struct mmu_notifier_mm *mmn_mm;
> +	int ret;
Hmmm, I think a later patch improperly changes the above to "int ret =
0;".
I'll check on that. It's correct here, though.
> +
> +	might_lock(&mm->mmap_sem);
> +
> +	mmn_mm = smp_load_acquire(&mm->mmu_notifier_mm);
What does the above pair with? Should have a comment that specifies that.

 
thanks,

John Hubbard
NVIDIA

On Wed, Nov 06, 2019 at 04:23:21PM -0800, John Hubbard
wrote:
> On 10/28/19 1:10 PM, Jason Gunthorpe wrote:
[...]
> >  /**
> >   * enum mmu_notifier_event - reason for the mmu notifier callback
> > @@ -32,6 +34,9 @@ struct mmu_notifier_range;
> >   * access flags). User should soft dirty the page in the end callback
to make
> >   * sure that anyone relying on soft dirtyness catch pages that might
be written
> >   * through non CPU mappings.
> > + *
> > + * @MMU_NOTIFY_RELEASE: used during mmu_range_notifier invalidate to
signal that
> > + * the mm refcount is zero and the range is no longer accessible.
> >   */
> >  enum mmu_notifier_event {
> >  	MMU_NOTIFY_UNMAP = 0,
> > @@ -39,6 +44,7 @@ enum mmu_notifier_event {
> >  	MMU_NOTIFY_PROTECTION_VMA,
> >  	MMU_NOTIFY_PROTECTION_PAGE,
> >  	MMU_NOTIFY_SOFT_DIRTY,
> > +	MMU_NOTIFY_RELEASE,
> >  };
> 
> 
> OK, let the naming debates begin! ha. Anyway, after careful study of the
overall
> patch, and some browsing of the larger patchset, it's clear that:
> 
> * The new "MMU range notifier" that you've created is,
approximately, a new
> object. It uses classic mmu notifiers inside, as an implementation detail,
and
> it does *similar* things (notifications) as mmn's. But it's
certainly not the same
> as mmn's, as shown later when you say the need to an entirely new ops
struct, and
> data struct too.
> 
> Therefore, you need a separate events enum as well. This is important.
MMN's
> won't be issuing MMN_NOTIFY_RELEASE events, nor will MNR's be
issuing the first
> four prexisting MMU_NOTIFY_* items. So it would be a design mistake to glom
them
> together, unless you ultimately decided to merge these MMN and MNR objects
(which
> I don't really see any intention of, and that's fine).
> 
> So this should read:
> 
> enum mmu_range_notifier_event {
> 	MMU_NOTIFY_RELEASE,
> };
> 
> ...assuming that we stay with "mmu_range_notifier" as a core name
for this
> whole thing.
> 
> Also, it is best moved down to be next to the new MNR structs, so that all
the
> MNR stuff is in one group.
> 
> Extra credit: IMHO, this clearly deserves to all be in a new
mmu_range_notifier.h
> header file, but I know that's extra work. Maybe later as a follow-up
patch,
> if anyone has the time.
The range notifier should get the event too, it would be a waste, i think it is
an oversight here. The release event is fine so NAK to you separate event. Event
is really an helper for notifier i had a set of patch for nouveau to leverage
this i need to resucite them. So no need to split thing, i would just forward
the event ie add event to mmu_range_notifier_ops.invalidate() i failed to catch
that in v1 sorry.


[...]
> > +struct mmu_range_notifier_ops {
> > +	bool (*invalidate)(struct mmu_range_notifier *mrn,
> > +			   const struct mmu_notifier_range *range,
> > +			   unsigned long cur_seq);
> > +};
> > +
> > +struct mmu_range_notifier {
> > +	struct interval_tree_node interval_tree;
> > +	const struct mmu_range_notifier_ops *ops;
> > +	struct hlist_node deferred_item;
> > +	unsigned long invalidate_seq;
> > +	struct mm_struct *mm;
> > +};
> > +
> 
> Again, now we have the new struct mmu_range_notifier, and the old 
> struct mmu_notifier_range, and it's not good.
> 
> Ideas:
> 
> a) Live with it.
> 
> b) (Discarded, too many callers): rename old one. Nope.
> 
> c) Rename new one. Ideas:
> 
>     struct mmu_interval_notifier
>     struct mmu_range_intersection
>     ...other ideas?
I vote for interval_notifier we do want notifier in name but i am also
fine with current name.

[...]
> > + *
> > + * Note that the core mm creates nested
invalidate_range_start()/end() regions
> > + * within the same thread, and runs invalidate_range_start()/end() in
parallel
> > + * on multiple CPUs. This is designed to not reduce concurrency or
block
> > + * progress on the mm side.
> > + *
> > + * As a secondary function, holding the full write side also serves
to prevent
> > + * writers for the itree, this is an optimization to avoid extra
locking
> > + * during invalidate_range_start/end notifiers.
> > + *
> > + * The write side has two states, fully excluded:
> > + *  - mm->active_invalidate_ranges != 0
> > + *  - mnn->invalidate_seq & 1 == True
> > + *  - some range on the mm_struct is being invalidated
> > + *  - the itree is not allowed to change
> > + *
> > + * And partially excluded:
> > + *  - mm->active_invalidate_ranges != 0
> 
> I assume this implies mnn->invalidate_seq & 1 == False in this case?
If so,
> let's say so. I'm probably getting that wrong, too.
Yes (mnn->invalidate_seq & 1) == 0
> 
> > + *  - some range on the mm_struct is being invalidated
> > + *  - the itree is allowed to change
> > + *
> > + * The later state avoids some expensive work on inv_end in the
common case of
> > + * no mrn monitoring the VA.
> > + */
> > +static bool mn_itree_is_invalidating(struct mmu_notifier_mm *mmn_mm)
> > +{
> > +	lockdep_assert_held(&mmn_mm->lock);
> > +	return mmn_mm->invalidate_seq & 1;
> > +}
> > +
> > +static struct mmu_range_notifier *
> > +mn_itree_inv_start_range(struct mmu_notifier_mm *mmn_mm,
> > +			 const struct mmu_notifier_range *range,
> > +			 unsigned long *seq)
> > +{
> > +	struct interval_tree_node *node;
> > +	struct mmu_range_notifier *res = NULL;
> > +
> > +	spin_lock(&mmn_mm->lock);
> > +	mmn_mm->active_invalidate_ranges++;
> > +	node = interval_tree_iter_first(&mmn_mm->itree,
range->start,
> > +					range->end - 1);
> > +	if (node) {
> > +		mmn_mm->invalidate_seq |= 1;
> 
> 
> OK, this either needs more documentation and assertions, or a different
> approach. Because I see addition, subtraction, AND, OR and booleans
> all being applied to this field, and it's darn near hopeless to figure
> out whether or not it really is even or odd at the right times.
> 
> Different approach: why not just add a mmn_mm->is_invalidating 
> member variable? It's not like you're short of space in that
struct.
The invalidate_seq scheme looks fine to me, maybe it can use more comments.

