Merge patch series "buffer: stop clearing BH_Uptodate when a write fails"

Chao Shi <coshi036@gmail.com> says:

When a metadata write fails, the buffer_head completion handlers clear
BH_Uptodate. The buffer still holds exactly the data the filesystem asked
to be written - it is the disk that is stale, not the buffer - and saying
otherwise has consequences:

  - mark_buffer_dirty() has a WARN_ON_ONCE(!buffer_uptodate(bh)), so a
    filesystem that dirties the buffer again to retry the write trips it.
    That is the warning that started this.

  - a buffer that is not up to date gets re-read from disk, which silently
    replaces the data the filesystem was trying to write with the stale
    on-disk copy.

  - the state is not self consistent while it lasts: the window between the
    write completing and BH_Uptodate being cleared is visible to anyone
    holding the folio lock.

BH_Write_EIO already records that the last write failed.  This series moves
every consumer over to it, then stops write completion touching BH_Uptodate
at all.

Patches 1-4 are groundwork.  Matthew's patch 1 removes b_page.  Patches 2
and 3 let a buffer_head point at memory outside the page cache and use that
for jbd2's shadow buffers, which today sit on a slab folio - a slab folio
overloads ->mapping, so mark_buffer_write_io_error() cannot be called on
them at all.  That is what blocked the jbd2 conversion.  Patch 4 then drops
the folio_mapping() call that was only there to cope with those slab
folios.

Patches 5-6 make BH_Write_EIO safe to leave set: clear it in bforget() and
discard it on invalidate, so a freed or reused block does not inherit
somebody else's write error.

Patches 7-19 convert the consumers, one filesystem at a time: the two core
helpers in fs/buffer.c, then adfs, ext2, omfs, exfat, fat, ext4, ocfs2,
gfs2 and jbd2.  Patch 20 stops write completion touching BH_Uptodate.
Patch 21 moves the point at which BH_Write_EIO is cleared from submission
to successful completion.

Every patch builds on its own and the tree behaves identically at each step
until patch 20, because a failed write currently sets BH_Write_EIO and
clears BH_Uptodate together.

* patches from https://patch.msgid.link/cover.1785951556.git.coshi036@gmail.com: (21 commits)
  buffer: clear BH_Write_EIO when a write succeeds, not when one starts
  buffer: stop touching BH_Uptodate on write completion
  jbd2: check for a fast commit write error with buffer_write_io_error()
  jbd2: say what jbd2_freeze_jh_data()'s assertion is actually checking
  jbd2: report journal write errors with BH_Write_EIO
  gfs2: check for a metadata write error with buffer_write_io_error()
  ocfs2: check for a stale write error before reusing a metadata buffer
  ocfs2: check for a metadata write error with buffer_write_io_error()
  ext4: check for a metadata write error with buffer_write_io_error()
  fat: check for a metadata write error with buffer_write_io_error()
  exfat: check for a directory write error with buffer_write_io_error()
  omfs: check for an inode write error with buffer_write_io_error()
  ext2: check for an xattr block write error with buffer_write_io_error()
  adfs: check for a directory write error with buffer_write_io_error()
  buffer: detect metadata write errors with buffer_write_io_error()
  buffer: discard BH_Write_EIO along with the rest of the buffer state
  buffer: clear BH_Write_EIO when a buffer is forgotten
  buffer: read the folio's mapping directly in buffer_set_crypto_ctx()
  jbd2: point the shadow buffer at the frozen data directly
  buffer: allow a buffer_head to point at memory outside the page cache
  ...

Link: https://patch.msgid.link/cover.1785951556.git.coshi036@gmail.com
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

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