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mm: delete unsafe BUG from page_cache_add_speculative()

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It is not easily reproducible, but on 5.16-rc I have several times hit
the VM_BUG_ON_PAGE(PageTail(page), page) in
page_cache_add_speculative(): usually from filemap_get_read_batch() for
an ext4 read, yesterday from next_uptodate_page() from
filemap_map_pages() for a shmem fault.

That BUG used to be placed where page_ref_add_unless() had succeeded,
but now it is placed before folio_ref_add_unless() is attempted: that is
not safe, since it is only the acquired reference which makes the page
safe from racing THP collapse or split.

We could keep the BUG, checking PageTail only when
folio_ref_try_add_rcu() has succeeded; but I don't think it adds much
value - just delete it.

Link: https://lkml.kernel.org/r/8b98fc6f-3439-8614-c3f3-945c659a1aba@google.com
Fixes: 020853b6f5 ("mm: Add folio_try_get_rcu()")
Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: William Kucharski <william.kucharski@oracle.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Hugh Dickins 2021-12-24 21:12:48 -08:00 committed by Linus Torvalds
parent e37e7b0b3b
commit 94ab10dd42
1 changed files with 0 additions and 1 deletions
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1
include/linux/pagemap.h
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@ -285,7 +285,6 @@ static inline struct inode *folio_inode(struct folio *folio)
static inline bool page_cache_add_speculative(struct page *page, int count) static inline bool page_cache_add_speculative(struct page *page, int count)
{ {
VM_BUG_ON_PAGE(PageTail(page), page);
return folio_ref_try_add_rcu((struct folio *)page, count); return folio_ref_try_add_rcu((struct folio *)page, count);
} }