> 
> 
> > +		res = container_of(node, struct mmu_range_notifier,
> > +				   interval_tree);
> > +	}
> > +
> > +	*seq = mmn_mm->invalidate_seq;
> > +	spin_unlock(&mmn_mm->lock);
> > +	return res;
> > +}
> > +
> > +static struct mmu_range_notifier *
> > +mn_itree_inv_next(struct mmu_range_notifier *mrn,
> > +		  const struct mmu_notifier_range *range)
> > +{
> > +	struct interval_tree_node *node;
> > +
> > +	node = interval_tree_iter_next(&mrn->interval_tree,
range->start,
> > +				       range->end - 1);
> > +	if (!node)
> > +		return NULL;
> > +	return container_of(node, struct mmu_range_notifier, interval_tree);
> > +}
> > +
> > +static void mn_itree_inv_end(struct mmu_notifier_mm *mmn_mm)
> > +{
> > +	struct mmu_range_notifier *mrn;
> > +	struct hlist_node *next;
> > +	bool need_wake = false;
> > +
> > +	spin_lock(&mmn_mm->lock);
> > +	if (--mmn_mm->active_invalidate_ranges ||
> > +	    !mn_itree_is_invalidating(mmn_mm)) {
> > +		spin_unlock(&mmn_mm->lock);
> > +		return;
> > +	}
> > +
> > +	mmn_mm->invalidate_seq++;
> 
> Is this the right place for an assertion that this is now an even value?
Yes at that point it should be even ie mmn_mm->active_invalidate_ranges == 0
and we are holding the lock thus nothing can set the lower bit of invalidate_seq
and ++ should lead to even number.
> 
> > +	need_wake = true;
> > +
> > +	/*
> > +	 * The inv_end incorporates a deferred mechanism like
> > +	 * rtnl_lock(). Adds and removes are queued until the final inv_end
> 
> Let me point out that rtnl_lock() itself is a one-liner that calls
mutex_lock().
> But I suppose if one studies that file closely there is more. :)
I think i commented in v1 about rtnl_lock() being something network people only
might be familiar, i think i saw it documented somewhere, maybe a lwn article.
But if you are familiar with network it is a think well understood ... for any
reasonable network scholar ;)
> ...
> 
> > +unsigned long mmu_range_read_begin(struct mmu_range_notifier *mrn)
> > +{
> > +	struct mmu_notifier_mm *mmn_mm = mrn->mm->mmu_notifier_mm;
> > +	unsigned long seq;
> > +	bool is_invalidating;
> > +
> > +	/*
> > +	 * If the mrn has a different seq value under the user_lock than we
> > +	 * started with then it has collided.
> > +	 *
> > +	 * If the mrn currently has the same seq value as the mmn_mm seq,
then
> > +	 * it is currently between invalidate_start/end and is colliding.
> > +	 *
> > +	 * The locking looks broadly like this:
> > +	 *   mn_tree_invalidate_start():          mmu_range_read_begin():
> > +	 *                                         spin_lock
> > +	 *                                          seq =
READ_ONCE(mrn->invalidate_seq);
> > +	 *                                          seq ==
mmn_mm->invalidate_seq
> > +	 *                                         spin_unlock
> > +	 *    spin_lock
> > +	 *     seq = ++mmn_mm->invalidate_seq
> > +	 *    spin_unlock
> > +	 *     op->invalidate_range():
> > +	 *       user_lock
> > +	 *        mmu_range_set_seq()
> > +	 *         mrn->invalidate_seq = seq
> > +	 *       user_unlock
> > +	 *
> > +	 *                          [Required: mmu_range_read_retry() ==
true]
> > +	 *
> > +	 *   mn_itree_inv_end():
> > +	 *    spin_lock
> > +	 *     seq = ++mmn_mm->invalidate_seq
> > +	 *    spin_unlock
> > +	 *
> > +	 *                                        user_lock
> > +	 *                                         mmu_range_read_retry():
> > +	 *                                          mrn->invalidate_seq
!= seq
> > +	 *                                        user_unlock
> > +	 *
> > +	 * Barriers are not needed here as any races here are closed by an
> > +	 * eventual mmu_range_read_retry(), which provides a barrier via the
> > +	 * user_lock.
> > +	 */
> > +	spin_lock(&mmn_mm->lock);
> > +	/* Pairs with the WRITE_ONCE in mmu_range_set_seq() */
> > +	seq = READ_ONCE(mrn->invalidate_seq);
> > +	is_invalidating = seq == mmn_mm->invalidate_seq;
> > +	spin_unlock(&mmn_mm->lock);
> > +
> > +	/*
> > +	 * mrn->invalidate_seq is always set to an odd value. This
ensures
> 
> This claim just looks wrong the first N times one reads the code, given
that
> there is mmu_range_set_seq() to set it to an arbitrary value!  Maybe you
mean
> 
> "is always set to an odd value when invalidating"??
No it is always odd, you must call mmu_range_set_seq() only from the
op->invalidate_range() callback at which point the seq is odd. As well
when mrn is added and its seq first set it is set to an odd value
always. Maybe the comment, should read:

 * mrn->invalidate_seq is always, yes always, set to an odd value. This
ensures

To stress that it is not an error.
> 
> > +	 * that if seq does wrap we will always clear the below sleep in
some
> > +	 * reasonable time as mmn_mm->invalidate_seq is even in the idle
> > +	 * state.
> > +	 */
> 
> Let's move that comment higher up. The code that follows it has nothing
to
> do with it, so it's confusing here.
No the comment is in the right place, the fact that it is odd and that
idle state is even explains why the wait() will never last forever.
Already had a discussion on this in v1.

