From: Zhang Yi truncate_inode_partial_folio() splits a large folio so that the caller's truncate loop can drop the in-range sub-folios while keeping the out-of-range tail. The first split at the punch start edge is non-uniform, which leaves the sub-folio at the truncation end edge as large as possible, this means it may still straddle the range, holding both zeroed in-range and valid out-of-range data. The function then attempts a second split at offset + length to isolate that tail. If the second split fails the straddling sub-folio stays merged. The function returned true unconditionally on all exit paths of the success block, telling the caller it was fully handled. The caller kept its default end and the truncate loop truncated every sub-folio below it, including the merged straddler, discarding the valid out-of-range tail. For example, a 4-page order-2 folio punched from offset 0 to the middle of the last page: truncate_inode_pages_range() truncate_inode_partial_folio() # same_folio == true 1st split at page0 -> [p0, p1, p2-3] # non-uniform, success folio2 = p2-3 # straddles: p2 zeroed, p3 tail valid 2nd split of folio2 fails / cannot lock return true # BUG: caller keeps default end end = 3 loop truncates p0, p1, p2-3 # p3's valid tail is lost This became reachable after commit 7460b470a131 ("mm/truncate: use folio_split() in truncate operation") replaced the atomic split_folio() with folio_split(), whose non-uniform split can partially split a folio and leave the end edge merged. It has gone unnoticed because a dirty large folio normally carries the filesystem's private data, for example buffer_head, so filemap_release_folio() -> iomap_release_folio() returns false on a dirty folio and folio_split() aborts with -EBUSY before any split, leaving the straddler safely unsplit. The bug is only reachable on paths that produce dirty large folios without filesystem private data, and it was caught on the upcoming ext4 iomap buffered I/O path when no ifs is attached. In addition, even when both splits succeed, data can still be lost when the mapping's minimum folio order (min_order) is non-zero. folio_split() stops at min_order instead of order 0, so the sub-folio containing a split point stays aligned to 1 << min_order rather than to a page. The original success path left start at the page-aligned head of the range and set end to the exact page index of the end edge, neither of which is a folio boundary in general. Either one could land inside the large folio at its edge, and the truncate loop would drop that straddling folio together with its valid out-of-range tail. For example, a 64K (order-4) folio with min_order = 2 punched from offset 0 to 36K: truncate_inode_pages_range() truncate_inode_partial_folio() # same_folio == true 1st split at p0 -> [p0-p3, p4-p7, p8-p15] # non-uniform, min_order folio2 = p8-p15 # straddles: p8 in range, p9-p15 tail valid 2nd split of folio2 -> [p8-p11, p12-p15] # success end = p9 # BUG: p9 inside [p8-p11] loop truncates ... p8-p11 # p9-p11's valid tail is lost Rework the contract so the caller is told the page range to discard: - Return true only when a split occurred, false otherwise. This clarifies the existing confusing return value semantics. - Add pgoff_t *pstart and *pend out-parameters that receive the page range fully covered by [lstart, lend] after any split (or none), i.e. the pages wholly within the range and safe to discard. They are aligned up (pstart) and down (pend) to the mapping's minimum folio order so they always fall on a folio boundary. - Rename the byte-range parameters start/end to lstart/lend to avoid clashing with the new outputs and to separate byte offsets from folio indices. Callers in truncate_inode_pages_range() and shmem_undo_range() pass &pstart for the folio at the start edge and &pend for the folio at the end edge, so the truncate loop drops exactly the fully covered pages and never touches a straddling folio that still holds valid out-of-range data. Suggested-by: Brian Foster