From 94ab10dd42a70acc5208a41325617e3d9cf81a70 Mon Sep 17 00:00:00 2001 From: Hugh Dickins Date: Fri, 24 Dec 2021 21:12:48 -0800 Subject: [PATCH] mm: delete unsafe BUG from page_cache_add_speculative() 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: 020853b6f5ea ("mm: Add folio_try_get_rcu()") Signed-off-by: Hugh Dickins Acked-by: Kirill A. Shutemov Reviewed-by: Matthew Wilcox (Oracle) Cc: Vlastimil Babka Cc: William Kucharski Cc: Christoph Hellwig Cc: Mike Rapoport Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- include/linux/pagemap.h | 1 - 1 file changed, 1 deletion(-) diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h index 605246452305..d150a9082b31 100644 --- a/include/linux/pagemap.h +++ b/include/linux/pagemap.h @@ -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) { - VM_BUG_ON_PAGE(PageTail(page), page); return folio_ref_try_add_rcu((struct folio *)page, count); } -- 2.11.0