| Message ID | 159032779896.957378.7852761411265662220.stgit@buzz (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [v2] mm: remove VM_BUG_ON(PageSlab()) from page_mapcount() | expand |
On Sun, 24 May 2020, 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). Ah, kudos to Vlastimil. > > Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru> Thanks, Acked-by: Hugh Dickins <hughd@google.com> For the BUG deletion. I got a bit confused by the comments at first: slight rewording suggested below. I had also intended a comment in page_mapcount(), on being used in that racy exceptional way by isolate_migratepages_block(): but now that the BUG is gone, I think you're right not to add that - it's not for the function to document its various callers, nor what's not in it. That kind of info belongs to the commit message, as you have done. > Fixes: 1d148e218a0d ("mm: add VM_BUG_ON_PAGE() to page_mapcount()") > Link: https://lore.kernel.org/lkml/557710E1.6060103@suse.cz/ > Link: https://lore.kernel.org/linux-mm/158937872515.474360.5066096871639561424.stgit@buzz/T/ (v1) > --- > include/linux/mm.h | 14 ++++++++++++-- > 1 file changed, 12 insertions(+), 2 deletions(-) > > diff --git a/include/linux/mm.h b/include/linux/mm.h > index 5a323422d783..95f777f482ac 100644 > --- a/include/linux/mm.h > +++ b/include/linux/mm.h > @@ -782,6 +782,11 @@ static inline void *kvcalloc(size_t n, size_t size, gfp_t flags) > > extern void kvfree(const void *addr); > > +/* > + * Mapcount of compound page as a whole, not includes mapped sub-pages. s/not includes/does not include/ > + * > + * Must be called only for compound pages or any their tail sub-pages. > + */ > static inline int compound_mapcount(struct page *page) > { > VM_BUG_ON_PAGE(!PageCompound(page), page); > @@ -801,10 +806,15 @@ static inline void page_mapcount_reset(struct page *page) > > int __page_mapcount(struct page *page); > > +/* > + * Mapcount of 0-order page, for sub-page includes compound_mapcount(). s/, for sub-page/; when compound sub-page,/ > + * > + * Result is undefined for pages which cannot be mapped into userspace. > + * For example SLAB or special types of pages. See function page_has_type(). > + * They use this place in struct page differently. > + */ > static inline int page_mapcount(struct page *page) > { > - VM_BUG_ON_PAGE(PageSlab(page), page); > - > if (unlikely(PageCompound(page))) > return __page_mapcount(page); > return atomic_read(&page->_mapcount) + 1;
On Sun, May 24, 2020 at 04:43:18PM +0300, 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). > > Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru> > Fixes: 1d148e218a0d ("mm: add VM_BUG_ON_PAGE() to page_mapcount()") > Link: https://lore.kernel.org/lkml/557710E1.6060103@suse.cz/ > Link: https://lore.kernel.org/linux-mm/158937872515.474360.5066096871639561424.stgit@buzz/T/ (v1) Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
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@yandex-team.ru> > Fixes: 1d148e218a0d ("mm: add VM_BUG_ON_PAGE() to page_mapcount()") > Link: https://lore.kernel.org/lkml/557710E1.6060103@suse.cz/ > Link: https://lore.kernel.org/linux-mm/158937872515.474360.5066096871639561424.stgit@buzz/T/ (v1) With Hugh's wording tweaks, Acked-by: Vlastimil Babka <vbabka@suse.cz> Thanks.
diff --git a/include/linux/mm.h b/include/linux/mm.h index 5a323422d783..95f777f482ac 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -782,6 +782,11 @@ static inline void *kvcalloc(size_t n, size_t size, gfp_t flags) extern void kvfree(const void *addr); +/* + * Mapcount of compound page as a whole, not includes mapped sub-pages. + * + * Must be called only for compound pages or any their tail sub-pages. + */ static inline int compound_mapcount(struct page *page) { VM_BUG_ON_PAGE(!PageCompound(page), page); @@ -801,10 +806,15 @@ static inline void page_mapcount_reset(struct page *page) int __page_mapcount(struct page *page); +/* + * Mapcount of 0-order page, for sub-page includes compound_mapcount(). + * + * Result is undefined for pages which cannot be mapped into userspace. + * For example SLAB or special types of pages. See function page_has_type(). + * They use this place in struct page differently. + */ static inline int page_mapcount(struct page *page) { - VM_BUG_ON_PAGE(PageSlab(page), page); - if (unlikely(PageCompound(page))) return __page_mapcount(page); return atomic_read(&page->_mapcount) + 1;
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). Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru> Fixes: 1d148e218a0d ("mm: add VM_BUG_ON_PAGE() to page_mapcount()") Link: https://lore.kernel.org/lkml/557710E1.6060103@suse.cz/ Link: https://lore.kernel.org/linux-mm/158937872515.474360.5066096871639561424.stgit@buzz/T/ (v1) --- include/linux/mm.h | 14 ++++++++++++-- 1 file changed, 12 insertions(+), 2 deletions(-)