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

[Matthew Wilcox <willy@infradead.org>]

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.