Re: [PATCH v2] mm: remove VM_BUG_ON(PageSlab()) from page_mapcount()

[Vlastimil Babka]

On 5/24/20 3:43 PM, Konstantin Khlebnikov wrote:
> Replace superfluous VM_BUG_ON() with comment about correct usage.
> 
> Technically reverts commit 1d148e218a0d0566b1c06f2f45f1436d53b049b2
> ("mm: add VM_BUG_ON_PAGE() to page_mapcount()"), but context have changed.
> 
> 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
> 
> 
> Code in isolate_migratepages_block() was added in commit 119d6d59dcc0
> ("mm, compaction: avoid isolating pinned pages") before adding VM_BUG_ON
> into page_mapcount().
> 
> This race has been predicted in 2015 by Vlastimil Babka (see link below).

Huh, looks like I made that prediction only half year after that patch has been
posted. Now if only I remembered why... I hope it was just a code inspection
while chasing something else. I most likely didn't actually see the bug happen,
as we don't compile with DEBUG_VM.

> Signed-off-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
> Fixes: 1d148e218a0d ("mm: add VM_BUG_ON_PAGE() to page_mapcount()")
> Link: https://lore.kernel.org/lkml/557710E1.6060103@xxxxxxx/
> Link: https://lore.kernel.org/linux-mm/158937872515.474360.5066096871639561424.stgit@buzz/T/ (v1)

With Hugh's wording tweaks,
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
Thanks.