From 170f37d6aa6ad4582eefd7459015de79e244536e Mon Sep 17 00:00:00 2001 From: "Matthew Wilcox (Oracle)" Date: Tue, 3 May 2022 00:09:31 -0400 Subject: block: Do not call folio_next() on an unreferenced folio It is unsafe to call folio_next() on a folio unless you hold a reference on it that prevents it from being split or freed. After returning from the iterator, iomap calls folio_end_writeback() which may drop the last reference to the page, or allow the page to be split. If that happens, the iterator will not advance far enough through the bio_vec, leading to assertion failures like the BUG() in folio_end_writeback() that checks we're not trying to end writeback on a page not currently under writeback. Other assertion failures were also seen, but they're all explained by this one bug. Fix the bug by remembering where the next folio starts before returning from the iterator. There are other ways of fixing this bug, but this seems the simplest. Reported-by: Darrick J. Wong Tested-by: Darrick J. Wong Reported-by: Brian Foster Tested-by: Brian Foster Signed-off-by: Matthew Wilcox (Oracle) --- include/linux/bio.h | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) (limited to 'include') diff --git a/include/linux/bio.h b/include/linux/bio.h index 278cc81cc1e7..00450fd86bb4 100644 --- a/include/linux/bio.h +++ b/include/linux/bio.h @@ -269,6 +269,7 @@ struct folio_iter { size_t offset; size_t length; /* private: for use by the iterator */ + struct folio *_next; size_t _seg_count; int _i; }; @@ -283,6 +284,7 @@ static inline void bio_first_folio(struct folio_iter *fi, struct bio *bio, PAGE_SIZE * (bvec->bv_page - &fi->folio->page); fi->_seg_count = bvec->bv_len; fi->length = min(folio_size(fi->folio) - fi->offset, fi->_seg_count); + fi->_next = folio_next(fi->folio); fi->_i = i; } @@ -290,9 +292,10 @@ static inline void bio_next_folio(struct folio_iter *fi, struct bio *bio) { fi->_seg_count -= fi->length; if (fi->_seg_count) { - fi->folio = folio_next(fi->folio); + fi->folio = fi->_next; fi->offset = 0; fi->length = min(folio_size(fi->folio), fi->_seg_count); + fi->_next = folio_next(fi->folio); } else if (fi->_i + 1 < bio->bi_vcnt) { bio_first_folio(fi, bio, fi->_i + 1); } else { -- cgit