Re: [PATCH v6 10/13] xfs: iomap COW-based atomic write support

John Garry
2025-03-17T08:54:26Z

>> + }
>> end_fsb = imap.br_startoff + imap.br_blockcount;
>> length = XFS_FSB_TO_B(mp, end_fsb) - offset;
>> }
>>
>> - if (imap_needs_alloc(inode, flags, &imap, nimaps))
>> + needs_alloc = imap_needs_alloc(inode, flags, &imap, nimaps);
>> +
>> + if (flags & IOMAP_ATOMIC) {
>> + error = -EAGAIN;
>> + /*
>> + * If we allocate less than what is required for the write
>> + * then we may end up with multiple mappings, which means that
>> + * REQ_ATOMIC-based cannot be used, so avoid this possibility.
>> + */
>> + if (needs_alloc && orig_end_fsb - offset_fsb > 1)
>> + goto out_unlock;
>
> I have a quick question here. Based on above check it looks like
> allocation requests on a hole or the 1st time allocation (append writes)
> for a given logical range will always be done using CoW fallback
> mechanism, isn't it?

Right, but...

> So that means HW based multi-fsblock atomic write
> request will only happen for over writes (non-discontigous extent),
> correct?

For an unwritten pre-allocated extent, we can use the REQ_ATOMIC method.

fallocate (without ZERO RANGE) would give a pre-allocated unwritten
extent, and a write there would not technically be an overwrite.

>
> Now, it's not always necessary that if we try to allocate an extent for
> the given range, it results into discontiguous extents. e.g. say, if the
> entire range being written to is a hole or append writes, then it might
> just allocate a single unwritten extent which is valid for doing an
> atomic write using HW/BIOs right?

Right

> And it is valid to write using unwritten extent as long as we don't have
> mixed mappings i.e. the entire range should either be unwritten or
> written for the atomic write to be untorned, correct?
>

We can't write to discontiguous extents, and a mixed mapping would mean
discontiguous extents.

And, as mentioned earlier, it is ok to use REQ_ATOMIC method on an
unwritten extent.

> I am guessing this is kept intentional?
>
Yes

Thanks,
John