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btrfs: keep folios locked inside run_delalloc_nocow()

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[BUG]
There is a very low chance that DEBUG_WARN() inside
btrfs_writepage_cow_fixup() can be triggered when
CONFIG_BTRFS_EXPERIMENTAL is enabled.

This only happens after run_delalloc_nocow() failed.

Unfortunately I haven't hit it for a while thus no real world dmesg for
now.

[CAUSE]
There is a race window where after run_delalloc_nocow() failed, error
handling can race with writeback thread.

Before we hit run_delalloc_nocow(), there is an inode with the following
dirty pages: (4K page size, 4K block size, no large folio)

  0         4K          8K          12K          16K
  |/////////|///////////|///////////|////////////|

The inode also have NODATACOW flag, and the above dirty range will go
through different extents during run_delalloc_range():

  0         4K          8K          12K          16K
  |  NOCOW  |    COW    |    COW    |   NOCOW    |

The race happen like this:

    writeback thread A            |        writeback thread B
----------------------------------+--------------------------------------
Writeback for folio 0             |
run_delalloc_nocow()              |
|- nocow_one_range()              |
|  For range [0, 4K), ret = 0     |
|                                 |
|- fallback_to_cow()              |
|  For range [4K, 8K), ret = 0    |
|  Folio 4K *UNLOCKED*            |
|                                 | Writeback for folio 4K
|- fallback_to_cow()              | extent_writepage()
|  For range [8K, 12K), failure   | |- writepage_delalloc()
|				  | |
|- btrfs_cleanup_ordered_extents()| |
   |- btrfs_folio_clear_ordered() | |
   |  Folio 0 still locked, safe  | |
   |                              | |  Ordered extent already allocated.
   |                              | |  Nothing to do.
   |                              | |- extent_writepage_io()
   |                              |    |- btrfs_writepage_cow_fixup()
   |- btrfs_folio_clear_ordered() |    |
      Folio 4K hold by thread B,  |    |
      UNSAFE!                     |    |- btrfs_test_ordered()
                                  |    |  Cleared by thread A,
				  |    |
                                  |    |- DEBUG_WARN();

This is only possible after run_delalloc_nocow() failure, as
cow_file_range() will keep all folios and io tree range locked, until
everything is finished or after error handling.

The root cause is we allow fallback_to_cow() and nocow_one_range() to
unlock the folios after a successful run, so that during error handling
we're no longer safe to use btrfs_cleanup_ordered_extents() as the
folios are already unlocked.

[FIX]
- Make fallback_to_cow() and nocow_one_range() to keep folios locked
  after a successful run

  For fallback_to_cow() we can pass COW_FILE_RANGE_KEEP_LOCKED flag
  into cow_file_range().

  For nocow_one_range() we have to remove the PAGE_UNLOCK flag from
  extent_clear_unlock_delalloc().

- Unlock folios if everything is fine in run_delalloc_nocow()

- Use extent_clear_unlock_delalloc() to handle range [@start,
  @cur_offset) inside run_delalloc_nocow()
  Since folios are still locked, we do not need
  cleanup_dirty_folios() to do the cleanup.

  extent_clear_unlock_delalloc() with "PAGE_START_WRITEBACK |
  PAGE_END_WRITEBACK" will clear the dirty flags.

- Remove cleanup_dirty_folios()

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
Qu Wenruo 2025-07-28 17:57:57 +09:30 committed by David Sterba
parent 13141df705
commit 737852c060
1 changed files with 22 additions and 51 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

73
fs/btrfs/inode.c
View File

@ -1772,9 +1772,15 @@ static int fallback_to_cow(struct btrfs_inode *inode,
* Don't try to create inline extents, as a mix of inline extent that
* is written out and unlocked directly and a normal NOCOW extent
* doesn't work.
*
* And here we do not unlock the folio after a successful run.
* The folios will be unlocked after everything is finished, or by error handling.
*
* This is to ensure error handling won't need to clear dirty/ordered flags without
* a locked folio, which can race with writeback.
*/
ret = cow_file_range(inode, locked_folio, start, end, NULL,
COW_FILE_RANGE_NO_INLINE);
COW_FILE_RANGE_NO_INLINE | COW_FILE_RANGE_KEEP_LOCKED);
ASSERT(ret != 1);
return ret;
}
@ -1917,53 +1923,6 @@ static int can_nocow_file_extent(struct btrfs_path *path,
return ret < 0 ? ret : can_nocow;
}
/*
* 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). 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 clearing their dirty flag, starting and
* finishing 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.
* The 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);
}
static int nocow_one_range(struct btrfs_inode *inode, struct folio *locked_folio,
struct extent_state **cached,
struct can_nocow_file_extent_args *nocow_args,
@ -2013,7 +1972,7 @@ static int nocow_one_range(struct btrfs_inode *inode, struct folio *locked_folio
extent_clear_unlock_delalloc(inode, file_pos, end, locked_folio, cached,
EXTENT_LOCKED | EXTENT_DELALLOC |
EXTENT_CLEAR_DATA_RESV,
PAGE_UNLOCK | PAGE_SET_ORDERED);
PAGE_SET_ORDERED);
return ret;
error:
@ -2248,6 +2207,14 @@ static noinline int run_delalloc_nocow(struct btrfs_inode *inode,
cow_start = (u64)-1;
}
/*
* Everything is finished without an error, can unlock the folios now.
*
* No need to touch the io tree range nor set folio ordered flag, as
* fallback_to_cow() and nocow_one_range() have already handled them.
*/
extent_clear_unlock_delalloc(inode, start, end, locked_folio, NULL, 0, PAGE_UNLOCK);
btrfs_free_path(path);
return 0;
@ -2306,9 +2273,13 @@ static noinline int run_delalloc_nocow(struct btrfs_inode *inode,
}
if (oe_cleanup_len) {
const u64 oe_cleanup_end = oe_cleanup_start + oe_cleanup_len - 1;
btrfs_cleanup_ordered_extents(inode, oe_cleanup_start, oe_cleanup_len);
cleanup_dirty_folios(inode, locked_folio, oe_cleanup_start,
oe_cleanup_start + oe_cleanup_len - 1, ret);
extent_clear_unlock_delalloc(inode, oe_cleanup_start, oe_cleanup_end,
locked_folio, NULL,
EXTENT_LOCKED | EXTENT_DELALLOC,
PAGE_UNLOCK | PAGE_START_WRITEBACK |
PAGE_END_WRITEBACK);
}
if (untouched_len) {