From: Konstantin Khlebnikov khlebnikov@yandex-team.ru Subject: mm: remove VM_BUG_ON(PageSlab()) from page_mapcount()

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).

[akpm@linux-foundation.org: comment tweaks, per Hugh] Link: http://lkml.kernel.org/r/159032779896.957378.7852761411265662220.stgit@buzz Link: https://lore.kernel.org/lkml/557710E1.6060103@suse.cz/ Link: https://lore.kernel.org/linux-mm/158937872515.474360.5066096871639561424.stg... (v1) Fixes: 1d148e218a0d ("mm: add VM_BUG_ON_PAGE() to page_mapcount()") Signed-off-by: Konstantin Khlebnikov khlebnikov@yandex-team.ru Acked-by: Hugh Dickins hughd@google.com Acked-by: Kirill A. Shutemov kirill.shutemov@linux.intel.com Acked-by: Vlastimil Babka vbabka@suse.cz Cc: David Rientjes rientjes@google.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

include/linux/mm.h | 15 +++++++++++++-- 1 file changed, 13 insertions(+), 2 deletions(-)

--- a/include/linux/mm.h~mm-remove-vm_bug_onpageslab-from-page_mapcount +++ a/include/linux/mm.h @@ -782,6 +782,11 @@ static inline void *kvcalloc(size_t n, s

extern void kvfree(const void *addr);

+/* + * Mapcount of compound page as a whole, does not include 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 +806,16 @@ static inline void page_mapcount_reset(s

int __page_mapcount(struct page *page);

+/* + * Mapcount of 0-order page; when compound 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; _