* Re: [PATCH] mm/compaction: avoid VM_BUG_ON(PageSlab()) in page_mapcount()
@ 2020-05-24 1:01 ` Hugh Dickins
0 siblings, 0 replies; 16+ messages in thread
From: Hugh Dickins @ 2020-05-24 1:01 UTC (permalink / raw)
To: Konstantin Khlebnikov
Cc: Andrew Morton, Vlastimil Babka, David Rientjes, Hugh Dickins,
Kirill A. Shutemov, linux-kernel, linux-mm
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@yandex-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.
> ---
> 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);
^ permalink raw reply [flat|nested] 16+ messages in thread
* Re: [PATCH] mm/compaction: avoid VM_BUG_ON(PageSlab()) in page_mapcount()
2020-05-24 1:01 ` Hugh Dickins
(?)
@ 2020-05-24 11:21 ` Konstantin Khlebnikov
2020-05-24 19:12 ` Hugh Dickins
-1 siblings, 1 reply; 16+ messages in thread
From: Konstantin Khlebnikov @ 2020-05-24 11:21 UTC (permalink / raw)
To: Hugh Dickins
Cc: Andrew Morton, Vlastimil Babka, David Rientjes,
Kirill A. Shutemov, linux-kernel, linux-mm
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@yandex-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);
^ permalink raw reply [flat|nested] 16+ messages in thread
* Re: [PATCH] mm/compaction: avoid VM_BUG_ON(PageSlab()) in page_mapcount()
2020-05-24 11:21 ` Konstantin Khlebnikov
@ 2020-05-24 19:12 ` Hugh Dickins
0 siblings, 0 replies; 16+ messages in thread
From: Hugh Dickins @ 2020-05-24 19:12 UTC (permalink / raw)
To: Konstantin Khlebnikov
Cc: Hugh Dickins, Andrew Morton, Vlastimil Babka, David Rientjes,
Kirill A. Shutemov, linux-kernel, linux-mm
On Sun, 24 May 2020, Konstantin Khlebnikov wrote:
> 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.
Yes, I bet you're right.
>
> >
> > >
> > >
> > > 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@yandex-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.
Thanks.
>
> >
> > > ---
> > > 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)))
Certainly it was fixable, but I'm too lazy to want to think through
the correct answer; and though I'm often out of sympathy with helper
functions (why do people want an inline bool function for every simple
flag test?!?!), here is a place that cries out for a helper, if you
complicate it beyond page_count > page_mapcount (especially when
driven into that detail of adding 1 to _mapcount).
>
> 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.
Yes, once the gup/pin conversion is widespread, I expect that it will
allow a better implementation of this compaction test, one not limited
to the anonymous pages. (We do internally use a patch extending the
current test to file pages, which in practice has saved a lot of time
wasted on failing compactions: but, last I looked anyway, it gets some
cases wrong - cases we happen not to care about ourselves, but would
be unacceptable upstream. So I hope the distinction of pinned pages
will work out well here later.)
Hugh
^ permalink raw reply [flat|nested] 16+ messages in thread
* Re: [PATCH] mm/compaction: avoid VM_BUG_ON(PageSlab()) in page_mapcount()
@ 2020-05-24 19:12 ` Hugh Dickins
0 siblings, 0 replies; 16+ messages in thread
From: Hugh Dickins @ 2020-05-24 19:12 UTC (permalink / raw)
To: Konstantin Khlebnikov
Cc: Hugh Dickins, Andrew Morton, Vlastimil Babka, David Rientjes,
Kirill A. Shutemov, linux-kernel, linux-mm
On Sun, 24 May 2020, Konstantin Khlebnikov wrote:
> 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.
Yes, I bet you're right.
>
> >
> > >
> > >
> > > 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@yandex-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.
Thanks.
>
> >
> > > ---
> > > 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)))
Certainly it was fixable, but I'm too lazy to want to think through
the correct answer; and though I'm often out of sympathy with helper
functions (why do people want an inline bool function for every simple
flag test?!?!), here is a place that cries out for a helper, if you
complicate it beyond page_count > page_mapcount (especially when
driven into that detail of adding 1 to _mapcount).
