Re: [PATCH 2/4] xfs: skip online discard during eofblocks trims

[Dave Chinner <david@fromorbit.com>]

On Mon, May 07, 2018 at 02:11:36PM -0400, Brian Foster wrote:
> We've had reports of online discard operations being sent from XFS
> on write-only workloads. These discards occur as a result of
> eofblocks trims that can occur after a large file copy completes.
> 
> These discards are slightly confusing for users who might be paying
> close attention to online discards (i.e., vdo) due to performance
> sensitivity. They also happen to be spurious because freed post-eof
> blocks by definition have not been written to during the current
> allocation cycle.
> 
> Update xfs_free_eofblocks() to skip discards that are purely
> attributed to eofblocks trims. This cuts down the number of spurious
> discards that may occur on write-only workloads due to normal
> preallocation activity.
> 
> Note that discards of post-eof extents can still occur from other
> codepaths that do not isolate handling of post-eof blocks from those
> within eof. For example, file unlinks and truncates may still cause
> discards for any file blocks affected by the operation.
> 
> Signed-off-by: Brian Foster <bfoster@redhat.com>
> ---
>  fs/xfs/xfs_attr_inactive.c |  3 ++-
>  fs/xfs/xfs_bmap_util.c     |  2 +-
>  fs/xfs/xfs_inode.c         | 11 ++++++++---
>  fs/xfs/xfs_inode.h         |  2 +-
>  fs/xfs/xfs_iops.c          |  3 ++-
>  fs/xfs/xfs_qm_syscalls.c   |  2 +-
>  6 files changed, 15 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/xfs/xfs_attr_inactive.c b/fs/xfs/xfs_attr_inactive.c
> index 52818ea2eb50..639311e60c5d 100644
> --- a/fs/xfs/xfs_attr_inactive.c
> +++ b/fs/xfs/xfs_attr_inactive.c
> @@ -434,7 +434,8 @@ xfs_attr_inactive(
>  		if (error)
>  			goto out_cancel;
>  
> -		error = xfs_itruncate_extents(&trans, dp, XFS_ATTR_FORK, 0);
> +		error = xfs_itruncate_extents(&trans, dp, XFS_ATTR_FORK, 0,
> +					      false);
>  		if (error)
>  			goto out_cancel;
>  	}
> diff --git a/fs/xfs/xfs_bmap_util.c b/fs/xfs/xfs_bmap_util.c
> index 8cd8c412f52d..7b95d8976488 100644
> --- a/fs/xfs/xfs_bmap_util.c
> +++ b/fs/xfs/xfs_bmap_util.c
> @@ -872,7 +872,7 @@ xfs_free_eofblocks(
>  		 * may be full of holes (ie NULL files bug).
>  		 */
>  		error = xfs_itruncate_extents(&tp, ip, XFS_DATA_FORK,
> -					      XFS_ISIZE(ip));
> +					      XFS_ISIZE(ip), true);

xfs_itruncate_extents_nodiscard(), and then all the other
xfs_itruncate_extents() can remain unchanged.

>  		if (error) {
>  			/*
>  			 * If we get an error at this point we simply don't
> diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
> index 2b70c8b4cee2..03b2b7099f90 100644
> --- a/fs/xfs/xfs_inode.c
> +++ b/fs/xfs/xfs_inode.c
> @@ -1538,7 +1538,8 @@ xfs_itruncate_extents(
>  	struct xfs_trans	**tpp,
>  	struct xfs_inode	*ip,
>  	int			whichfork,
> -	xfs_fsize_t		new_size)
> +	xfs_fsize_t		new_size,
> +	bool			skip_discard)
>  {
>  	struct xfs_mount	*mp = ip->i_mount;
>  	struct xfs_trans	*tp = *tpp;
> @@ -1549,6 +1550,7 @@ xfs_itruncate_extents(
>  	xfs_filblks_t		unmap_len;
>  	int			error = 0;
>  	int			done = 0;
> +	int			flags = 0;
>  
>  	ASSERT(xfs_isilocked(ip, XFS_ILOCK_EXCL));
>  	ASSERT(!atomic_read(&VFS_I(ip)->i_count) ||
> @@ -1561,6 +1563,9 @@ xfs_itruncate_extents(
>  
>  	trace_xfs_itruncate_extents_start(ip, new_size);
>  
> +	if (skip_discard)
> +		flags |= XFS_BMAPI_NODISCARD;
> +

	flags = xfs_bmapi_aflag(whichfork);
	if (skip_discard)
		flags |= XFS_BMAPI_NODISCARD;

>  	/*
>  	 * Since it is possible for space to become allocated beyond
>  	 * the end of the file (in a crash where the space is allocated
> @@ -1581,7 +1586,7 @@ xfs_itruncate_extents(
>  		xfs_defer_init(&dfops, &first_block);
>  		error = xfs_bunmapi(tp, ip,
>  				    first_unmap_block, unmap_len,
> -				    xfs_bmapi_aflag(whichfork),
> +				    xfs_bmapi_aflag(whichfork) | flags,

so this gets simpler and easier to read.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com