[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 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@xxxxxxxxxxxxxxx
Signed-off-by: Qu Wenruo <wqu@xxxxxxxx>
---
fs/btrfs/inode.c | 93 ++++++++++++++++++++++++++++++++++++++++++++----
1 file changed, 86 insertions(+), 7 deletions(-)
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 57e9b7deee88..75b7956a7b4c 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -1961,6 +1961,48 @@ static int can_nocow_file_extent(struct btrfs_path *path,
return ret < 0 ? ret : can_nocow;
}
+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_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);
+
+ 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_clear_dirty(fs_info, folio, start, len);
+ btrfs_folio_clamp_set_writeback(fs_info, folio, start, len);
+ btrfs_folio_clamp_clear_writeback(fs_info, 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.
@@ -1976,6 +2018,11 @@ 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;
@@ -2136,6 +2183,7 @@ 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;
}
@@ -2209,11 +2257,12 @@ 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);
@@ -2221,12 +2270,42 @@ 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_start end
+ * |/////////////| |
+ *
+ * For range [start, cur_start) 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_start cow_end end
+ * |/////////////|-----------| |
+ *
+ * For range [start, cur_start) it's the same as case 1).
+ * But for range [cur_start, cow_end), the folios have dirty flag
+ * cleared and unlocked, EXTENT_DEALLLOC cleared.
+ * There may or may not be any ordered extents/extent maps allocated.
+ *
+ * We should not call extent_clear_unlock_delalloc() on range [cur_start,
+ * 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
--
2.47.1