>
> 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.
Yes, once the gup/pin conversion is widespread, I expect that it will
allow a better implementation of this compaction test, one not limited
to the anonymous pages. (We do internally use a patch extending the
current test to file pages, which in practice has saved a lot of time
wasted on failing compactions: but, last I looked anyway, it gets some
cases wrong - cases we happen not to care about ourselves, but would
be unacceptable upstream. So I hope the distinction of pinned pages
will work out well here later.)
Hugh
^ permalink raw reply [flat|nested] 16+ messages in thread
* Re: [PATCH] mm/compaction: avoid VM_BUG_ON(PageSlab()) in page_mapcount()
2020-05-24 1:01 ` Hugh Dickins
@ 2020-06-02 4:05 ` Hugh Dickins
-1 siblings, 0 replies; 16+ messages in thread
From: Hugh Dickins @ 2020-06-02 4:05 UTC (permalink / raw)
To: Andrew Morton
Cc: Konstantin Khlebnikov, Vlastimil Babka, David Rientjes,
Hugh Dickins, Kirill A. Shutemov, Alex Shi, Johannes Weiner,
linux-kernel, linux-mm
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@yandex-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.
>
> > ---
> > 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
^ permalink raw reply [flat|nested] 16+ messages in thread
* Re: [PATCH] mm/compaction: avoid VM_BUG_ON(PageSlab()) in page_mapcount()
@ 2020-06-02 4:05 ` Hugh Dickins
0 siblings, 0 replies; 16+ messages in thread
From: Hugh Dickins @ 2020-06-02 4:05 UTC (permalink / raw)
To: Andrew Morton
Cc: Konstantin Khlebnikov, Vlastimil Babka, David Rientjes,
Hugh Dickins, Kirill A. Shutemov, Alex Shi, Johannes Weiner,
linux-kernel, linux-mm
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@yandex-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.
>
> > ---
> > 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
^ permalink raw reply [flat|nested] 16+ messages in thread
* Re: [PATCH] mm/compaction: avoid VM_BUG_ON(PageSlab()) in page_mapcount()
2020-06-02 4:05 ` Hugh Dickins
(?)
@ 2020-06-02 4:13 ` Andrew Morton
-1 siblings, 0 replies; 16+ messages in thread
From: Andrew Morton @ 2020-06-02 4:13 UTC (permalink / raw)
To: Hugh Dickins
Cc: Konstantin Khlebnikov, Vlastimil Babka, David Rientjes,
Kirill A. Shutemov, Alex Shi, Johannes Weiner, linux-kernel,
linux-mm
On Mon, 1 Jun 2020 21:05:25 -0700 (PDT) Hugh Dickins <hughd@google.com> wrote:
> 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.
Thanks, gone.
^ permalink raw reply [flat|nested] 16+ messages in thread
* Re: [PATCH] mm/compaction: avoid VM_BUG_ON(PageSlab()) in page_mapcount()
2020-06-02 4:05 ` Hugh Dickins
(?)
(?)
@ 2020-06-02 11:28 ` Alex Shi
-1 siblings, 0 replies; 16+ messages in thread
From: Alex Shi @ 2020-06-02 11:28 UTC (permalink / raw)
To: Hugh Dickins, Andrew Morton
Cc: Konstantin Khlebnikov, Vlastimil Babka, David Rientjes,
Kirill A. Shutemov, Johannes Weiner, linux-kernel, linux-mm
在 2020/6/2 下午12:05, Hugh Dickins 写道:
> 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.
Hi Hugh,
Yes, this could make a very tricky change on compaction behavior. I will update
the patchset after next update.
Thanks a lot for notice this! And looking forward to more comments on per memcg
lru_lock patchset! :)
Thanks
Alex
^ permalink raw reply [flat|nested] 16+ messages in thread