Nouveau - Jun 2021 - [PATCH v10 07/10] mm: Device exclusive memory access

On Friday, 11 June 2021 11:00:34 AM AEST Peter Xu wrote:
> On Fri, Jun 11, 2021 at 09:17:14AM +1000, Alistair Popple wrote:
> > On Friday, 11 June 2021 9:04:19 AM AEST Peter Xu wrote:
> > > On Fri, Jun 11, 2021 at 12:21:26AM +1000, Alistair Popple wrote:
> > > > > Hmm, the thing is.. to me FOLL_SPLIT_PMD should have
similar effect to explicit
> > > > > call split_huge_pmd_address(), afaict.  Since both of
them use __split_huge_pmd()
> > > > > internally which will generate that unwanted CLEAR
notify.
> > > >
> > > > Agree that gup calls __split_huge_pmd() via
split_huge_pmd_address()
> > > > which will always CLEAR. However gup only calls
split_huge_pmd_address() if it
> > > > finds a thp pmd. In follow_pmd_mask() we have:
> > > >
> > > >       if (likely(!pmd_trans_huge(pmdval)))
> > > >               return follow_page_pte(vma, address, pmd,
flags, &ctx->pgmap);
> > > >
> > > > So I don't think we have a problem here.
> > >
> > > Sorry I didn't follow here..  We do FOLL_SPLIT_PMD after this
check, right?  I
> > > mean, if it's a thp for the current mm, afaict
pmd_trans_huge() should return
> > > true above, so we'll skip follow_page_pte(); then we'll
check FOLL_SPLIT_PMD
> > > and do the split, then the CLEAR notify.  Hmm.. Did I miss
something?
> >
> > That seems correct - if the thp is not mapped with a pmd we won't
split and we
> > won't CLEAR. If there is a thp pmd we will split and CLEAR, but in
that case it
> > is fine - we will retry, but the retry will won't CLEAR because
the pmd has
> > already been split.
> 
> Aha!
> 
> >
> > The issue arises with doing it unconditionally in make device
exclusive is that
> > you *always* CLEAR even if there is no thp pmd to split. Or at least
that's my
> > understanding, please let me know if it doesn't make sense.
> 
> Exactly.  But if you see what I meant here, even if it can work like this,
it
> sounds still fragile, isn't it?  I just feel something is slightly off
there..
> 
> IMHO split_huge_pmd() checked pmd before calling __split_huge_pmd() for
> performance, afaict, because if it's not a thp even without locking,
then it
> won't be, so further __split_huge_pmd() is not necessary.
> 
> IOW, it's very legal if someday we'd like to let split_huge_pmd()
call
> __split_huge_pmd() directly, then AFAIU device exclusive API will be the
1st
> one to be broken with that seems-to-be-irrelevant change I'm afraid..
Well I would argue the performance of memory notifiers is becoming increasingly
important, and a change that causes them to be called unnecessarily is
therefore not very legal. Likely the correct fix here is to optimise
__split_huge_pmd() to only call the notifier if it's actually going to split
a
pmd. As you said though that's a completely different story which I think
would
be best done as a separate series.
> This lets me goes back a step to think about why do we need this notifier
at
> all to cover this whole range of make_device_exclusive() procedure..
> 
> What I am thinking is, we're afraid some CPU accesses this page so the
pte got
> quickly restored when device atomic operation is carrying on.  Then with
this
> notifier we'll be able to cancel it.  Makes perfect sense.
> 
> However do we really need to register this notifier so early?  The thing is
the
> GPU driver still has all the page locks, so even if there's a race to
restore
> the ptes, they'll block at taking the page lock until the driver
releases it.
> 
> IOW, I'm wondering whether the "non-fragile" way to do this
is not do
> mmu_interval_notifier_insert() that early: what if we register that
notifier
> after make_device_exclusive_range() returns but before page_unlock()
somehow?
> So before page_unlock(), race is protected fully by the lock itself; after
> that, it's done by mmu notifier.  Then maybe we don't need to worry
about all
> these notifications during marking exclusive (while we shouldn't)?
The notifier is needed to protect against races with pte changes. Once a page
has been marked for exclusive access the driver will update it's page tables
to
allow atomic access to the page. However in the meantime the page could become
unmapped entirely or write protected.

