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

[Hugh Dickins]

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);