Re: [PATCH RFC v4 2/3] iomap: use BIO_COMPLETE_IN_TASK for dropbehind writeback

[Christoph Hellwig <hch@infradead.org>]

On Thu, Mar 26, 2026 at 07:34:45AM +1100, Dave Chinner wrote:
> At this point, I'd suggest that we should not be making random
> one-off changes to the iomap and filesystem layers like this just
> for one operation that needs deferred IO completion work. This needs
> to considered from the overall perspective of how we defer
> completion work -  there are lots of different paths through
> filesystems and/or iomap that require/use task deferal for IO
> completion. We want them all to use the same mechanism - splitting
> deferal between multiple layers depending on IO type is not a
> particularly nice thing to be doing...

Yes and no.  The XFS/iomap write completions needs special handling
for merging operation, using different workqueues, and also the
serialization provided by the per-inode list.

Everything that just needs a dumb user context should be the same,
though.  And this mechanism should work just fine for the T10 PI
checksums.  It does not currently work for the defer to user on error
used by the fserror reporting, but should be adaptable to that by
allowing to also defer an I/O completion from an already running
end_io handler, although that might get ugly.

It should work really well for other places that defer bio completions
like the erofs decompression handler that recently came up, and it will
be very useful to implement actually working REQ_NOWAIT support for
file system writes.  So yes, I think we need to look more at the whole
picture, and I think this is a good building block considering the
whole picture.  I don't think we can coverge on just a single mechanism,
but having few and generic ones is good.