[BUG] With CONFIG_DEBUG_VM set, test case generic/476 has some chance to crash with the following VM_BUG_ON_FOLIO():
BTRFS error (device dm-3): cow_file_range failed, start 1146880 end 1253375 len 106496 ret -28 BTRFS error (device dm-3): run_delalloc_nocow failed, start 1146880 end 1253375 len 106496 ret -28 page: refcount:4 mapcount:0 mapping:00000000592787cc index:0x12 pfn:0x10664 aops:btrfs_aops [btrfs] ino:101 dentry name(?):"f1774" flags: 0x2fffff80004028(uptodate|lru|private|node=0|zone=2|lastcpupid=0xfffff) page dumped because: VM_BUG_ON_FOLIO(!folio_test_locked(folio)) ------------[ cut here ]------------ kernel BUG at mm/page-writeback.c:2992! Internal error: Oops - BUG: 00000000f2000800 [#1] SMP CPU: 2 UID: 0 PID: 3943513 Comm: kworker/u24:15 Tainted: G OE 6.12.0-rc7-custom+ #87 Tainted: [O]=OOT_MODULE, [E]=UNSIGNED_MODULE Hardware name: QEMU KVM Virtual Machine, BIOS unknown 2/2/2022 Workqueue: events_unbound btrfs_async_reclaim_data_space [btrfs] pc : folio_clear_dirty_for_io+0x128/0x258 lr : folio_clear_dirty_for_io+0x128/0x258 Call trace: folio_clear_dirty_for_io+0x128/0x258 btrfs_folio_clamp_clear_dirty+0x80/0xd0 [btrfs] __process_folios_contig+0x154/0x268 [btrfs] extent_clear_unlock_delalloc+0x5c/0x80 [btrfs] run_delalloc_nocow+0x5f8/0x760 [btrfs] btrfs_run_delalloc_range+0xa8/0x220 [btrfs] writepage_delalloc+0x230/0x4c8 [btrfs] extent_writepage+0xb8/0x358 [btrfs] extent_write_cache_pages+0x21c/0x4e8 [btrfs] btrfs_writepages+0x94/0x150 [btrfs] do_writepages+0x74/0x190 filemap_fdatawrite_wbc+0x88/0xc8 start_delalloc_inodes+0x178/0x3a8 [btrfs] btrfs_start_delalloc_roots+0x174/0x280 [btrfs] shrink_delalloc+0x114/0x280 [btrfs] flush_space+0x250/0x2f8 [btrfs] btrfs_async_reclaim_data_space+0x180/0x228 [btrfs] process_one_work+0x164/0x408 worker_thread+0x25c/0x388 kthread+0x100/0x118 ret_from_fork+0x10/0x20 Code: 910a8021 a90363f7 a9046bf9 94012379 (d4210000) ---[ end trace 0000000000000000 ]---
[CAUSE] The first two lines of extra debug messages show the problem is caused by the error handling of run_delalloc_nocow().
E.g. we have the following dirtied range (4K blocksize 4K page size):
0 16K 32K |//////////////////////////////////////| | Pre-allocated |
And the range [0, 16K) has a preallocated extent.
- Enter run_delalloc_nocow() for range [0, 16K) Which found range [0, 16K) is preallocated, can do the proper NOCOW write.
- Enter fallback_to_fow() for range [16K, 32K) Since the range [16K, 32K) is not backed by preallocated extent, we have to go COW.
- cow_file_range() failed for range [16K, 32K) So cow_file_range() will do the clean up by clearing folio dirty, unlock the folios.
Now the folios in range [16K, 32K) is unlocked.
- Enter extent_clear_unlock_delalloc() from run_delalloc_nocow() Which is called with PAGE_START_WRITEBACK to start page writeback. But folios can only be marked writeback when it's properly locked, thus this triggered the VM_BUG_ON_FOLIO().
Furthermore there is another hidden but common bug that run_delalloc_nocow() is not clearing the folio dirty flags in its error handling path. This is the common bug shared between run_delalloc_nocow() and cow_file_range().
[FIX] - Clear folio dirty for range [@start, @cur_offset) Introduce a helper, cleanup_dirty_folios(), which will find and lock the folio in the range, clear the dirty flag and start/end the writeback, with the extra handling for the @locked_folio.
- Introduce a helper to clear folio dirty, start and end writeback
- Introduce a helper to record the last failed COW range end This is to trace which range we should skip, to avoid double unlocking.
