[0/2] btrfs: fix corner cases for subpage generic/363 failures

Message

Qu Wenruo April 11, 2025, 5:13 a.m. UTC

Test case generic/363 always fail on subpage (fs block fs < page size)
btrfses, there are mostly two kinds of problems here:

All examples are based on 64K page size and 4K fs block size.

1) EOF is polluted and btrfs_truncate_block() only zeros the block that
   needs to be written back

   
   0                           32K                           64K
   |                           |              |GGGGGGGGGGGGGG|
                                              50K EOF
   The original file is 50K sized (not 4K aligned), and fsx polluted the
   range beyond EOF through memory mapped write.
   And since memory mapped write is page based, and our page size is
   larger than block size, the page range [0, 64K) covere blocks beyond
   EOF.

   Those polluted range will not be written back, but will still affect
   our page cache.

   Then some operation happens to expand the inode to size 64K.

   In that case btrfs_truncate_block() is called to trim the block
   [48K, 52K), and that block will be marked dirty for written back.

   But the range [52K, 64K) is untouched at all, left the garbage
   hanging there, triggering `fsx -e 1` failure.

   Fix this case by force btrfs_truncate_block() to zeroing any involved
   blocks. (Meanwhile still only one block [48K, 52K) will be written
   back)

2) EOF is polluted and the original size is block aligned so
   btrfs_truncate_block() does nothing

   0                           32K                           64K
   |                           |                |GGGGGGGGGGGG|
                                                52K EOF

   Mostly the same as case 1, but this time since the inode size is
   block aligned, btrfs_truncate_block() will do nothing.

   Leaving the garbage range [52K, 64K) untouched and fail `fsx -e 1`
   runs.

   Fix this case by force btrfs_truncate_block() to zeroing any involved
   blocks when the btrfs is subpage and the range is aligned.
   This will not cause any new dirty blocks, but purely zeroing out EOF
   to pass `fsx -e 1` runs.

Qu Wenruo (2):
  btrfs: make btrfs_truncate_block() to zero involved blocks in a folio
  btrfs: make btrfs_truncate_block() zero folio range for certain
    subpage corner cases

 fs/btrfs/btrfs_inode.h |  10 ++-
 fs/btrfs/file.c        |  33 ++++++---
 fs/btrfs/inode.c       | 148 ++++++++++++++++++++++++++++++++++-------
 3 files changed, 155 insertions(+), 36 deletions(-)