Linux Virtualization - Mar 2018 - [PATCH v29 1/4] mm: support reporting free page blocks

On Tue 27-03-18 14:23:51, Wei Wang wrote:
> On 03/27/2018 05:22 AM, Andrew Morton wrote:
> > On Mon, 26 Mar 2018 10:39:51 +0800 Wei Wang <wei.w.wang at
intel.com> wrote:
> > 
> > > This patch adds support to walk through the free page blocks in
the
> > > system and report them via a callback function. Some page blocks
may
> > > leave the free list after zone->lock is released, so it is the
caller's
> > > responsibility to either detect or prevent the use of such pages.
> > > 
> > > One use example of this patch is to accelerate live migration by
skipping
> > > the transfer of free pages reported from the guest. A popular
method used
> > > by the hypervisor to track which part of memory is written during
live
> > > migration is to write-protect all the guest memory. So, those
pages that
> > > are reported as free pages but are written after the report
function
> > > returns will be captured by the hypervisor, and they will be
added to the
> > > next round of memory transfer.
> > > 
> > > ...
> > > 
> > > --- a/mm/page_alloc.c
> > > +++ b/mm/page_alloc.c
> > > @@ -4912,6 +4912,102 @@ void show_free_areas(unsigned int filter,
nodemask_t *nodemask)
> > >   	show_swap_cache_info();
> > >   }
> > > +/*
> > > + * Walk through a free page list and report the found pfn range
via the
> > > + * callback.
> > > + *
> > > + * Return 0 if it completes the reporting. Otherwise, return the
non-zero
> > > + * value returned from the callback.
> > > + */
> > > +static int walk_free_page_list(void *opaque,
> > > +			       struct zone *zone,
> > > +			       int order,
> > > +			       enum migratetype mt,
> > > +			       int (*report_pfn_range)(void *,
> > > +						       unsigned long,
> > > +						       unsigned long))
> > > +{
> > > +	struct page *page;
> > > +	struct list_head *list;
> > > +	unsigned long pfn, flags;
> > > +	int ret = 0;
> > > +
> > > +	spin_lock_irqsave(&zone->lock, flags);
> > > +	list = &zone->free_area[order].free_list[mt];
> > > +	list_for_each_entry(page, list, lru) {
> > > +		pfn = page_to_pfn(page);
> > > +		ret = report_pfn_range(opaque, pfn, 1 << order);
> > > +		if (ret)
> > > +			break;
> > > +	}
> > > +	spin_unlock_irqrestore(&zone->lock, flags);
> > > +
> > > +	return ret;
> > > +}
> > > +
> > > +/**
> > > + * walk_free_mem_block - Walk through the free page blocks in
the system
> > > + * @opaque: the context passed from the caller
> > > + * @min_order: the minimum order of free lists to check
> > > + * @report_pfn_range: the callback to report the pfn range of
the free pages
> > > + *
> > > + * If the callback returns a non-zero value, stop iterating the
list of free
> > > + * page blocks. Otherwise, continue to report.
> > > + *
> > > + * Please note that there are no locking guarantees for the
callback and
> > > + * that the reported pfn range might be freed or disappear after
the
> > > + * callback returns so the caller has to be very careful how it
is used.
> > > + *
> > > + * The callback itself must not sleep or perform any operations
which would
> > > + * require any memory allocations directly (not even
GFP_NOWAIT/GFP_ATOMIC)
> > > + * or via any lock dependency. It is generally advisable to
implement
> > > + * the callback as simple as possible and defer any heavy
lifting to a
> > > + * different context.
> > > + *
> > > + * There is no guarantee that each free range will be reported
only once
> > > + * during one walk_free_mem_block invocation.
> > > + *
> > > + * pfn_to_page on the given range is strongly discouraged and if
there is
> > > + * an absolute need for that make sure to contact MM people to
discuss
> > > + * potential problems.
> > > + *
> > > + * The function itself might sleep so it cannot be called from
atomic
> > > + * contexts.
> > I don't see how walk_free_mem_block() can sleep.
> 
> OK, it would be better to remove this sentence for the current version. But
> I think we could probably keep it if we decide to add cond_resched() below.
The point of this sentence was to make any user aware that the function
might sleep from the very begining rather than chase existing callers
when we need to add cond_resched or sleep for any other reason. So I
would rather keep it.
-- 
Michal Hocko
SUSE Labs

On Tue, Mar 27, 2018 at 08:33:22AM +0200, Michal Hocko
wrote:
> > > > + * The function itself might sleep so it cannot be called
from atomic
> > > > + * contexts.
> > > I don't see how walk_free_mem_block() can sleep.
> > 
> > OK, it would be better to remove this sentence for the current
version. But
> > I think we could probably keep it if we decide to add cond_resched()
below.
> 
> The point of this sentence was to make any user aware that the function
> might sleep from the very begining rather than chase existing callers
> when we need to add cond_resched or sleep for any other reason. So I
> would rather keep it.
Let's say what it is then - "will be changed to sleep in the
future".
> -- 
> Michal Hocko
> SUSE Labs

On Tue 27-03-18 19:07:22, Michael S. Tsirkin wrote:
> On Tue, Mar 27, 2018 at 08:33:22AM +0200, Michal Hocko wrote:
> > > > > + * The function itself might sleep so it cannot be
called from atomic
> > > > > + * contexts.
> > > > I don't see how walk_free_mem_block() can sleep.
> > > 
> > > OK, it would be better to remove this sentence for the current
version. But
> > > I think we could probably keep it if we decide to add
cond_resched() below.
> > 
> > The point of this sentence was to make any user aware that the
function
> > might sleep from the very begining rather than chase existing callers
> > when we need to add cond_resched or sleep for any other reason. So I
> > would rather keep it.
> 
> Let's say what it is then - "will be changed to sleep in the
future".
Do we really want to describe the precise implementation in the
documentation? I thought the main purpose of the documentation is to
describe the _contract_. If I am curious about the implementation I can
look at the code. As I've said earlier in this patchset lifetime. This
interface is rather dangerous because we are exposing guts of our
internal data structures. So we better set expectations of what can and
cannot be done right from the beginning. I definitely do not want
somebody to simply look at the code and see that the interface is
sleepable and abuse that fact.
-- 
Michal Hocko
SUSE Labs