[...]
> > +	/*
> > +	 * If some invalidate_range_start/end region is going on in parallel
> > +	 * we don't know what VA ranges are affected, so we must assume
this
> > +	 * new range is included.
> > +	 *
> > +	 * If the itree is invalidating then we are not allowed to change
> > +	 * it. Retrying until invalidation is done is tricky due to the
> > +	 * possibility for live lock, instead defer the add to the unlock so
> > +	 * this algorithm is deterministic.
> > +	 *
> > +	 * In all cases the value for the mrn->mr_invalidate_seq should
be
> > +	 * odd, see mmu_range_read_begin()
> > +	 */
> > +	spin_lock(&mmn_mm->lock);
> > +	if (mmn_mm->active_invalidate_ranges) {
> > +		if (mn_itree_is_invalidating(mmn_mm))
> > +			hlist_add_head(&mrn->deferred_item,
> > +				       &mmn_mm->deferred_list);
> > +		else {
> > +			mmn_mm->invalidate_seq |= 1;
> > +			interval_tree_insert(&mrn->interval_tree,
> > +					     &mmn_mm->itree);
> > +		}
> > +		mrn->invalidate_seq = mmn_mm->invalidate_seq;
> > +	} else {
> > +		WARN_ON(mn_itree_is_invalidating(mmn_mm));
> > +		mrn->invalidate_seq = mmn_mm->invalidate_seq - 1;
> 
> Ohhh, checkmate. I lose. Why is *subtracting* the right thing to do
> for seq numbers here?  I'm acutely unhappy trying to figure this out.
> I suspect it's another unfortunate side effect of trying to use the
> lower bit of the seq number (even/odd) for something else.
If there is no mmn_mm->active_invalidate_ranges then it means that
mmn_mm->invalidate_seq is even and thus mmn_mm->invalidate_seq - 1
is an odd number which means that mrn->invalidate_seq is initialized
to odd value and if you follow the rule for calling mmu_range_set_seq()
then it will _always_ be an odd number and this close the loop with
the above comments :)
> 
> > +		interval_tree_insert(&mrn->interval_tree,
&mmn_mm->itree);
> > +	}
> > +	spin_unlock(&mmn_mm->lock);
> > +	return 0;
> > +}
> > +
> > +/**
> > + * mmu_range_notifier_insert - Insert a range notifier
> > + * @mrn: Range notifier to register
> > + * @start: Starting virtual address to monitor
> > + * @length: Length of the range to monitor
> > + * @mm : mm_struct to attach to
> > + *
> > + * This function subscribes the range notifier for notifications from
the mm.
> > + * Upon return the ops related to mmu_range_notifier will be called
whenever
> > + * an event that intersects with the given range occurs.
> > + *
> > + * Upon return the range_notifier may not be present in the interval
tree yet.
> > + * The caller must use the normal range notifier locking flow via
> > + * mmu_range_read_begin() to establish SPTEs for this range.
> > + */
> > +int mmu_range_notifier_insert(struct mmu_range_notifier *mrn,
> > +			      unsigned long start, unsigned long length,
> > +			      struct mm_struct *mm)
> > +{
> > +	struct mmu_notifier_mm *mmn_mm;
> > +	int ret;
> 
> Hmmm, I think a later patch improperly changes the above to "int ret =
0;".
> I'll check on that. It's correct here, though.
> 
> > +
> > +	might_lock(&mm->mmap_sem);
> > +
> > +	mmn_mm = smp_load_acquire(&mm->mmu_notifier_mm);
> 
> What does the above pair with? Should have a comment that specifies that.
It was discussed in v1 but maybe a comment of what was said back then would
be helpful. Something like:

/*
 * We need to insure that all writes to mm->mmu_notifier_mm are visible
before
 * any checks we do on mmn_mm below as otherwise CPU might re-order write done
 * by another CPU core to mm->mmu_notifier_mm structure fields after the read
 * belows.
 */

Cheers,
J?r?me

On Wed, Nov 06, 2019 at 04:23:21PM -0800, John Hubbard wrote:
 
> Nice design, I love the seq foundation! So far, I'm not able to spot
anything
> actually wrong with the implementation, sorry about that. 
Alas :( I feel there must be a bug in here still, but onwards!

One of the main sad points was it didn't make sense to use the
existing seqlock/seqcount primitives as they have both the wrong write
concurrancy model and extra barriers that are not needed when it is
always manipulated under a spinlock
 
> 1. There is a rather severe naming overlap (not technically a naming
conflict,
> but still) with existing mmn work, which already has, for example:
> 
>     struct mmu_notifier_range
> 
> ...and you're adding:
> 
>     struct mmu_range_notifier
> 
> ...so I'll try to help sort that out.
Yes, I've been sad about this too.
> So this should read:
> 
> enum mmu_range_notifier_event {
> 	MMU_NOTIFY_RELEASE,
> };
> 
> ...assuming that we stay with "mmu_range_notifier" as a core name
for this
> whole thing.
> 
> Also, it is best moved down to be next to the new MNR structs, so that all
the
> MNR stuff is in one group.
I agree with Jerome, this enum is part of the 'struct
mmu_notifier_range' (ie the description of the invalidation) and it
doesn't really matter that only these new notifiers can be called with
this type, it is still part of the mmu_notifier_range.

The comment already says it only applies to the mmu_range_notifier
scheme..
> >  #define MMU_NOTIFIER_RANGE_BLOCKABLE (1 << 0)
> > @@ -222,6 +228,26 @@ struct mmu_notifier {
> >  	unsigned int users;
> >  };
> >  
> 
> That should also be moved down, next to the new structs.
Which this?
> > +/**
> > + * struct mmu_range_notifier_ops
> > + * @invalidate: Upon return the caller must stop using any SPTEs
within this
> > + *              range, this function can sleep. Return false if
blocking was
> > + *              required but range is non-blocking
> > + */
> 
> How about this (I'm not sure I fully understand the return value,
though):
> 
> /**
>  * struct mmu_range_notifier_ops
>  * @invalidate: Upon return the caller must stop using any SPTEs within
this
>  * 		range.
>  *
>  * 		This function is permitted to sleep.
>  *
>  *      	@Return: false if blocking was required, but @range is
>  *			non-blocking.
>  *
>  */
Is this kdoc format for function pointers?
 
> 
> > +struct mmu_range_notifier_ops {
> > +	bool (*invalidate)(struct mmu_range_notifier *mrn,
> > +			   const struct mmu_notifier_range *range,
> > +			   unsigned long cur_seq);
> > +};
> > +
> > +struct mmu_range_notifier {
> > +	struct interval_tree_node interval_tree;
> > +	const struct mmu_range_notifier_ops *ops;
> > +	struct hlist_node deferred_item;
> > +	unsigned long invalidate_seq;
> > +	struct mm_struct *mm;
> > +};
> > +
> 
> Again, now we have the new struct mmu_range_notifier, and the old 
> struct mmu_notifier_range, and it's not good.
> 
> Ideas:
> 
> a) Live with it.
> 
> b) (Discarded, too many callers): rename old one. Nope.
> 
> c) Rename new one. Ideas:
> 
>     struct mmu_interval_notifier
>     struct mmu_range_intersection
>     ...other ideas?
This odd duality has already cause some confusion, but names here are
hard.  mmu_interval_notifier is the best alternative I've heard.

