Re: [PATCH] mm/compaction: avoid VM_BUG_ON(PageSlab()) in page_mapcount()

[Hugh Dickins]

On Sat, 23 May 2020, Hugh Dickins wrote:
> On Wed, 13 May 2020, Konstantin Khlebnikov wrote:
> 
> > Function isolate_migratepages_block() runs some checks out of lru_lock
> > when choose pages for migration. After checking PageLRU() it checks extra
> > page references by comparing page_count() and page_mapcount(). Between
> > these two checks page could be removed from lru, freed and taken by slab.
> > 
> > As a result this race triggers VM_BUG_ON(PageSlab()) in page_mapcount().
> > Race window is tiny. For certain workload this happens around once a year.
> 
> Around once a year, that was my guess too. I have no record of us ever
> hitting this, but yes it could happen when you have CONFIG_DEBUG_VM=y
> (which I too like to run with, but would not recommend for users).
> 
> > 
> > 
> >  page:ffffea0105ca9380 count:1 mapcount:0 mapping:ffff88ff7712c180 index:0x0 compound_mapcount: 0
> >  flags: 0x500000000008100(slab|head)
> >  raw: 0500000000008100 dead000000000100 dead000000000200 ffff88ff7712c180
> >  raw: 0000000000000000 0000000080200020 00000001ffffffff 0000000000000000
> >  page dumped because: VM_BUG_ON_PAGE(PageSlab(page))
> >  ------------[ cut here ]------------
> >  kernel BUG at ./include/linux/mm.h:628!
> >  invalid opcode: 0000 [#1] SMP NOPTI
> >  CPU: 77 PID: 504 Comm: kcompactd1 Tainted: G        W         4.19.109-27 #1
> >  Hardware name: Yandex T175-N41-Y3N/MY81-EX0-Y3N, BIOS R05 06/20/2019
> >  RIP: 0010:isolate_migratepages_block+0x986/0x9b0
> > 
> > 
> > To fix just opencode page_mapcount() in racy check for 0-order case and
> > recheck carefully under lru_lock when page cannot escape from lru.
> > 
> > Also add checking extra references for file pages and swap cache.
> > 
> > Signed-off-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
> > Fixes: 119d6d59dcc0 ("mm, compaction: avoid isolating pinned pages")
> 
> Not really, that commit was correct at the time it went in.
> 
> > Fixes: 1d148e218a0d ("mm: add VM_BUG_ON_PAGE() to page_mapcount()")
> 
> Exactly, that commit was well-intentioned, but did not allow for this
> (admittedly very exceptional) usage.  How many developers actually
> make the mistake of applying page_mapcount() to their slab pages?
> None, I expect.  That VM_BUG_ON_PAGE() is there for documentation,
> and could just be replaced by a comment - and Linus would be happy
> with that.
> 
> > ---
> >  mm/compaction.c |   17 +++++++++++++----
> >  1 file changed, 13 insertions(+), 4 deletions(-)
> > 
> > diff --git a/mm/compaction.c b/mm/compaction.c
> > index 46f0fcc93081..91bb87fd9420 100644
> > --- a/mm/compaction.c
> > +++ b/mm/compaction.c
> > @@ -935,12 +935,16 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
> >  		}
> >  
> >  		/*
> > -		 * Migration will fail if an anonymous page is pinned in memory,
> > +		 * Migration will fail if an page is pinned in memory,
> >  		 * so avoid taking lru_lock and isolating it unnecessarily in an
> > -		 * admittedly racy check.
> > +		 * admittedly racy check simplest case for 0-order pages.
> > +		 *
> > +		 * Open code page_mapcount() to avoid VM_BUG_ON(PageSlab(page)).
> 
> But open coding page_mapcount() is not all that you did.  You have
> (understandably) chosen to avoid calling page_mapping(page), but...
> 
> > +		 * Page could have extra reference from mapping or swap cache.
> >  		 */
> > -		if (!page_mapping(page) &&
> > -		    page_count(page) > page_mapcount(page))
> > +		if (!PageCompound(page) &&
> > +		    page_count(page) > atomic_read(&page->_mapcount) + 1 +
> > +				(!PageAnon(page) || PageSwapCache(page)))
> >  			goto isolate_fail;
> 
> Isn't that test going to send all the file cache pages with buffer heads
> in page->private, off to isolate_fail when they're actually great
> candidates for migration?
> 
> Given that the actual bug spotted was with the VM_BUG_ON_PAGE(PageSlab),
> and nobody has reported any crash from the use of page_mapping() there
> (and we only need the test to be right most of the time: all of this 
> knowingly racy, as you explain in other mail): I'd go for just replacing
> the VM_BUG_ON_PAGE in page_mapcount() by a comment about this case.
> 
> But if you think developers are really in danger of coding page_mapcount()
> on their slab pages, then you could add a _page_mapcount() to linux/mm.h,
> which omits the VM_BUG_ON_PAGE, for use here only.
> 
> Then we wouldn't have to think so hard about the counting above!
> 
> >  
> >  		/*
> > @@ -975,6 +979,11 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
> >  				low_pfn += compound_nr(page) - 1;
> >  				goto isolate_fail;
> >  			}
> > +
> > +			/* Recheck page extra references under lock */
> > +			if (page_count(page) > page_mapcount(page) +
> > +				    (!PageAnon(page) || PageSwapCache(page)))
> > +				goto isolate_fail;
> 
> Well, that lru_lock (and the intervening PageLRU check after getting it)
> may restrict PageAnon and PageSwapCache transitions to some extent, but
> it certainly has no effect on page_count and page_mapcount: so I think
> such an additional check here is rather superfluous, and we should just
> rely on the final checks in migrate_page_move_mapping(), as before.
> 
> >  		}
> >  
> >  		lruvec = mem_cgroup_page_lruvec(page, pgdat);

Andrew, I've noticed that this buggy
mm-compaction-avoid-vm_bug_onpageslab-in-page_mapcount.patch
was still in Friday's mmotm 2020-05-29-16-09, despite its replacement
6988f31d558a ("mm: remove VM_BUG_ON(PageSlab()) from page_mapcount()")
getting into 5.7, thanks to your "incoming" to Linus on that day.

Please be sure to remove this patch to mm/compaction.c from your tree
and queue to Linus for 5.8: it imposes an unintended and significant
limitation on the current behavior of compaction. (And in some loads,
some of that additional limitation may actually be beneficial: but if
so, must be argued separately, not as page_mapcount BUG avoidance).

Cc'ing Alex Shi, because I noticed this when trying his v11 per-memcg
lru_lock series (which appears to be a big improvement over earlier
versions, thanks in particular to Johannes's memcg swap simplifications);
and Alex's 12/16 makes a change on top of Konstantin's latter check,
which will now just be reverted. I'm not yet confident in Alex's
isolate_migratepages_block(), in part because this muddle.

Thanks,
Hugh