- Skip the failed COW range for the error handling
Cc: stable@vger.kernel.org Reviewed-by: Boris Burkov boris@bur.io Signed-off-by: Qu Wenruo wqu@suse.com --- fs/btrfs/inode.c | 98 ++++++++++++++++++++++++++++++++++++++++++---- fs/btrfs/subpage.h | 13 ++++++ 2 files changed, 104 insertions(+), 7 deletions(-)
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c index 1afbeaf3cf4c..a450fc080ca3 100644 --- a/fs/btrfs/inode.c +++ b/fs/btrfs/inode.c @@ -1958,6 +1958,54 @@ static int can_nocow_file_extent(struct btrfs_path *path, return ret < 0 ? ret : can_nocow; }
+/* + * To cleanup the dirty folios which will never be submitted due to + * error. + * + * When running a delalloc range, we may need to split the ranges (due to + * fragmentation or NOCOW limit). If we hit an error in the later part, + * we will error out and previously successfully executed range will never + * be submitted, thus we have to cleanup those folios by clear their + * dirty flag, start and finish the writeback. + */ +static void cleanup_dirty_folios(struct btrfs_inode *inode, + struct folio *locked_folio, + u64 start, u64 end, int error) +{ + struct btrfs_fs_info *fs_info = inode->root->fs_info; + struct address_space *mapping = inode->vfs_inode.i_mapping; + pgoff_t start_index = start >> PAGE_SHIFT; + pgoff_t end_index = end >> PAGE_SHIFT; + u32 len; + + ASSERT(end + 1 - start < U32_MAX); + ASSERT(IS_ALIGNED(start, fs_info->sectorsize) && + IS_ALIGNED(end + 1, fs_info->sectorsize)); + len = end + 1 - start; + + /* + * Handle the locked folio first. + * btrfs_folio_clamp_*() helpers can handle range out of the folio case. + */ + btrfs_folio_clamp_finish_io(fs_info, locked_folio, start, len); + + for (pgoff_t index = start_index; index <= end_index; index++) { + struct folio *folio; + + /* Already handled at the beginning. */ + if (index == locked_folio->index) + continue; + folio = __filemap_get_folio(mapping, index, FGP_LOCK, GFP_NOFS); + /* Cache already dropped, no need to do any cleanup. */ + if (IS_ERR(folio)) + continue; + btrfs_folio_clamp_finish_io(fs_info, locked_folio, start, len); + folio_unlock(folio); + folio_put(folio); + } + mapping_set_error(mapping, error); +} + /* * when nowcow writeback call back. This checks for snapshots or COW copies * of the extents that exist in the file, and COWs the file as required. @@ -1973,6 +2021,11 @@ static noinline int run_delalloc_nocow(struct btrfs_inode *inode, struct btrfs_root *root = inode->root; struct btrfs_path *path; u64 cow_start = (u64)-1; + /* + * If not 0, represents the inclusive end of the last fallback_to_cow() + * range. Only for error handling. + */ + u64 cow_end = 0; u64 cur_offset = start; int ret; bool check_prev = true; @@ -2133,6 +2186,7 @@ static noinline int run_delalloc_nocow(struct btrfs_inode *inode, found_key.offset - 1); cow_start = (u64)-1; if (ret) { + cow_end = found_key.offset - 1; btrfs_dec_nocow_writers(nocow_bg); goto error; } @@ -2206,11 +2260,12 @@ static noinline int run_delalloc_nocow(struct btrfs_inode *inode, cow_start = cur_offset;
if (cow_start != (u64)-1) { - cur_offset = end; ret = fallback_to_cow(inode, locked_folio, cow_start, end); cow_start = (u64)-1; - if (ret) + if (ret) { + cow_end = end; goto error; + } }
btrfs_free_path(path); @@ -2218,12 +2273,41 @@ static noinline int run_delalloc_nocow(struct btrfs_inode *inode,
error: /* - * If an error happened while a COW region is outstanding, cur_offset - * needs to be reset to cow_start to ensure the COW region is unlocked - * as well. + * There are several error cases: + * + * 1) Failed without falling back to COW + * start cur_offset end + * |/////////////| | + * + * For range [start, cur_offset) the folios are already unlocked (except + * @locked_folio), EXTENT_DELALLOC already removed. + * Only need to clear the dirty flag as they will never be submitted. + * Ordered extent and extent maps are handled by + * btrfs_mark_ordered_io_finished() inside run_delalloc_range(). + * + * 2) Failed with error from fallback_to_cow() + * start cur_offset cow_end end + * |/////////////|-----------| | + * + * For range [start, cur_offset) it's the same as case 1). + * But for range [cur_offset, cow_end), the folios have dirty flag + * cleared and unlocked, EXTENT_DEALLLOC cleared by cow_file_range(). + * + * Thus we should not call extent_clear_unlock_delalloc() on range + * [cur_offset, cow_end), as the folios are already unlocked. + * + * So clear the folio dirty flags for [start, cur_offset) first. */ - if (cow_start != (u64)-1) - cur_offset = cow_start; + if (cur_offset > start) + cleanup_dirty_folios(inode, locked_folio, start, cur_offset - 1, ret); + + /* + * If an error happened while a COW region is outstanding, cur_offset + * needs to be reset to @cow_end + 1 to skip the COW range, as + * cow_file_range() will do the proper cleanup at error. + */ + if (cow_end) + cur_offset = cow_end + 1;
/* * We need to lock the extent here because we're clearing DELALLOC and diff --git a/fs/btrfs/subpage.h b/fs/btrfs/subpage.h index 428fa9389fd4..44fff1f4eac4 100644 --- a/fs/btrfs/subpage.h +++ b/fs/btrfs/subpage.h @@ -137,6 +137,19 @@ DECLARE_BTRFS_SUBPAGE_OPS(writeback); DECLARE_BTRFS_SUBPAGE_OPS(ordered); DECLARE_BTRFS_SUBPAGE_OPS(checked);
+/* + * Helper for error cleanup, where a folio will have its dirty flag cleared, + * with writeback started and finished. + */ +static inline void btrfs_folio_clamp_finish_io(struct btrfs_fs_info *fs_info, + struct folio *locked_folio, + u64 start, u32 len) +{ + btrfs_folio_clamp_clear_dirty(fs_info, locked_folio, start, len); + btrfs_folio_clamp_set_writeback(fs_info, locked_folio, start, len); + btrfs_folio_clamp_clear_writeback(fs_info, locked_folio, start, len); +} + bool btrfs_subpage_clear_and_test_dirty(const struct btrfs_fs_info *fs_info, struct folio *folio, u64 start, u32 len);