Changing this name is a lot of work - are we happy
'mmu_interval_notifier' is the right choice?
> > +/**
> > + * mmu_range_set_seq - Save the invalidation sequence
> 
> How about:
> 
>  * mmu_range_set_seq - Set the .invalidate_seq to a new value.
It is not a 'new value', it is a value that is provided to the
invalidate callback
> 
> > + * @mrn - The mrn passed to invalidate
> > + * @cur_seq - The cur_seq passed to invalidate
> > + *
> > + * This must be called unconditionally from the invalidate callback
of a
> > + * struct mmu_range_notifier_ops under the same lock that is used to
call
> > + * mmu_range_read_retry(). It updates the sequence number for later
use by
> > + * mmu_range_read_retry().
> > + *
> > + * If the user does not call mmu_range_read_begin() or
mmu_range_read_retry()
> 
> nit: "caller" is better than "user", when referring
to...well, callers. "user"
> most often refers to user space, whereas a call stack and function calling
is
> clearly what you're referring to here (and in other places, especially
"user lock").
Done
> > +/**
> > + * mmu_range_check_retry - Test if a collision has occurred
> > + * mrn: The range under lock
> > + * seq: The return of the matching mmu_range_read_begin()
> > + *
> > + * This can be used in the critical section between
mmu_range_read_begin() and
> > + * mmu_range_read_retry().  A return of true indicates an
invalidation has
> > + * collided with this lock and a future mmu_range_read_retry() will
return
> > + * true.
> > + *
> > + * False is not reliable and only suggests a collision has not
happened. It
> 
> let's say "suggests that a collision *may* not have
occurred."
Sure
> > +/*
> > + * This is a collision-retry read-side/write-side 'lock', a
lot like a
> > + * seqcount, however this allows multiple write-sides to hold it at
> > + * once. Conceptually the write side is protecting the values of the
PTEs in
> > + * this mm, such that PTES cannot be read into SPTEs while any writer
exists.
> 
> Just to be kind, can we say "SPTEs (shadow PTEs)", just this
once? :)
Haha, sure, why not
> > + * The write side has two states, fully excluded:
> > + *  - mm->active_invalidate_ranges != 0
> > + *  - mnn->invalidate_seq & 1 == True
> > + *  - some range on the mm_struct is being invalidated
> > + *  - the itree is not allowed to change
> > + *
> > + * And partially excluded:
> > + *  - mm->active_invalidate_ranges != 0
> 
> I assume this implies mnn->invalidate_seq & 1 == False in this case?
If so,
> let's say so. I'm probably getting that wrong, too.
Yes that is right, done
> 
> > + *  - some range on the mm_struct is being invalidated
> > + *  - the itree is allowed to change
> > + *
> > + * The later state avoids some expensive work on inv_end in the
common case of
> > + * no mrn monitoring the VA.
> > + */
> > +static bool mn_itree_is_invalidating(struct mmu_notifier_mm *mmn_mm)
> > +{
> > +	lockdep_assert_held(&mmn_mm->lock);
> > +	return mmn_mm->invalidate_seq & 1;
> > +}
> > +
> > +static struct mmu_range_notifier *
> > +mn_itree_inv_start_range(struct mmu_notifier_mm *mmn_mm,
> > +			 const struct mmu_notifier_range *range,
> > +			 unsigned long *seq)
> > +{
> > +	struct interval_tree_node *node;
> > +	struct mmu_range_notifier *res = NULL;
> > +
> > +	spin_lock(&mmn_mm->lock);
> > +	mmn_mm->active_invalidate_ranges++;
> > +	node = interval_tree_iter_first(&mmn_mm->itree,
range->start,
> > +					range->end - 1);
> > +	if (node) {
> > +		mmn_mm->invalidate_seq |= 1;
> 
> 
> OK, this either needs more documentation and assertions, or a different
> approach. Because I see addition, subtraction, AND, OR and booleans
> all being applied to this field, and it's darn near hopeless to figure
> out whether or not it really is even or odd at the right times.
This is a standard design for a seqlock scheme and follows the
existing design of the linux seq lock.

The lower bit indicates the lock'd state and the upper bits indicate
the generation of the lock

The operations on the lock itself are then:
   seq |= 1  # Take the lock
   seq++     # Release an acquired lock
   seq & 1   # True if locked

Which is how this is written
> Different approach: why not just add a mmn_mm->is_invalidating 
> member variable? It's not like you're short of space in that
struct.
Splitting it makes alot of stuff more complex and unnatural.

The ops above could be put in inline wrappers, but they only occur
only in functions already called mn_itree_inv_start_range() and
mn_itree_inv_end() and mn_itree_is_invalidating().

There is the one 'take the lock' outlier in
__mmu_range_notifier_insert() though
> > +static void mn_itree_inv_end(struct mmu_notifier_mm *mmn_mm)
> > +{
> > +	struct mmu_range_notifier *mrn;
> > +	struct hlist_node *next;
> > +	bool need_wake = false;
> > +
> > +	spin_lock(&mmn_mm->lock);
> > +	if (--mmn_mm->active_invalidate_ranges ||
> > +	    !mn_itree_is_invalidating(mmn_mm)) {
> > +		spin_unlock(&mmn_mm->lock);
> > +		return;
> > +	}
> > +
> > +	mmn_mm->invalidate_seq++;
> 
> Is this the right place for an assertion that this is now an even value?
Yes, but I'm reluctant to add such a runtime check on this fastish path..
How about a comment?
> > +	need_wake = true;
> > +
> > +	/*
> > +	 * The inv_end incorporates a deferred mechanism like
> > +	 * rtnl_lock(). Adds and removes are queued until the final inv_end
> 
> Let me point out that rtnl_lock() itself is a one-liner that calls
mutex_lock().
> But I suppose if one studies that file closely there is more. :)
Lets change that to rtnl_unlock() then
> > +	spin_lock(&mmn_mm->lock);
> > +	/* Pairs with the WRITE_ONCE in mmu_range_set_seq() */
> > +	seq = READ_ONCE(mrn->invalidate_seq);
> > +	is_invalidating = seq == mmn_mm->invalidate_seq;
> > +	spin_unlock(&mmn_mm->lock);
> > +
> > +	/*
> > +	 * mrn->invalidate_seq is always set to an odd value. This
ensures
> 
> This claim just looks wrong the first N times one reads the code, given
that
> there is mmu_range_set_seq() to set it to an arbitrary value!  Maybe
> you mean
mmu_range_set_seq() is NOT to be used to set to an arbitary value, it
must only be used to set to the value provided in the invalidate()
callback and that value is always odd. Lets make this super clear:

	/*
	 * mrn->invalidate_seq must always be set to an odd value via
	 * mmu_range_set_seq() using the provided cur_seq from
	 * mn_itree_inv_start_range(). This ensures that if seq does wrap we
	 * will always clear the below sleep in some reasonable time as
	 * mmn_mm->invalidate_seq is even in the idle state.
	 */

The invarient is that the 'struct mmu_range_notifier' always has an
odd 'seq'
> > +	 * that if seq does wrap we will always clear the below sleep in
some
> > +	 * reasonable time as mmn_mm->invalidate_seq is even in the idle
> > +	 * state.
> > +	 */
> 
> Let's move that comment higher up. The code that follows it has nothing
to
> do with it, so it's confusing here.
The comment is explaining why the wait_event is safe, even if we wrap
the sequence number, which is a significant and very subtle corner
case. This is really why we have the even/odd thing at all.
> > +	spin_lock(&mmn_mm->lock);
> > +	if (mmn_mm->active_invalidate_ranges) {
> > +		if (mn_itree_is_invalidating(mmn_mm))
> > +			hlist_add_head(&mrn->deferred_item,
> > +				       &mmn_mm->deferred_list);
> > +		else {
> > +			mmn_mm->invalidate_seq |= 1;
> > +			interval_tree_insert(&mrn->interval_tree,
> > +					     &mmn_mm->itree);
> > +		}
> > +		mrn->invalidate_seq = mmn_mm->invalidate_seq;
> > +	} else {
> > +		WARN_ON(mn_itree_is_invalidating(mmn_mm));
> > +		mrn->invalidate_seq = mmn_mm->invalidate_seq - 1;
> 
> Ohhh, checkmate. I lose. Why is *subtracting* the right thing to do
> for seq numbers here?  I'm acutely unhappy trying to figure this out.
> I suspect it's another unfortunate side effect of trying to use the
> lower bit of the seq number (even/odd) for something else.
No, this is actually done for the seq number itself. We need to
generate a seq number that is != the current invalidate_seq as this
new mrn is not invalidating.

