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

[Konstantin Khlebnikov]

On 13/05/2020 21.32, Andrew Morton 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.
>
> It sounds like a cc:stable is appropriate?

Yep, but probably I'm missing something.

It seems bug is there for a long time and nobody seen it.
Am I the only one using COONFIG_DEBUG_VM=y everywhere? =)

> > --- 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;
> >   
> >   		/*
> > @@ -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);