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

[Christoph Hellwig <hch@lst.de>]

On Tue, Mar 18, 2025 at 08:22:53AM +0000, John Garry wrote:
>> Is xfs_reflink_allocate_cow even the right helper to use?  We know we
>> absolutely want a a COW fork extent, we know there can't be delalloc
>> extent to convert as we flushed dirty data, so most of the logic in it
>> is pointless.
>
> Well xfs_reflink_allocate_cow gives us what we want when we set some flag 
> (XFS_REFLINK_FORCE_COW).
>
> Are you hinting at a dedicated helper? Note that 
> xfs_reflink_fill_cow_hole() also handles the XFS_REFLINK_FORCE_COW flag.

We might not even need much of a helper.  This all comes from my
experience with the zoned code that always writes out of place as well.
I initially tried to reuse the existing iomap_begin methods and all
these helpers, but in the end it turned out to much, much simpler to
just open code the logic.  Now the atomic cow code will be a little
more complex in some aspect (unwritten extents, speculative
preallocations), but also simpler in others (no need to support buffered
I/O including zeroing and sub-block writes that require the ugly
srcmap based read-modify-write), but I suspect the overall trade-off
will be similar.

After all the high-level logic for the atomic COW iomap_begin really
should just be:

 - take the ilock
 - check the COW fork if there is an extent for the start of the range.
 - If yes:
     - if the extent is unwritten, convert the part overlapping with
       the range to written
     - return the extent
 - If no:
     - allocate a new extent covering the range in the COW fork

Doing this without going down multiple levels of helpers designed for
an entirely different use case will probably simplify things
significantly.