The best seq to use is one that the invalidate_seq will not reach for
a long time, ie 'invalidate_seq + MAX' which is expressed as -1

The even/odd thing just takes care of itself naturally here as
invalidate_seq is guarenteed even and -1 creates both an odd mrn value
and a good seq number.

The algorithm would actually work correctly if this was
'mrn->invalidate_seq = 1', but occasionally things would block when
they don't need to block.

Lets add a comment:

		/*
		 * The starting seq for a mrn not under invalidation should be
		 * odd, not equal to the current invalidate_seq and
		 * invalidate_seq should not 'wrap' to the new seq any time
		 * soon.
		 */
> > +int mmu_range_notifier_insert(struct mmu_range_notifier *mrn,
> > +			      unsigned long start, unsigned long length,
> > +			      struct mm_struct *mm)
> > +{
> > +	struct mmu_notifier_mm *mmn_mm;
> > +	int ret;
> 
> Hmmm, I think a later patch improperly changes the above to "int ret =
0;".
> I'll check on that. It's correct here, though.
Looks OK in my tree?
> > +	might_lock(&mm->mmap_sem);
> > +
> > +	mmn_mm = smp_load_acquire(&mm->mmu_notifier_mm);
> 
> What does the above pair with? Should have a comment that specifies that.
smp_load_acquire() always pairs with smp_store_release() to the same
memory, there is only one store, is a comment really needed?

Below are the comment updates I made, thanks!

Jason

diff --git a/include/linux/mmu_notifier.h b/include/linux/mmu_notifier.h
index 51b92ba013ddce..065c95002e9602 100644
--- a/include/linux/mmu_notifier.h
+++ b/include/linux/mmu_notifier.h
@@ -302,15 +302,15 @@ void mmu_range_notifier_remove(struct mmu_range_notifier
*mrn);
 /**
  * mmu_range_set_seq - Save the invalidation sequence
  * @mrn - The mrn passed to invalidate
- * @cur_seq - The cur_seq passed to invalidate
+ * @cur_seq - The cur_seq passed to the invalidate() callback
  *
  * This must be called unconditionally from the invalidate callback of a
  * struct mmu_range_notifier_ops under the same lock that is used to call
  * mmu_range_read_retry(). It updates the sequence number for later use by
- * mmu_range_read_retry().
+ * mmu_range_read_retry(). The provided cur_seq will always be odd.
  *
- * If the user does not call mmu_range_read_begin() or mmu_range_read_retry()
- * then this call is not required.
+ * If the caller does not call mmu_range_read_begin() or
+ * mmu_range_read_retry() then this call is not required.
  */
 static inline void mmu_range_set_seq(struct mmu_range_notifier *mrn,
 				     unsigned long cur_seq)
@@ -348,8 +348,9 @@ static inline bool mmu_range_read_retry(struct
mmu_range_notifier *mrn,
  * collided with this lock and a future mmu_range_read_retry() will return
  * true.
  *
- * False is not reliable and only suggests a collision has not happened. It
- * can be called many times and does not have to hold the user provided lock.
+ * False is not reliable and only suggests a collision may not have
+ * occured. It can be called many times and does not have to hold the user
+ * provided lock.
  *
  * This call can be used as part of loops and other expensive operations to
  * expedite a retry.
diff --git a/mm/mmu_notifier.c b/mm/mmu_notifier.c
index 2b7485919ecfeb..afe1e2d94183f8 100644
--- a/mm/mmu_notifier.c
+++ b/mm/mmu_notifier.c
@@ -51,7 +51,8 @@ struct mmu_notifier_mm {
  * This is a collision-retry read-side/write-side 'lock', a lot like a
  * seqcount, however this allows multiple write-sides to hold it at
  * once. Conceptually the write side is protecting the values of the PTEs in
- * this mm, such that PTES cannot be read into SPTEs while any writer exists.
+ * this mm, such that PTES cannot be read into SPTEs (shadow PTEs) while any
+ * writer exists.
  *
  * Note that the core mm creates nested invalidate_range_start()/end() regions
  * within the same thread, and runs invalidate_range_start()/end() in parallel
@@ -64,12 +65,13 @@ struct mmu_notifier_mm {
  *
  * The write side has two states, fully excluded:
  *  - mm->active_invalidate_ranges != 0
- *  - mnn->invalidate_seq & 1 == True
+ *  - mnn->invalidate_seq & 1 == True (odd)
  *  - some range on the mm_struct is being invalidated
  *  - the itree is not allowed to change
  *
  * And partially excluded:
  *  - mm->active_invalidate_ranges != 0
+ *  - mnn->invalidate_seq & 1 == False (even)
  *  - some range on the mm_struct is being invalidated
  *  - the itree is allowed to change
  *
@@ -131,12 +133,13 @@ static void mn_itree_inv_end(struct mmu_notifier_mm
*mmn_mm)
 		return;
 	}
 
+	/* Make invalidate_seq even */
 	mmn_mm->invalidate_seq++;
 	need_wake = true;
 
 	/*
 	 * The inv_end incorporates a deferred mechanism like
-	 * rtnl_lock(). Adds and removes are queued until the final inv_end
+	 * rtnl_unlock(). Adds and removes are queued until the final inv_end
 	 * happens then they are progressed. This arrangement for tree updates
 	 * is used to avoid using a blocking lock during
 	 * invalidate_range_start.
@@ -230,10 +233,11 @@ unsigned long mmu_range_read_begin(struct
mmu_range_notifier *mrn)
 	spin_unlock(&mmn_mm->lock);
 
 	/*
-	 * mrn->invalidate_seq is always set to an odd value. This ensures
-	 * that if seq does wrap we will always clear the below sleep in some
-	 * reasonable time as mmn_mm->invalidate_seq is even in the idle
-	 * state.
+	 * mrn->invalidate_seq must always be set to an odd value via
+	 * mmu_range_set_seq() using the provided cur_seq from
+	 * mn_itree_inv_start_range(). This ensures that if seq does wrap we
+	 * will always clear the below sleep in some reasonable time as
+	 * mmn_mm->invalidate_seq is even in the idle state.
 	 */
 	lock_map_acquire(&__mmu_notifier_invalidate_range_start_map);
 	lock_map_release(&__mmu_notifier_invalidate_range_start_map);
@@ -892,6 +896,12 @@ static int __mmu_range_notifier_insert(struct
mmu_range_notifier *mrn,
 		mrn->invalidate_seq = mmn_mm->invalidate_seq;
 	} else {
 		WARN_ON(mn_itree_is_invalidating(mmn_mm));
+		/*
+		 * The starting seq for a mrn not under invalidation should be
+		 * odd, not equal to the current invalidate_seq and
+		 * invalidate_seq should not 'wrap' to the new seq any time
+		 * soon.
+		 */
 		mrn->invalidate_seq = mmn_mm->invalidate_seq - 1;
 		interval_tree_insert(&mrn->interval_tree, &mmn_mm->itree);
 	}

