The patch titled Subject: mm: delete unsafe BUG from page_cache_add_speculative() has been added to the -mm tree. Its filename is mm-delete-unsafe-bug-from-page_cache_add_speculative.patch

This patch should soon appear at https://ozlabs.org/~akpm/mmots/broken-out/mm-delete-unsafe-bug-from-page_cac... and later at https://ozlabs.org/~akpm/mmotm/broken-out/mm-delete-unsafe-bug-from-page_cac...

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------------------------------------------------------ From: Hugh Dickins hughd@google.com Subject: 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 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 Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

include/linux/pagemap.h | 1 - 1 file changed, 1 deletion(-)

--- a/include/linux/pagemap.h~mm-delete-unsafe-bug-from-page_cache_add_speculative +++ a/include/linux/pagemap.h @@ -285,7 +285,6 @@ static inline struct inode *folio_inode(

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); }

_

Patches currently in -mm which might be from hughd@google.com are

mm-delete-unsafe-bug-from-page_cache_add_speculative.patch