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