On Wed, Nov 06, 2019 at 09:08:07PM -0500, Jerome Glisse wrote:
> > 
> > Extra credit: IMHO, this clearly deserves to all be in a new
mmu_range_notifier.h
> > header file, but I know that's extra work. Maybe later as a
follow-up patch,
> > if anyone has the time.
> 
> The range notifier should get the event too, it would be a waste, i think
it is
> an oversight here. The release event is fine so NAK to you separate event.
Event
> is really an helper for notifier i had a set of patch for nouveau to
leverage
> this i need to resucite them. So no need to split thing, i would just
forward
> the event ie add event to mmu_range_notifier_ops.invalidate() i failed to
catch
> that in v1 sorry.
I think what you mean is already done?

struct mmu_range_notifier_ops {
	bool (*invalidate)(struct mmu_range_notifier *mrn,
			   const struct mmu_notifier_range *range,
			   unsigned long cur_seq);
> No it is always odd, you must call mmu_range_set_seq() only from the
> op->invalidate_range() callback at which point the seq is odd. As well
> when mrn is added and its seq first set it is set to an odd value
> always. Maybe the comment, should read:
> 
>  * mrn->invalidate_seq is always, yes always, set to an odd value. This
ensures
> 
> To stress that it is not an error.
I went with this:

	/*
	 * mrn->invalidate_seq must always be set to an odd value via
	 * mmu_range_set_seq() using the provided cur_seq from
	 * mn_itree_inv_start_range(). This ensures that if seq does wrap we
	 * will always clear the below sleep in some reasonable time as
	 * mmn_mm->invalidate_seq is even in the idle state.
	 */
> > > +	spin_lock(&mmn_mm->lock);
> > > +	if (mmn_mm->active_invalidate_ranges) {
> > > +		if (mn_itree_is_invalidating(mmn_mm))
> > > +			hlist_add_head(&mrn->deferred_item,
> > > +				       &mmn_mm->deferred_list);
> > > +		else {
> > > +			mmn_mm->invalidate_seq |= 1;
> > > +			interval_tree_insert(&mrn->interval_tree,
> > > +					     &mmn_mm->itree);
> > > +		}
> > > +		mrn->invalidate_seq = mmn_mm->invalidate_seq;
> > > +	} else {
> > > +		WARN_ON(mn_itree_is_invalidating(mmn_mm));
> > > +		mrn->invalidate_seq = mmn_mm->invalidate_seq - 1;
> > 
> > Ohhh, checkmate. I lose. Why is *subtracting* the right thing to do
> > for seq numbers here?  I'm acutely unhappy trying to figure this
out.
> > I suspect it's another unfortunate side effect of trying to use
the
> > lower bit of the seq number (even/odd) for something else.
> 
> If there is no mmn_mm->active_invalidate_ranges then it means that
> mmn_mm->invalidate_seq is even and thus mmn_mm->invalidate_seq - 1
> is an odd number which means that mrn->invalidate_seq is initialized
> to odd value and if you follow the rule for calling mmu_range_set_seq()
> then it will _always_ be an odd number and this close the loop with
> the above comments :)
The key thing is that it is an odd value that will take a long time
before mmn_mm->invalidate seq reaches it
> > > +	might_lock(&mm->mmap_sem);
> > > +
> > > +	mmn_mm = smp_load_acquire(&mm->mmu_notifier_mm);
> > 
> > What does the above pair with? Should have a comment that specifies
that.
> 
> It was discussed in v1 but maybe a comment of what was said back then would
> be helpful. Something like:
> 
> /*
>  * We need to insure that all writes to mm->mmu_notifier_mm are visible
before
>  * any checks we do on mmn_mm below as otherwise CPU might re-order write
done
>  * by another CPU core to mm->mmu_notifier_mm structure fields after the
read
>  * belows.
>  */
This comment made it, just at the store side:

	/*
	 * Serialize the update against mmu_notifier_unregister. A
	 * side note: mmu_notifier_release can't run concurrently with
	 * us because we hold the mm_users pin (either implicitly as
	 * current->mm or explicitly with get_task_mm() or similar).
	 * We can't race against any other mmu notifier method either
	 * thanks to mm_take_all_locks().
	 *
	 * release semantics on the initialization of the mmu_notifier_mm's
         * contents are provided for unlocked readers.  acquire can only be
         * used while holding the mmgrab or mmget, and is safe because once
         * created the mmu_notififer_mm is not freed until the mm is
         * destroyed.  As above, users holding the mmap_sem or one of the
         * mm_take_all_locks() do not need to use acquire semantics.
	 */
	if (mmu_notifier_mm)
		smp_store_release(&mm->mmu_notifier_mm, mmu_notifier_mm);

Which I think is really overly belaboring the typical smp
store/release pattern, but people do seem unfamiliar with them...

Thanks,
Jason

On 11/7/19 12:06 PM, Jason Gunthorpe wrote:
...
>>
>> Also, it is best moved down to be next to the new MNR structs, so that
all the
>> MNR stuff is in one group.
> 
> I agree with Jerome, this enum is part of the 'struct
> mmu_notifier_range' (ie the description of the invalidation) and it
> doesn't really matter that only these new notifiers can be called with
> this type, it is still part of the mmu_notifier_range.
> 
OK.
> The comment already says it only applies to the mmu_range_notifier
> scheme..
> 
>>>   #define MMU_NOTIFIER_RANGE_BLOCKABLE (1 << 0)
>>> @@ -222,6 +228,26 @@ struct mmu_notifier {
>>>   	unsigned int users;
>>>   };
>>>   
>>
>> That should also be moved down, next to the new structs.
> 
> Which this?
I was referring to MMU_NOTIFIER_RANGE_BLOCKABLE, above. Trying
to put all the new range notifier stuff in one place. But maybe not,
if these are really not as separate as I thought.
> 
>>> +/**
>>> + * struct mmu_range_notifier_ops
>>> + * @invalidate: Upon return the caller must stop using any SPTEs
within this
>>> + *              range, this function can sleep. Return false if
blocking was
>>> + *              required but range is non-blocking
>>> + */
>>
>> How about this (I'm not sure I fully understand the return value,
though):
>>
>> /**
>>   * struct mmu_range_notifier_ops
>>   * @invalidate: Upon return the caller must stop using any SPTEs
within this
>>   * 		range.
>>   *
>>   * 		This function is permitted to sleep.
>>   *
>>   *      	@Return: false if blocking was required, but @range is
>>   *			non-blocking.
>>   *
>>   */
> 
> Is this kdoc format for function pointers?
heh, I'm sort of winging it, I'm not sure how function pointers are
supposed
to be documented in kdoc. Actually the only key take-away here is to write

"This function can sleep"

as a separate sentence..