As I understand things the page lock won't protect against these kind of pte
changes, hence the need for mmu_interval_read_begin/retry which allows the
driver to hold a mutex protecting against invalidations via blocking the
notifier until the device page tables have been updated.
> Sorry in advance if I overlooked anything as I know little on device side
(even
> less than mm itself).  Also sorry to know that this series got marked
> to-be-update in -mm; hopefully it'll still land soon even if it still
needs
> some rebase to other more important bugfixes - I definitely jumped in too
late
> even if to mess this all up. :-)
I was thinking that was probably coming anyway, but I'm still hoping it will
be
just a rebase on Hugh's work which wasn't too bad last time I tried it
:-)
> --
> Peter Xu
>

On Fri, Jun 11, 2021 at 01:43:20PM +1000, Alistair Popple
wrote:
> On Friday, 11 June 2021 11:00:34 AM AEST Peter Xu wrote:
> > On Fri, Jun 11, 2021 at 09:17:14AM +1000, Alistair Popple wrote:
> > > On Friday, 11 June 2021 9:04:19 AM AEST Peter Xu wrote:
> > > > On Fri, Jun 11, 2021 at 12:21:26AM +1000, Alistair Popple
wrote:
> > > > > > Hmm, the thing is.. to me FOLL_SPLIT_PMD should
have similar effect to explicit
> > > > > > call split_huge_pmd_address(), afaict.  Since both
of them use __split_huge_pmd()
> > > > > > internally which will generate that unwanted CLEAR
notify.
> > > > >
> > > > > Agree that gup calls __split_huge_pmd() via
split_huge_pmd_address()
> > > > > which will always CLEAR. However gup only calls
split_huge_pmd_address() if it
> > > > > finds a thp pmd. In follow_pmd_mask() we have:
> > > > >
> > > > >       if (likely(!pmd_trans_huge(pmdval)))
> > > > >               return follow_page_pte(vma, address, pmd,
flags, &ctx->pgmap);
> > > > >
> > > > > So I don't think we have a problem here.
> > > >
> > > > Sorry I didn't follow here..  We do FOLL_SPLIT_PMD after
this check, right?  I
> > > > mean, if it's a thp for the current mm, afaict
pmd_trans_huge() should return
> > > > true above, so we'll skip follow_page_pte(); then
we'll check FOLL_SPLIT_PMD
> > > > and do the split, then the CLEAR notify.  Hmm.. Did I miss
something?
> > >
> > > That seems correct - if the thp is not mapped with a pmd we
won't split and we
> > > won't CLEAR. If there is a thp pmd we will split and CLEAR,
but in that case it
> > > is fine - we will retry, but the retry will won't CLEAR
because the pmd has
> > > already been split.
> > 
> > Aha!
> > 
> > >
> > > The issue arises with doing it unconditionally in make device
exclusive is that
> > > you *always* CLEAR even if there is no thp pmd to split. Or at
least that's my
> > > understanding, please let me know if it doesn't make sense.
> > 
> > Exactly.  But if you see what I meant here, even if it can work like
this, it
> > sounds still fragile, isn't it?  I just feel something is slightly
off there..
> > 
> > IMHO split_huge_pmd() checked pmd before calling __split_huge_pmd()
for
> > performance, afaict, because if it's not a thp even without
locking, then it
> > won't be, so further __split_huge_pmd() is not necessary.
> > 
> > IOW, it's very legal if someday we'd like to let
split_huge_pmd() call
> > __split_huge_pmd() directly, then AFAIU device exclusive API will be
the 1st
> > one to be broken with that seems-to-be-irrelevant change I'm
afraid..
> 
> Well I would argue the performance of memory notifiers is becoming
increasingly
> important, and a change that causes them to be called unnecessarily is
> therefore not very legal. Likely the correct fix here is to optimise
> __split_huge_pmd() to only call the notifier if it's actually going to
split a
> pmd. As you said though that's a completely different story which I
think would
> be best done as a separate series.
Right, maybe I can look a bit more into that later; but my whole point was to
express that one functionality shouldn't depend on such a trivial detail of
implementation of other modules (thp split in this case).
> 
> > This lets me goes back a step to think about why do we need this
notifier at
> > all to cover this whole range of make_device_exclusive() procedure..
> > 
> > What I am thinking is, we're afraid some CPU accesses this page so
the pte got
> > quickly restored when device atomic operation is carrying on.  Then
with this
> > notifier we'll be able to cancel it.  Makes perfect sense.
> > 
> > However do we really need to register this notifier so early?  The
thing is the
> > GPU driver still has all the page locks, so even if there's a race
to restore
> > the ptes, they'll block at taking the page lock until the driver
releases it.
> > 
> > IOW, I'm wondering whether the "non-fragile" way to do
this is not do
> > mmu_interval_notifier_insert() that early: what if we register that
notifier
> > after make_device_exclusive_range() returns but before page_unlock()
somehow?
> > So before page_unlock(), race is protected fully by the lock itself;
after
> > that, it's done by mmu notifier.  Then maybe we don't need to
worry about all
> > these notifications during marking exclusive (while we shouldn't)?
> 
> The notifier is needed to protect against races with pte changes. Once a
page
> has been marked for exclusive access the driver will update it's page
tables to
> allow atomic access to the page. However in the meantime the page could
become
> unmapped entirely or write protected.
> 
> As I understand things the page lock won't protect against these kind
of pte
> changes, hence the need for mmu_interval_read_begin/retry which allows the
> driver to hold a mutex protecting against invalidations via blocking the
> notifier until the device page tables have been updated.
Indeed, I suppose you mean change_pte_range() and zap_pte_range()
correspondingly.

