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. Rework the contract so the caller is told where to stop instead of silently truncating the straddler: - Return true only when a split occurred, false otherwise. This clarifies the existing confusing return value semantics. - Add an optional out-parameter pgoff_t *end, set to the index of the folio that contains @lend and must be kept by the caller's loop. It is only written when the folio actually straddles @lend. On the success path it defaults to the page index of the end edge and is refined to folio2->index when the second split fails to isolate the tail. - Rename the byte-range parameters start/end to lstart/lend to avoid clashing with the new @end output and to separate byte offsets from folio indices. Callers in truncate_inode_pages_range() and shmem_undo_range() pass &end only when the folio straddles lend. After all, no caller discards a straddling folio anymore, the in-range cleanly-split sub-folios below it are still dropped. 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 --- mm/internal.h | 4 ++-- mm/shmem.c | 12 +++++------ mm/truncate.c | 57 ++++++++++++++++++++++++++++++--------------------- 3 files changed, 41 insertions(+), 32 deletions(-) diff --git a/mm/internal.h b/mm/internal.h index 68db5abd0a4c..db7d9d9fb5c9 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 *end); 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..cc1548ff509a 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -1175,11 +1175,9 @@ 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)) { + if (!truncate_inode_partial_folio(folio, lstart, lend, + same_folio ? &end : NULL)) start = folio_next_index(folio); - if (same_folio) - end = folio->index; - } folio_unlock(folio); folio_put(folio); folio = NULL; @@ -1189,8 +1187,7 @@ 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, &end); folio_unlock(folio); folio_put(folio); } @@ -1258,7 +1255,8 @@ 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)) { /* * 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..2ebb00f6c379 100644 --- a/mm/truncate.c +++ b/mm/truncate.c @@ -206,15 +206,18 @@ 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 @end non-NULL, set to the index of the folio that contains @lend + * and must be kept by the caller's truncate loop. Return %true if the + * folio was split, %false otherwise, in which case the folio is dropped or + * may still straddle the range, so the caller must not discard it. */ -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 *end) { loff_t pos = folio_pos(folio); size_t size = folio_size(folio); @@ -222,19 +225,22 @@ bool truncate_inode_partial_folio(struct folio *folio, loff_t start, loff_t end) struct page *split_at, *split_at2; unsigned int min_order; - if (pos < start) - offset = start - pos; + if (end && pos + size > (u64)lend) + *end = folio->index; + + 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; + return false; } /* @@ -248,7 +254,7 @@ 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; + return false; min_order = mapping_min_folio_order(folio->mapping); split_at = folio_page(folio, PAGE_ALIGN_DOWN(offset) / PAGE_SIZE); @@ -259,6 +265,10 @@ 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 (end) + *end = (pos + offset + length) >> PAGE_SHIFT; if (offset + length == size) goto no_split; @@ -273,24 +283,28 @@ bool truncate_inode_partial_folio(struct folio *folio, loff_t start, loff_t end) if (!folio_test_large(folio2)) goto out; - if (!folio_trylock(folio2)) + if (!folio_trylock(folio2)) { + tail_isolated = false; goto out; + } /* 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 && end) + *end = folio2->index; folio_put(folio2); no_split: 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); + return false; } /* @@ -413,11 +427,9 @@ 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)) { + if (!truncate_inode_partial_folio(folio, lstart, lend, + same_folio ? &end : NULL)) start = folio_next_index(folio); - if (same_folio) - end = folio->index; - } folio_unlock(folio); folio_put(folio); folio = NULL; @@ -427,8 +439,7 @@ 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, &end); folio_unlock(folio); folio_put(folio); } -- 2.52.0