...
>> c) Rename new one. Ideas:
>>
>>      struct mmu_interval_notifier
>>      struct mmu_range_intersection
>>      ...other ideas?
> 
> This odd duality has already cause some confusion, but names here are
> hard.  mmu_interval_notifier is the best alternative I've heard.
> 
> Changing this name is a lot of work - are we happy
> 'mmu_interval_notifier' is the right choice?

Yes, it's my favorite too. I'd vote for going with that.

...
>>
>>
>> OK, this either needs more documentation and assertions, or a different
>> approach. Because I see addition, subtraction, AND, OR and booleans
>> all being applied to this field, and it's darn near hopeless to
figure
>> out whether or not it really is even or odd at the right times.
> 
> This is a standard design for a seqlock scheme and follows the
> existing design of the linux seq lock.
> 
> The lower bit indicates the lock'd state and the upper bits indicate
> the generation of the lock
> 
> The operations on the lock itself are then:
>     seq |= 1  # Take the lock
>     seq++     # Release an acquired lock
>     seq & 1   # True if locked
> 
> Which is how this is written
Very nice, would you be open to putting that into (any) one of the comment
headers? That's an unusually clear and concise description:

/*
  * This is a standard design for a seqlock scheme and follows the
  * existing design of the linux seq lock.
  *
  * The lower bit indicates the lock'd state and the upper bits indicate
  * the generation of the lock
  *
  * The operations on the lock itself are then:
  *    seq |= 1  # Take the lock
  *    seq++     # Release an acquired lock
  *    seq & 1   # True if locked
  */

> 
>> Different approach: why not just add a mmn_mm->is_invalidating
>> member variable? It's not like you're short of space in that
struct.
> 
> Splitting it makes alot of stuff more complex and unnatural.
> 
OK, agreed.
> The ops above could be put in inline wrappers, but they only occur
> only in functions already called mn_itree_inv_start_range() and
> mn_itree_inv_end() and mn_itree_is_invalidating().
> 
> There is the one 'take the lock' outlier in
> __mmu_range_notifier_insert() though
> 
>>> +static void mn_itree_inv_end(struct mmu_notifier_mm *mmn_mm)
>>> +{
>>> +	struct mmu_range_notifier *mrn;
>>> +	struct hlist_node *next;
>>> +	bool need_wake = false;
>>> +
>>> +	spin_lock(&mmn_mm->lock);
>>> +	if (--mmn_mm->active_invalidate_ranges ||
>>> +	    !mn_itree_is_invalidating(mmn_mm)) {
>>> +		spin_unlock(&mmn_mm->lock);
>>> +		return;
>>> +	}
>>> +
>>> +	mmn_mm->invalidate_seq++;
>>
>> Is this the right place for an assertion that this is now an even
value?
> 
> Yes, but I'm reluctant to add such a runtime check on this fastish
path..
> How about a comment?
Sure.
> 
>>> +	need_wake = true;
>>> +
>>> +	/*
>>> +	 * The inv_end incorporates a deferred mechanism like
>>> +	 * rtnl_lock(). Adds and removes are queued until the final
inv_end
>>
>> Let me point out that rtnl_lock() itself is a one-liner that calls
mutex_lock().
>> But I suppose if one studies that file closely there is more. :)
> 
> Lets change that to rtnl_unlock() then

Thanks :)


...
>>> +	 * mrn->invalidate_seq is always set to an odd value. This
ensures
>>
>> This claim just looks wrong the first N times one reads the code, given
that
>> there is mmu_range_set_seq() to set it to an arbitrary value!  Maybe
>> you mean
> 
> mmu_range_set_seq() is NOT to be used to set to an arbitary value, it
> must only be used to set to the value provided in the invalidate()
> callback and that value is always odd. Lets make this super clear:
> 
> 	/*
> 	 * mrn->invalidate_seq must always be set to an odd value via
> 	 * mmu_range_set_seq() using the provided cur_seq from
> 	 * mn_itree_inv_start_range(). This ensures that if seq does wrap we
> 	 * will always clear the below sleep in some reasonable time as
> 	 * mmn_mm->invalidate_seq is even in the idle state.
> 	 */
> 
OK, that helps a lot.

...
>>> +		mrn->invalidate_seq = mmn_mm->invalidate_seq - 1;
>>
>> Ohhh, checkmate. I lose. Why is *subtracting* the right thing to do
>> for seq numbers here?  I'm acutely unhappy trying to figure this
out.
>> I suspect it's another unfortunate side effect of trying to use the
>> lower bit of the seq number (even/odd) for something else.
> 
> No, this is actually done for the seq number itself. We need to
> generate a seq number that is != the current invalidate_seq as this
> new mrn is not invalidating.
> 
> The best seq to use is one that the invalidate_seq will not reach for
> a long time, ie 'invalidate_seq + MAX' which is expressed as -1
> 
> The even/odd thing just takes care of itself naturally here as
> invalidate_seq is guarenteed even and -1 creates both an odd mrn value
> and a good seq number.
> 
> The algorithm would actually work correctly if this was
> 'mrn->invalidate_seq = 1', but occasionally things would block
when
> they don't need to block.
> 
> Lets add a comment:
> 
> 		/*
> 		 * The starting seq for a mrn not under invalidation should be
> 		 * odd, not equal to the current invalidate_seq and
> 		 * invalidate_seq should not 'wrap' to the new seq any time
> 		 * soon.
> 		 */
Very helpful. How about this additional tweak:

/*
  * The starting seq for a mrn not under invalidation should be
  * odd, not equal to the current invalidate_seq and
  * invalidate_seq should not 'wrap' to the new seq any time
  * soon. Subtracting 1 from the current (even) value achieves that.
  */

> 
>>> +int mmu_range_notifier_insert(struct mmu_range_notifier *mrn,
>>> +			      unsigned long start, unsigned long length,
>>> +			      struct mm_struct *mm)
>>> +{
>>> +	struct mmu_notifier_mm *mmn_mm;
>>> +	int ret;
>>
>> Hmmm, I think a later patch improperly changes the above to "int
ret = 0;".
>> I'll check on that. It's correct here, though.
> 
> Looks OK in my tree?
Nope, that's how I found it. The top of your mmu_notifier branch has this:

int __mmu_notifier_invalidate_range_start(struct mmu_notifier_range *range)
{
         struct mmu_notifier_mm *mmn_mm = range->mm->mmu_notifier_mm;
         int ret = 0;

         if (mmn_mm->has_interval) {
                 ret = mn_itree_invalidate(mmn_mm, range);
                 if (ret)
                         return ret;
         }
         if (!hlist_empty(&mmn_mm->list))
                 return mn_hlist_invalidate_range_start(mmn_mm, range);
         return 0;
}