Do you think we can restore pte right before wr-protect or zap?  Then all
things serializes with page lock (btw: it's already an insane userspace to
either unmap a page or wr-protect a page if it knows the device is using it!).
If these are the only two cases, it still sounds a cleaner approach to me than
the current approach.

This also reminded me that right now the cpu pgtable recovery is lazy - it
happens either from fork() or a cpu page fault.  Even after device finished
using it, swap ptes keep there.

What if the device tries to do atomic op on the same page twice?  I am not sure
whether it means we may also want to teach both GUP (majorly follow_page_pte()
for now before pmd support) and process of page_make_device_exclusive() with
understanding the device exclusive entries too?  Another option seems to be
restoring pte after device finish using it, as long as the device knows when.

-- 
Peter Xu

On Saturday, 12 June 2021 1:01:42 AM AEST Peter Xu
wrote:
> On Fri, Jun 11, 2021 at 01:43:20PM +1000, Alistair Popple wrote:
> > On Friday, 11 June 2021 11:00:34 AM AEST Peter Xu wrote:
> > > On Fri, Jun 11, 2021 at 09:17:14AM +1000, Alistair Popple wrote:
> > > > On Friday, 11 June 2021 9:04:19 AM AEST Peter Xu wrote:
> > > > > On Fri, Jun 11, 2021 at 12:21:26AM +1000, Alistair
Popple wrote:
> > > > > > > Hmm, the thing is.. to me FOLL_SPLIT_PMD
should have similar effect to explicit
> > > > > > > call split_huge_pmd_address(), afaict.  Since
both of them use __split_huge_pmd()
> > > > > > > internally which will generate that unwanted
CLEAR notify.
> > > > > >
> > > > > > Agree that gup calls __split_huge_pmd() via
split_huge_pmd_address()
> > > > > > which will always CLEAR. However gup only calls
split_huge_pmd_address() if it
> > > > > > finds a thp pmd. In follow_pmd_mask() we have:
> > > > > >
> > > > > >       if (likely(!pmd_trans_huge(pmdval)))
> > > > > >               return follow_page_pte(vma, address,
pmd, flags, &ctx->pgmap);
> > > > > >
> > > > > > So I don't think we have a problem here.
> > > > >
> > > > > Sorry I didn't follow here..  We do FOLL_SPLIT_PMD
after this check, right?  I
> > > > > mean, if it's a thp for the current mm, afaict
pmd_trans_huge() should return
> > > > > true above, so we'll skip follow_page_pte(); then
we'll check FOLL_SPLIT_PMD
> > > > > and do the split, then the CLEAR notify.  Hmm.. Did I
miss something?
> > > >
> > > > That seems correct - if the thp is not mapped with a pmd we
won't split and we
> > > > won't CLEAR. If there is a thp pmd we will split and
CLEAR, but in that case it
> > > > is fine - we will retry, but the retry will won't CLEAR
because the pmd has
> > > > already been split.
> > >
> > > Aha!
> > >
> > > >
> > > > The issue arises with doing it unconditionally in make
device exclusive is that
> > > > you *always* CLEAR even if there is no thp pmd to split. Or
at least that's my
> > > > understanding, please let me know if it doesn't make
sense.
> > >
> > > Exactly.  But if you see what I meant here, even if it can work
like this, it
> > > sounds still fragile, isn't it?  I just feel something is
slightly off there..
> > >
> > > IMHO split_huge_pmd() checked pmd before calling
__split_huge_pmd() for
> > > performance, afaict, because if it's not a thp even without
locking, then it
> > > won't be, so further __split_huge_pmd() is not necessary.
> > >
> > > IOW, it's very legal if someday we'd like to let
split_huge_pmd() call
> > > __split_huge_pmd() directly, then AFAIU device exclusive API will
be the 1st
> > > one to be broken with that seems-to-be-irrelevant change I'm
afraid..
> >
> > Well I would argue the performance of memory notifiers is becoming
increasingly
> > important, and a change that causes them to be called unnecessarily is
> > therefore not very legal. Likely the correct fix here is to optimise
> > __split_huge_pmd() to only call the notifier if it's actually
going to split a
> > pmd. As you said though that's a completely different story which
I think would
> > be best done as a separate series.
> 
> Right, maybe I can look a bit more into that later; but my whole point was
to
> express that one functionality shouldn't depend on such a trivial
detail of
> implementation of other modules (thp split in this case).
> 
> >
