Re: [PATCH 3/3] xfs: Don't free EOF blocks on sync write close

[Dave Chinner <david@fromorbit.com>]

Ugh forgot to rename patch. should be:

Subject: [PATCH 0/3] xfs: Don't free EOF blocks on O_RDONLY close

On Thu, Feb 07, 2019 at 04:08:13PM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
> 
> When we have a workload that does open/read/close in parallel with
> other synchronous buffered writes to long term open files, the file
> becomes rapidly fragmented. This is due to close() after read
> calling xfs_release() and removing the speculative preallocation
> beyond EOF.
> 
> The existing open/write/close hueristic in xfs_release() does not
> catch this as sync writes do not leave delayed allocation blocks
> allocated on the inode for later writeback that can be detected in
> xfs_release() and hence XFS_IDIRTY_RELEASE never gets set.
> 
> Further, the close context here is for a file opened O_RDONLY, and
> so /modifying/ the file metadata on close doesn't pass muster.
> Fortunately, we can tell in xfs_file_release() whether the release
> context was a read-only context, and so we need to communicate this
> to xfs_release() so it can do the right thing here and skip EOF
> block truncation, hence ensuring that only contexts with write
> permissions will remove post-EOF blocks from the file.
> 
> Before:
> 
> Test 3: Open/read/close loop fragmentation counts
> 
> /mnt/scratch/file.0: 150
> /mnt/scratch/file.1: 342
> /mnt/scratch/file.2: 113
> /mnt/scratch/file.3: 165
> /mnt/scratch/file.4: 86
> /mnt/scratch/file.5: 363
> /mnt/scratch/file.6: 129
> /mnt/scratch/file.7: 233
> 
> After:
> 
> Test 3: Open/read/close loop fragmentation counts
> 
> /mnt/scratch/file.0: 12
> /mnt/scratch/file.1: 12
> /mnt/scratch/file.2: 12
> /mnt/scratch/file.3: 12
> /mnt/scratch/file.4: 12
> /mnt/scratch/file.5: 12
> /mnt/scratch/file.6: 12
> /mnt/scratch/file.7: 12
> 
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---
>  fs/xfs/xfs_file.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> index 02f76b8e6c03..e2d8a0b7f891 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -1023,6 +1023,10 @@ xfs_dir_open(
>   * When we release the file, we don't want it to trim EOF blocks for synchronous
>   * write contexts as this leads to severe fragmentation when applications do
>   * repeated open/appending sync write/close to a file amongst other file IO.
> + *
> + * We also don't want to trim the EOF blocks if it is a read only context. This
> + * prevents open/read/close workloads from removing EOF blocks that other
> + * writers are depending on to prevent fragmentation.
>   */
>  STATIC int
>  xfs_file_release(
> @@ -1031,8 +1035,9 @@ xfs_file_release(
>  {
>  	bool		free_eof_blocks = true;
>  
> -	if ((file->f_mode & FMODE_WRITE) &&
> -	    (file->f_flags & O_DSYNC))
> +	if ((file->f_mode & FMODE_WRITE|FMODE_READ) == FMODE_READ)
> +		free_eof_blocks = false;
> +	else if ((file->f_mode & FMODE_WRITE) && (file->f_flags & O_DSYNC))
>  		free_eof_blocks = false;
>  
>  	return xfs_release(XFS_I(inode), free_eof_blocks);
> -- 
> 2.20.1
> 
> 

-- 
Dave Chinner
david@fromorbit.com