Re: [PATCH 15/15] xfs: Use the new iomap infrastructure for CoW

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On Thu, Sep 05, 2019 at 10:06:50AM -0500, Goldwyn Rodrigues wrote:
> diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
> index 8321733c16c3..13495d8a1ee2 100644
> --- a/fs/xfs/xfs_iomap.c
> +++ b/fs/xfs/xfs_iomap.c
> @@ -1006,7 +1006,10 @@ xfs_file_iomap_begin(
>  		 */
>  		if (directio || imap.br_startblock == HOLESTARTBLOCK)
>  			imap = cmap;
> +		else
> +			xfs_bmbt_to_iomap(ip, srcmap, &cmap, false);
>  
> +		iomap->flags |= IOMAP_F_COW;

I don't think this is correct.  We should only set IOMAP_F_COW
when we actually fill out the srcmap.  Which is a very good agument
for Darrick's suggestion to check for a non-emptry srcmap.

Also this is missing the actually interesting part in
xfs_file_iomap_begin_delay.

I ended up spending the better half of the day trying to implement
that and did run into a few bugs in the core iomap changes, mostly
due to a confusion which iomap to use.  So I ended up reworking those
a bit to:

  a) check srcmap->type to see if there is a valid srcmap
  b) set the srcmap pointer to iomap so that it doesn't need to
     be special cased all over
  c) fixed up a few more places to use the srcmap

This now at least survives xfstests -g quick on a 4k xfs file system
for.  Here is my current tree:

http://git.infradead.org/users/hch/xfs.git/shortlog/refs/heads/xfs-cow-iomap



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