> > > This lets me goes back a step to think about why do we need this
notifier at
> > > all to cover this whole range of make_device_exclusive()
procedure..
> > >
> > > What I am thinking is, we're afraid some CPU accesses this
page so the pte got
> > > quickly restored when device atomic operation is carrying on. 
Then with this
> > > notifier we'll be able to cancel it.  Makes perfect sense.
> > >
> > > However do we really need to register this notifier so early? 
The thing is the
> > > GPU driver still has all the page locks, so even if there's a
race to restore
> > > the ptes, they'll block at taking the page lock until the
driver releases it.
> > >
> > > IOW, I'm wondering whether the "non-fragile" way to
do this is not do
> > > mmu_interval_notifier_insert() that early: what if we register
that notifier
> > > after make_device_exclusive_range() returns but before
page_unlock() somehow?
> > > So before page_unlock(), race is protected fully by the lock
itself; after
> > > that, it's done by mmu notifier.  Then maybe we don't
need to worry about all
> > > these notifications during marking exclusive (while we
shouldn't)?
> >
> > The notifier is needed to protect against races with pte changes. Once
a page
> > has been marked for exclusive access the driver will update it's
page tables to
> > allow atomic access to the page. However in the meantime the page
could become
> > unmapped entirely or write protected.
> >
> > As I understand things the page lock won't protect against these
kind of pte
> > changes, hence the need for mmu_interval_read_begin/retry which allows
the
> > driver to hold a mutex protecting against invalidations via blocking
the
> > notifier until the device page tables have been updated.
> 
> Indeed, I suppose you mean change_pte_range() and zap_pte_range()
> correspondingly.
Right.
> Do you think we can restore pte right before wr-protect or zap?  Then all
> things serializes with page lock (btw: it's already an insane userspace
to
> either unmap a page or wr-protect a page if it knows the device is using
it!).
> If these are the only two cases, it still sounds a cleaner approach to me
than
> the current approach.
Perhaps we could but it would make {zap|change}_pte_range() much more complex as
we can't sleep taking the page lock whilst holding the ptl, so we'd have
to
implement a retry scheme similar to copy_pte_range() in both those functions as
well. Given mmu_interval_read_begin/retry was IMHO added to solve this type of
problem (freezing pte's to safely program device pte's) it seems like
the
better option rather than adding more complex code to generic mm paths.

It's also worth noting i915 seems to use mmu_interval_read_begin/retry()
with
gup to sync mappings so this isn't an entirely new concept. I'm not an
expert
in that driver but I imagine changing gup to generate unconditional mmu notifier
invalidates would also cause issues there. So I think overall this is the
cleanest solution as it reduces the amount of code (particularly in generic mm
paths).
> This also reminded me that right now the cpu pgtable recovery is lazy - it
> happens either from fork() or a cpu page fault.  Even after device finished
> using it, swap ptes keep there.
> 
> What if the device tries to do atomic op on the same page twice?  I am not
sure
> whether it means we may also want to teach both GUP (majorly
follow_page_pte()
> for now before pmd support) and process of page_make_device_exclusive()
with
> understanding the device exclusive entries too?  Another option seems to be
> restoring pte after device finish using it, as long as the device knows
when.
I don't think we need to complicate follow_page_pte() with knowledge of
exclusive entries. GUP will just restore the original pte via the normal
fault path - follow_page_pte() will return NULL for an exclusive entry,
resulting in handle_mm_path() getting called via faultin_page(). Therefore
a driver calling make_device_exclusive() twice on the same page won't cause
an
issue. Also the device shouldn't fault on subsequent accesses if the
exclusive
entry is still in place anyway.

We can't restore the pte when the device is finished with it because there
is
no way of knowing when a device is done using an exclusive entry - device
pte's work much the same as cpu pte's in that regard.

 - Alistair
> --
> Peter Xu
>