Re: [patch v2] mm, compaction: avoid isolating pinned pages

[David Rientjes]

On Thu, 6 Feb 2014, Vlastimil Babka wrote:

> > Page migration will fail for memory that is pinned in memory with, for
> > example, get_user_pages().  In this case, it is unnecessary to take
> > zone->lru_lock or isolating the page and passing it to page migration
> > which will ultimately fail.
> > 
> > This is a racy check, the page can still change from under us, but in
> > that case we'll just fail later when attempting to move the page.
> > 
> > This avoids very expensive memory compaction when faulting transparent
> > hugepages after pinning a lot of memory with a Mellanox driver.
> > 
> > On a 128GB machine and pinning ~120GB of memory, before this patch we
> > see the enormous disparity in the number of page migration failures
> > because of the pinning (from /proc/vmstat):
> > 
> > 	compact_pages_moved 8450
> > 	compact_pagemigrate_failed 15614415
> > 
> > 0.05% of pages isolated are successfully migrated and explicitly
> > triggering memory compaction takes 102 seconds.  After the patch:
> > 
> > 	compact_pages_moved 9197
> > 	compact_pagemigrate_failed 7
> > 
> > 99.9% of pages isolated are now successfully migrated in this
> > configuration and memory compaction takes less than one second.
> > 
> > Signed-off-by: David Rientjes <rientjes@xxxxxxxxxx>
> > ---
> >   v2: address page count issue per Joonsoo
> > 
> >   mm/compaction.c | 9 +++++++++
> >   1 file changed, 9 insertions(+)
> > 
> > diff --git a/mm/compaction.c b/mm/compaction.c
> > --- a/mm/compaction.c
> > +++ b/mm/compaction.c
> > @@ -578,6 +578,15 @@ isolate_migratepages_range(struct zone *zone, struct
> > compact_control *cc,
> >   			continue;
> >   		}
> >   +		/*
> > +		 * Migration will fail if an anonymous page is pinned in
> > memory,
> > +		 * so avoid taking lru_lock and isolating it unnecessarily in
> > an
> > +		 * admittedly racy check.
> > +		 */
> > +		if (!page_mapping(page) &&
> > +		    page_count(page) > page_mapcount(page))
> > +			continue;
> > +
> 
> Hm this page_count() seems it could substantially increase the chance of race
> with prep_compound_page that your patch "mm, page_alloc: make first_page
> visible before PageTail" tries to fix :)
> 

That's why I sent the fix for page_count().

The "racy check" the comment eludes to above concerns the fact that 
page_count() and page_mapcount() can change out from under us before 
isolation and if we had not avoided isolating them that they would have 
been migratable later.  We accept that as a consequence of doing this in a 
lockless way without page references.
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