Re: [PATCH v2 04/11] iomap: Support CoW-based atomic writes

John Garry
2025-02-25T10:20:13Z

On 24/02/2025 19:59, Darrick J. Wong wrote:
>> + * ``IOMAP_ATOMIC_COW``: This write is being issued with torn-write
>> + protection based on CoW support.
> I think using "COW" here results in a misnamed flag. Consider:
>
> "IOMAP_ATOMIC_SW:

ok, fine

> This write is being issued with torn-write protection
> via a software fallback provided by the filesystem."

I'm not sure that we really need to even mention software fallback.
Indeed, xfs could just use IOMAP_ATOMIC_SW always when the bdev does not
support HW offload. Maybe I can mention that typically it can be used as
a software fallback when HW offload is not possible.

>
> iomap itself doesn't care*how* the filesystem guarantees that the
> direct write isn't torn, right?

Correct. iomap just ensures that for IOMAP_ATOMIC_HW we produce a single
bio - that's the only check really.

> The fs' io completion handler has to
> ensure that the mapping update(s) are either applied fully or discarded
> fully.

right

>
> In theory if you had a bunch of physical space mapped to the same
> file but with different unwritten states, you could gang together all
> the unwritten extent conversions in a single transaction, which would
> provide the necessary tearing prevention without the out of place write.
> Nobody does that right now, but I think that's the only option for ext4.

ok, maybe. But ext4 still does have bigalloc or opportunity to support
forcealign (to always use IOMAP_ATOMIC_HW for large untorn writes).

Thanks,
John