Link: https://lore.kernel.org/linux-fsdevel/anH-WKA1coW6wtfG@bfoster/ Fixes: 7460b470a131 ("mm/truncate: use folio_split() in truncate operation") Signed-off-by: Zhang Yi --- v1->v2: - Export pstart as a new parameter so that the generic and shmem truncate paths don't need to recompute the start value from the return value. (Brian) - When min_order is nonzero, align [pstart, pend] to the inner boundaries of the folio to ensure they do not point into the middle of a large folio, which could otherwise cause valid data within the folio to be incorrectly cleared. (Joanne) v1: https://lore.kernel.org/linux-mm/20260903115018.2034541-1-yi.zhang@huaweicloud.com/ mm/internal.h | 4 +-- mm/shmem.c | 13 +++----- mm/truncate.c | 88 +++++++++++++++++++++++++++++++++++---------------- 3 files changed, 67 insertions(+), 38 deletions(-) diff --git a/mm/internal.h b/mm/internal.h index 68db5abd0a4c..6e6ad3187378 100644 --- a/mm/internal.h +++ b/mm/internal.h @@ -627,8 +627,8 @@ unsigned find_lock_entries(struct address_space *mapping, pgoff_t *start, unsigned find_get_entries(struct address_space *mapping, pgoff_t *start, pgoff_t end, struct folio_batch *fbatch, pgoff_t *indices); int truncate_inode_folio(struct address_space *mapping, struct folio *folio); -bool truncate_inode_partial_folio(struct folio *folio, loff_t start, - loff_t end); +bool truncate_inode_partial_folio(struct folio *folio, loff_t lstart, + loff_t lend, pgoff_t *pstart, pgoff_t *pend); long mapping_evict_folio(struct address_space *mapping, struct folio *folio); unsigned long mapping_try_invalidate(struct address_space *mapping, pgoff_t start, pgoff_t end, unsigned long *nr_failed); diff --git a/mm/shmem.c b/mm/shmem.c index 89a1495e55f7..4efbbbba5da5 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -1175,11 +1175,8 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, uoff_t lend, if (folio) { same_folio = lend < folio_next_pos(folio); folio_mark_dirty(folio); - if (!truncate_inode_partial_folio(folio, lstart, lend)) { - start = folio_next_index(folio); - if (same_folio) - end = folio->index; - } + truncate_inode_partial_folio(folio, lstart, lend, &start, + same_folio ? &end : NULL); folio_unlock(folio); folio_put(folio); folio = NULL; @@ -1189,8 +1186,7 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, uoff_t lend, folio = shmem_get_partial_folio(inode, lend >> PAGE_SHIFT); if (folio) { folio_mark_dirty(folio); - if (!truncate_inode_partial_folio(folio, lstart, lend)) - end = folio->index; + truncate_inode_partial_folio(folio, lstart, lend, NULL, &end); folio_unlock(folio); folio_put(folio); } @@ -1258,7 +1254,8 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, uoff_t lend, if (!folio_test_large(folio)) { truncate_inode_folio(mapping, folio); - } else if (truncate_inode_partial_folio(folio, lstart, lend)) { + } else if (truncate_inode_partial_folio(folio, + lstart, lend, NULL, NULL)) { /* * If we split a page, reset the loop so * that we pick up the new sub pages. diff --git a/mm/truncate.c b/mm/truncate.c index b58ba940be47..d88a1b159084 100644 --- a/mm/truncate.c +++ b/mm/truncate.c @@ -206,35 +206,41 @@ static int folio_split_or_unmap(struct folio *folio, struct page *split_at, /* * Handle partial folios. The folio may be entirely within the * range if a split has raced with us. If not, we zero the part of the - * folio that's within the [start, end] range, and then split the folio if + * folio that's within the [lstart, lend] range, and then split the folio if * it's large. split_page_range() will discard pages which now lie beyond * i_size, and we rely on the caller to discard pages which lie within a * newly created hole. * - * Returns false if splitting failed so the caller can avoid - * discarding the entire folio which is stubbornly unsplit. + * When @pstart and/or @pend are non-NULL they receive the indexes of the + * page range fully covered by [lstart, lend] after any split (or none), + * i.e. the range of pages that are wholly within [lstart, lend] and so safe + * to discard. + * + * Return %true if the folio was split, %false otherwise. */ -bool truncate_inode_partial_folio(struct folio *folio, loff_t start, loff_t end) +bool truncate_inode_partial_folio(struct folio *folio, loff_t lstart, + loff_t lend, pgoff_t *pstart, pgoff_t *pend) { loff_t pos = folio_pos(folio); size_t size = folio_size(folio); unsigned int offset, length; struct page *split_at, *split_at2; + unsigned long min_nrbytes; unsigned int min_order; - if (pos < start) - offset = start - pos; + if (pos < lstart) + offset = lstart - pos; else offset = 0; - if (pos + size <= (u64)end) + if (pos + size <= (u64)lend) length = size - offset; else - length = end + 1 - pos - offset; + length = lend + 1 - pos - offset; folio_wait_writeback(folio); if (length == size) { truncate_inode_folio(folio->mapping, folio); - return true; + goto no_split; } /* @@ -248,9 +254,10 @@ bool truncate_inode_partial_folio(struct folio *folio, loff_t start, loff_t end) if (folio_needs_release(folio)) folio_invalidate(folio, offset, length); if (!folio_test_large(folio)) - return true; + goto no_split; min_order = mapping_min_folio_order(folio->mapping); + min_nrbytes = mapping_min_folio_nrbytes(folio->mapping); split_at = folio_page(folio, PAGE_ALIGN_DOWN(offset) / PAGE_SIZE); if (!folio_split_or_unmap(folio, split_at, min_order)) { /* @@ -259,38 +266,67 @@ bool truncate_inode_partial_folio(struct folio *folio, loff_t start, loff_t end) * for shmem truncate */ struct folio *folio2; + bool tail_isolated = true; + + if (pend) + *pend = round_down(pos + offset + length, + min_nrbytes) >> PAGE_SHIFT; if (offset + length == size) - goto no_split; - + goto split; +retry: split_at2 = folio_page(folio, PAGE_ALIGN_DOWN(offset + length) / PAGE_SIZE); folio2 = page_folio(split_at2); if (!folio_try_get(folio2)) - goto no_split; + goto split; if (!folio_test_large(folio2)) goto out; - if (!folio_trylock(folio2)) + if (!folio_trylock(folio2)) { + tail_isolated = false; goto out; + } + + /* + * split_at2 may no longer belong to folio2 due to concurrent + * split. Retry to find the correct folio in case it's still + * large. + */ + if (page_folio(split_at2) != folio2) { + folio_unlock(folio2); + folio_put(folio2); + goto retry; + } /* make sure folio2 is large and does not change its mapping */ if (folio_test_large(folio2) && - folio2->mapping == folio->mapping) - folio_split_or_unmap(folio2, split_at2, min_order); + folio2->mapping == folio->mapping && + folio_split_or_unmap(folio2, split_at2, min_order)) + tail_isolated = false; folio_unlock(folio2); out: + if (!tail_isolated && pend) + *pend = folio2->index; folio_put(folio2); -no_split: +split: + if (pstart) + *pstart = round_up(pos + offset, + min_nrbytes) >> PAGE_SHIFT; return true; } - if (folio_test_dirty(folio)) - return false; - truncate_inode_folio(folio->mapping, folio); - return true; + if (!folio_test_dirty(folio)) + truncate_inode_folio(folio->mapping, folio); +no_split: + if (pstart) + *pstart = offset ? folio_next_index(folio) : folio->index; + if (pend) + *pend = (pos + size > (u64)lend) ? folio->index : + folio_next_index(folio); + return false; } /* @@ -413,11 +449,8 @@ void truncate_inode_pages_range(struct address_space *mapping, folio = __filemap_get_folio(mapping, lstart >> PAGE_SHIFT, FGP_LOCK, 0); if (!IS_ERR(folio)) { same_folio = lend < folio_next_pos(folio); - if (!truncate_inode_partial_folio(folio, lstart, lend)) { - start = folio_next_index(folio); - if (same_folio) - end = folio->index; - } + truncate_inode_partial_folio(folio, lstart, lend, &start, + same_folio ? &end : NULL); folio_unlock(folio); folio_put(folio); folio = NULL; @@ -427,8 +460,7 @@ void truncate_inode_pages_range(struct address_space *mapping, folio = __filemap_get_folio(mapping, lend >> PAGE_SHIFT, FGP_LOCK, 0); if (!IS_ERR(folio)) { - if (!truncate_inode_partial_folio(folio, lstart, lend)) - end = folio->index; + truncate_inode_partial_folio(folio, lstart, lend, NULL, &end); folio_unlock(folio); folio_put(folio); } -- 2.52.0