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Date: Sun, 24 May 2020 14:21:13 +0300
From: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
To: Hugh Dickins <hughd@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Vlastimil Babka <vbabka@...e.cz>,
David Rientjes <rientjes@...gle.com>,
"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm/compaction: avoid VM_BUG_ON(PageSlab()) in
page_mapcount()
On 24/05/2020 04.01, 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).
Yep, but for large cluster and pinpointed workload this happens surprisingly
frequently =) I've believed into this race only after seeing statistics for
count of compactions and how it correlates with incidents.
Probably the key component is a slab allocation from network irq/bh context
which interrupts compaction exactly at this spot.
>
>>
>>
>> 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@...dex-team.ru>
>> 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.
Ok, I'll redo the fix in this way.
>
>> ---
>> 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?
Yes. What a shame. Adding page_has_private() could fix that?
Kind of
page_count(page) > page_mapcount(page) +
(PageAnon(page) ? PageSwapCache(page) : (1 + page_has_private(page)))
or probably something like this:
page_count(page) > page_mapcount(page) +
(PageAnon(page) ? PageSwapCache(page) : GUP_PIN_COUNTING_BIAS)
I.e. skip only file pages pinned by dma or something slower.
I see some movements in this direction in recent changes.
of course that's independent matter.
>
> 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);
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