btrfs: allow buffered write to avoid full page read if it's block aligned

[BUG]
Since the support of block size (sector size) < page size for btrfs,
test case generic/563 fails with 4K block size and 64K page size:

  --- tests/generic/563.out	2024-04-25 18:13:45.178550333 +0930
  +++ /home/adam/xfstests-dev/results//generic/563.out.bad	2024-09-30 09:09:16.155312379 +0930
  @@ -3,7 +3,8 @@
   read is in range
   write is in range
   write -> read/write
  -read is in range
  +read has value of 8388608
  +read is NOT in range -33792 .. 33792
   write is in range
  ...

[CAUSE]
The test case creates a 8MiB file, then does buffered write into the 8MiB
using 4K block size, to overwrite the whole file.

On 4K page sized systems, since the write range covers the full block and
page, btrfs will not bother reading the page, just like what XFS and EXT4
do.

But on 64K page sized systems, although the 4K sized write is still block
aligned, it's not page aligned anymore, thus btrfs will read the full
page, which will be accounted by cgroup and fail the test.

As the test case itself expects such 4K block aligned write should not
trigger any read.

Such expected behavior is an optimization to reduce folio reads when
possible, and unfortunately btrfs does not implement such optimization.

[FIX]
To skip the full page read, we need to do the following modification:

- Do not trigger full page read as long as the buffered write is block
  aligned
  This is pretty simple by modifying the check inside
  prepare_uptodate_page().

- Skip already uptodate blocks during full page read
  Or we can lead to the following data corruption:

  0       32K        64K
  |///////|          |

  Where the file range [0, 32K) is dirtied by buffered write, the
  remaining range [32K, 64K) is not.

  When reading the full page, since [0,32K) is only dirtied but not
  written back, there is no data extent map for it, but a hole covering
  [0, 64k).

  If we continue reading the full page range [0, 64K), the dirtied range
  will be filled with 0 (since there is only a hole covering the whole
  range).
  This causes the dirtied range to get lost.

With this optimization, btrfs can pass generic/563 even if the page size
is larger than fs block size.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

0 files changed, 0 insertions, 0 deletions