Re: [PATCH v5 05/10] xfs: Iomap SW-based atomic write support

[Christoph Hellwig <hch@infradead.org>]

On Wed, Mar 12, 2025 at 09:00:52AM +0000, John Garry wrote:
> > How is -EAGAIN going to work here given that it is also used to defer
> > non-blocking requests to the caller blocking context?
> 
> You are talking about IOMAP_NOWAIT handling, right?

Yes.

> If so, we handle that in
> xfs_file_dio_write_atomic(), similar to xfs_file_dio_write_unaligned(), i.e.
> if IOMAP_NOWAIT is set and we get -EAGAIN, then we will return -EAGAIN
> directly to the caller.

Can you document this including the interaction between the different
cases of -EAGAIN somewhere?

> > What is the probem with only setting the flag that causes REQ_ATOMIC
> > to be set from the file system instead of forcing it when calling
> > iomap_dio_rw?
> 
> We have this in __iomap_dio_rw():
> 
> 	if (dio_flags & IOMAP_DIO_ATOMIC_SW)
> 		iomi.flags |= IOMAP_ATOMIC_SW;
> 	else if (iocb->ki_flags & IOCB_ATOMIC)
>  		iomi.flags |= IOMAP_ATOMIC_HW;
> 
> I do admit that the checks are a bit uneven, i.e. check vs
> IOMAP_DIO_ATOMIC_SW and IOCB_ATOMIC
> 
> If we want a flag to set REQ_ATOMIC from the FS then we need
> IOMAP_DIO_BIO_ATOMIC, and that would set IOMAP_BIO_ATOMIC. Is that better?

My expectation from a very cursory view is that iomap would be that
there is a IOMAP_F_REQ_ATOMIC that is set in ->iomap_begin and which
would make the core iomap code set REQ_ATOMIC on the bio for that
iteration.

> > Also how you ensure this -EAGAIN only happens on the first extent
> > mapped and you doesn't cause double writes?
> 
> When we find that a mapping does not suit REQ_ATOMIC-based atomic write,
> then we immediately bail and retry with FS-based atomic write. And that
> check should cover all requirements for a REQ_ATOMIC-based atomic write:
> - aligned
> - contiguous blocks, i.e. the mapping covers the full write
> 
> And we also have the check in iomap_dio_bit_iter() to ensure that the
> mapping covers the full write (for REQ_ATOMIC-based atomic write).

Ah, I guess that's the

	if (bio_atomic && length != iter->len)
		return -EINVAL;

So yes, please adda comment there that this is about a single iteration
covering the entire write.