Re: [PATCH RFC] ext4: never fall back to buffered I/O for atomic direct writes

[Tal Zussman]

On 9/10/26 6:58 AM, Jan Kara wrote:
> I agree your patch makes things better but I have a question: are short
> writes (even 0) allowed for atomic writes? I don't think so as far as my
> understanding of atomic writes goes (and based on what I could find in the
> code). But after your change we'd just return short write (including ret ==
> 0) now which doesn't look ideal and also doesn't match your "so any future
> bug yields an error rather than a torn write with a WARN()" description. So
> as a futureproofing I'd maybe suggest just:
> 
> 	if (WARN_ON_ONCE(iocb->ki_flags & IOCB_ATOMIC && ret >= 0 &&
> 			 iov_iter_count(from)))
> 		ret = -EIO; /* or maybe -ENOPROTOOPT??? */
> 
> before the fallback to buffered IO as a more robust solution.
> 

Short writes shouldn't be allowed, so this is definitely better in terms of
future-proofing. I'll do this with EIO but with the if (WARN_ON_ONCE())
inside the existing ret and iov_iter_count() check to avoid duplicating the
checks.

It looks like Sashiko also found a pre-existing issue where an atomic request
can be done non-atomically [1]. I'll see if I can reproduce it and add a fix
as a second patch in v2.

Thanks,
Tal

[1] https://sashiko.dev/#/patchset/20260909-ext4-atomic-no-fallback-v1-1-5a6b9539c002%40columbia.edu?part=1