Re: [PATCH 3/4] xfs: don't discard on free of unwritten extents

[Dave Chinner <david@fromorbit.com>]

On Mon, May 07, 2018 at 02:11:37PM -0400, Brian Foster wrote:
> Unwritten extents by definition have not been written to until they
> are converted to normal written extents. If unwritten extents are
> freed from a file, it is therefore guaranteed that the blocks have
> not been written to since allocation (note that zero range punches
> and reallocates blocks).
> 
> To cut down on online discards generated from workloads that make
> use of preallocation, skip discards of extents if they are in the
> unwritten state when the extent is freed.
> 
> Note that this optimization does not apply to log recovery, during
> which all freed extents are discarded if online discard is enabled.
> Also note that it may be possible for a filesystem crash to occur
> after write completion of an unwritten extent but before unwritten
> conversion such that the extent remains unwritten after log
> recovery. Since this pseudo-inconsistency may already be possible
> after a crash (consider writing to recently allocated blocks where
> the allocation transaction is lost after a crash), this change
> shouldn't introduce any fundamental limitations that don't already
> exist. In short, on storage stacks where discards are important,
> it's good practice to run an occasional fstrim even with online
> discard enabled in the filesystem, particularly after a crash.
> 
> Signed-off-by: Brian Foster <bfoster@redhat.com>
> ---
>  fs/xfs/libxfs/xfs_bmap.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
> index b171f4185adb..a50c197d426f 100644
> --- a/fs/xfs/libxfs/xfs_bmap.c
> +++ b/fs/xfs/libxfs/xfs_bmap.c
> @@ -5107,7 +5107,8 @@ xfs_bmap_del_extent_real(
>  			if (error)
>  				goto done;
>  		} else {
> -			if (bflags & XFS_BMAPI_NODISCARD)
> +			if (bflags & XFS_BMAPI_NODISCARD ||
> +			    del->br_state == XFS_EXT_UNWRITTEN)

Doesn't gcc warn about parenthesis challenged logic like this by
default these days?

I also find code with multi-line function calls easier to follow if
there are {} aroudn them like so:

			if ((bflags & XFS_BMAPI_NODISCARD) ||
			    del->br_state == XFS_EXT_UNWRITTEN) {
>  				xfs_bmap_add_free_nodiscard(mp, dfops,
>  					del->br_startblock, del->br_blockcount,
>  					NULL);
			}


Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com