> 
>>> +	might_lock(&mm->mmap_sem);
>>> +
>>> +	mmn_mm = smp_load_acquire(&mm->mmu_notifier_mm);
>>
>> What does the above pair with? Should have a comment that specifies
that.
> 
> smp_load_acquire() always pairs with smp_store_release() to the same
> memory, there is only one store, is a comment really needed?
> 
> Below are the comment updates I made, thanks!
> 
> Jason
> 
> diff --git a/include/linux/mmu_notifier.h b/include/linux/mmu_notifier.h
> index 51b92ba013ddce..065c95002e9602 100644
> --- a/include/linux/mmu_notifier.h
> +++ b/include/linux/mmu_notifier.h
> @@ -302,15 +302,15 @@ void mmu_range_notifier_remove(struct
mmu_range_notifier *mrn);
>   /**
>    * mmu_range_set_seq - Save the invalidation sequence
>    * @mrn - The mrn passed to invalidate
> - * @cur_seq - The cur_seq passed to invalidate
> + * @cur_seq - The cur_seq passed to the invalidate() callback
>    *
>    * This must be called unconditionally from the invalidate callback of a
>    * struct mmu_range_notifier_ops under the same lock that is used to call
>    * mmu_range_read_retry(). It updates the sequence number for later use
by
> - * mmu_range_read_retry().
> + * mmu_range_read_retry(). The provided cur_seq will always be odd.
>    *
> - * If the user does not call mmu_range_read_begin() or
mmu_range_read_retry()
> - * then this call is not required.
> + * If the caller does not call mmu_range_read_begin() or
> + * mmu_range_read_retry() then this call is not required.
>    */
>   static inline void mmu_range_set_seq(struct mmu_range_notifier *mrn,
>   				     unsigned long cur_seq)
> @@ -348,8 +348,9 @@ static inline bool mmu_range_read_retry(struct
mmu_range_notifier *mrn,
>    * collided with this lock and a future mmu_range_read_retry() will
return
>    * true.
>    *
> - * False is not reliable and only suggests a collision has not happened.
It
> - * can be called many times and does not have to hold the user provided
lock.
> + * False is not reliable and only suggests a collision may not have
> + * occured. It can be called many times and does not have to hold the user
> + * provided lock.
>    *
>    * This call can be used as part of loops and other expensive operations
to
>    * expedite a retry.
> diff --git a/mm/mmu_notifier.c b/mm/mmu_notifier.c
> index 2b7485919ecfeb..afe1e2d94183f8 100644
> --- a/mm/mmu_notifier.c
> +++ b/mm/mmu_notifier.c
> @@ -51,7 +51,8 @@ struct mmu_notifier_mm {
>    * This is a collision-retry read-side/write-side 'lock', a lot
like a
>    * seqcount, however this allows multiple write-sides to hold it at
>    * once. Conceptually the write side is protecting the values of the PTEs
in
> - * this mm, such that PTES cannot be read into SPTEs while any writer
exists.
> + * this mm, such that PTES cannot be read into SPTEs (shadow PTEs) while
any
> + * writer exists.
>    *
>    * Note that the core mm creates nested invalidate_range_start()/end()
regions
>    * within the same thread, and runs invalidate_range_start()/end() in
parallel
> @@ -64,12 +65,13 @@ struct mmu_notifier_mm {
>    *
>    * The write side has two states, fully excluded:
>    *  - mm->active_invalidate_ranges != 0
> - *  - mnn->invalidate_seq & 1 == True
> + *  - mnn->invalidate_seq & 1 == True (odd)
>    *  - some range on the mm_struct is being invalidated
>    *  - the itree is not allowed to change
>    *
>    * And partially excluded:
>    *  - mm->active_invalidate_ranges != 0
> + *  - mnn->invalidate_seq & 1 == False (even)
>    *  - some range on the mm_struct is being invalidated
>    *  - the itree is allowed to change
>    *
> @@ -131,12 +133,13 @@ static void mn_itree_inv_end(struct mmu_notifier_mm
*mmn_mm)
>   		return;
>   	}
>   
> +	/* Make invalidate_seq even */
>   	mmn_mm->invalidate_seq++;
>   	need_wake = true;
>   
>   	/*
>   	 * The inv_end incorporates a deferred mechanism like
> -	 * rtnl_lock(). Adds and removes are queued until the final inv_end
> +	 * rtnl_unlock(). Adds and removes are queued until the final inv_end
>   	 * happens then they are progressed. This arrangement for tree updates
>   	 * is used to avoid using a blocking lock during
>   	 * invalidate_range_start.
> @@ -230,10 +233,11 @@ unsigned long mmu_range_read_begin(struct
mmu_range_notifier *mrn)
>   	spin_unlock(&mmn_mm->lock);
>   
>   	/*
> -	 * mrn->invalidate_seq is always set to an odd value. This ensures
> -	 * that if seq does wrap we will always clear the below sleep in some
> -	 * reasonable time as mmn_mm->invalidate_seq is even in the idle
> -	 * state.
> +	 * mrn->invalidate_seq must always be set to an odd value via
> +	 * mmu_range_set_seq() using the provided cur_seq from
> +	 * mn_itree_inv_start_range(). This ensures that if seq does wrap we
> +	 * will always clear the below sleep in some reasonable time as
> +	 * mmn_mm->invalidate_seq is even in the idle state.
>   	 */
>   	lock_map_acquire(&__mmu_notifier_invalidate_range_start_map);
>   	lock_map_release(&__mmu_notifier_invalidate_range_start_map);
> @@ -892,6 +896,12 @@ static int __mmu_range_notifier_insert(struct
mmu_range_notifier *mrn,
>   		mrn->invalidate_seq = mmn_mm->invalidate_seq;
>   	} else {
>   		WARN_ON(mn_itree_is_invalidating(mmn_mm));
> +		/*
> +		 * The starting seq for a mrn not under invalidation should be
> +		 * odd, not equal to the current invalidate_seq and
> +		 * invalidate_seq should not 'wrap' to the new seq any time
> +		 * soon.
> +		 */
>   		mrn->invalidate_seq = mmn_mm->invalidate_seq - 1;
>   		interval_tree_insert(&mrn->interval_tree,
&mmn_mm->itree);
>   	}
> 
Looks good. We're just polishing up minor points now, so you can add:

Reviewed-by: John Hubbard <jhubbard at nvidia.com>



thanks,
-- 
John Hubbard
NVIDIA

On Thu, Nov 07, 2019 at 08:06:08PM +0000, Jason Gunthorpe
wrote:
> > 
> > enum mmu_range_notifier_event {
> > 	MMU_NOTIFY_RELEASE,
> > };
> > 
> > ...assuming that we stay with "mmu_range_notifier" as a core
name for this
> > whole thing.
> > 
> > Also, it is best moved down to be next to the new MNR structs, so that
all the
> > MNR stuff is in one group.
> 
> I agree with Jerome, this enum is part of the 'struct
> mmu_notifier_range' (ie the description of the invalidation) and it
> doesn't really matter that only these new notifiers can be called with
> this type, it is still part of the mmu_notifier_range.
> 
> The comment already says it only applies to the mmu_range_notifier
> scheme..
In fact the enum is entirely unused.  We might as well just kill it off
entirely.