Re: [PATCH v2] fs/buffer: serialize set_buffer_uptodate against concurrent clears

[Chris S]

Yes, that works. And appreciate your reply.
  I prototyped it, and it survives a journal replay.

Six sites, with bio_add_virt_nofail() covering the submit side:

  bh_offset()                     b_page may be NULL
  buffer_set_crypto_ctx()         NULL guard
  __bh_submit()                   bio_add_virt_nofail() when !b_folio, and
                                  skip wbc_account_cgroup_owner()
  jbd2_journal_write_metadata_buffer()
                                  b_folio = NULL, b_data =
                                  jh->b_frozen_data, not folio_set_bh()
  jbd2_checksum_data(), jbd2_block_tag_csum_set()   both kmap the temp bh
                                  (fs/jbd2/commit.c:705 and :745)

4 files, +38/-12.  I'll send it as the first patch of the series.

Tested: ext4 with metadata_csum, mounted data=journal,journal_checksum,
writing files whose every block starts with the JBD2 magic so escaping
forces copy-out into b_frozen_data, plus repeated modification of the same
metadata while commits run.  Then sysrq-b without unmounting, reboot,
mount to replay. Journal recovery completed, md5sums of the escaped files
matched, e2fsck -fn clean, no warnings.  An instrumented build confirmed
the b_folio == NULL path was actually taken rather than silently skipped.

Best，
Chao

On Thu, Jul 30, 2026 at 4:20 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> On Thu, Jul 30, 2026 at 03:47:23PM -0400, Chris S wrote:
> > Hi Jan, I'm currently working on the changes we discussed.  Based on
> > vfs.all now.  Four things came out of writing it that I would rather
> > resolve before posting.
> >
> > 1. mark_buffer_write_io_error() is not safe for jbd2's buffers yet.  It
> > dereferences bh->b_folio->mapping directly (fs/buffer.c:1057), and
> > jbd2_journal_write_metadata_buffer() points the temporary bh at
> > virt_to_folio(jh->b_frozen_data), which is slab-backed.
>
> ... yeah.  That's one of the unclean things which keeps me awake at
> night.  Sorry you ran into it.
>
> My longterm plan for this particular usecase (pointing a bh at slab
> memory) is to have bh->b_folio = NULL, bh->b_data = (address of data).
> Maybe we could do that now?  That would make mark_buffer_write_io_error()
> work without change today.
>
> There might be a good reason I didn't do that yet, but I forget what it
> was.
>