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

[Konstantin Khlebnikov]

On Sat, May 23, 2020 at 4:34 AM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Wed, 13 May 2020 17:05:25 +0300 Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx> 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.
> >
> >
> >  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.
>
> I dunno, this code looks quite nasty.  I'm more thinking we should
> revert and rethink David's 119d6d59dcc0980dcd58 ("mm, compaction: avoid
> isolating pinned pages").
>
> > --- 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)).
> > +              * 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;
>
> OK, we happened to notice this because we happened to trigger a
> !PageSlab assertion.  But if this page has been freed and reused for
> slab, it could have been reused for *anything*?  Perhaps it was reused
> as a migratable page which we'll go ahead and migrate even though we no
> longer should.  There are various whacky parts of the kernel which
> (ab)use surprising struct page fields in surprising ways - how do we
> know it isn't one of those which now happens to look like a migratable
> page?

Here we just optimistically skip as much unwanted pages as possible.

This code rechecks PageLRU and other tests later, under lru_lock.
lru_lock blocks freeing path which should acquire it to remove from lru.

>
> I also worry about the next test:
>
>                 /*
>                  * Only allow to migrate anonymous pages in GFP_NOFS context
>                  * because those do not depend on fs locks.
>                  */
>                 if (!(cc->gfp_mask & __GFP_FS) && page_mapping(page))
>                         goto isolate_fail;
>
> This page isn't PageLocked(), is it?  It could be a recycled page which
> is will be getting its ->mapping set one nanosecond hence.

Yes, it's racy. I don't see how compaction rechecks  this later.
So it could try to unmap and migrate file page if race here with recycle.

Probably nobody starts direct-compaction without GFP_FS.

>
>
> >               /*
> > @@ -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;
> >               }
> >
> >               lruvec = mem_cgroup_page_lruvec(page, pgdat);
> >
>