Re: [PATCH] Btrfs: send, fix file hole not being preserved due to inline extent

[Liu Bo <bo.li.liu@oracle.com>]

On Thu, Apr 06, 2017 at 05:07:56PM +0100, fdmanana@kernel.org wrote:
> From: Filipe Manana <fdmanana@suse.com>
> 
> Normally we don't have inline extents followed by regular extents, but
> there's currently at least one harmless case where this happens. For
> example, when the page size is 4Kb and compression is enabled:
> 
>   $ mkfs.btrfs -f /dev/sdb
>   $ mount -o compress /dev/sdb /mnt
>   $ xfs_io -f -c "pwrite -S 0xaa 0 4K" -c "fsync" /mnt/foobar
>   $ xfs_io -c "pwrite -S 0xbb 8K 4K" -c "fsync" /mnt/foobar
> 
> In this case we get a compressed inline extent, representing 4Kb of
> data, followed by a hole extent and then a regular data extent. The
> inline extent was not expanded/converted to a regular extent exactly
> because it represents 4Kb of data. This does not cause any apparent
> problem (such as the issue solved by commit e1699d2d7bf6
> ("btrfs: add missing memset while reading compressed inline extents"))
> except trigger an unexpected case in the incremental send code path
> that makes us issue an operation to write a hole when it's not needed,
> resulting in more writes at the receiver and wasting space at the
> receiver.
> 
> So teach the incremental send code to deal with this particular case.
> 
> The issue can be currently triggered by running fstests btrfs/137 with
> compression enabled (MOUNT_OPTIONS="-o compress" ./check btrfs/137).
> 

Reviewed-by: Liu Bo <bo.li.liu@oracle.com>

Thanks,

-liubo
> Signed-off-by: Filipe Manana <fdmanana@suse.com>
> ---
>  fs/btrfs/send.c | 23 +++++++++++++++++++++--
>  1 file changed, 21 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/btrfs/send.c b/fs/btrfs/send.c
> index 456c890..f66095a 100644
> --- a/fs/btrfs/send.c
> +++ b/fs/btrfs/send.c
> @@ -5184,13 +5184,19 @@ static int is_extent_unchanged(struct send_ctx *sctx,
>  	while (key.offset < ekey->offset + left_len) {
>  		ei = btrfs_item_ptr(eb, slot, struct btrfs_file_extent_item);
>  		right_type = btrfs_file_extent_type(eb, ei);
> -		if (right_type != BTRFS_FILE_EXTENT_REG) {
> +		if (right_type != BTRFS_FILE_EXTENT_REG &&
> +		    right_type != BTRFS_FILE_EXTENT_INLINE) {
>  			ret = 0;
>  			goto out;
>  		}
>  
>  		right_disknr = btrfs_file_extent_disk_bytenr(eb, ei);
> -		right_len = btrfs_file_extent_num_bytes(eb, ei);
> +		if (right_type == BTRFS_FILE_EXTENT_INLINE) {
> +			right_len = btrfs_file_extent_inline_len(eb, slot, ei);
> +			right_len = PAGE_ALIGN(right_len);
> +		} else {
> +			right_len = btrfs_file_extent_num_bytes(eb, ei);
> +		}
>  		right_offset = btrfs_file_extent_offset(eb, ei);
>  		right_gen = btrfs_file_extent_generation(eb, ei);
>  
> @@ -5204,6 +5210,19 @@ static int is_extent_unchanged(struct send_ctx *sctx,
>  			goto out;
>  		}
>  
> +		/*
> +		 * We just wanted to see if when we have an inline extent, what
> +		 * follows it is a regular extent (wanted to check the above
> +		 * condition for inline extents too). This should normally not
> +		 * happen but it's possible for example when we have an inline
> +		 * compressed extent representing data with a size matching
> +		 * the page size (currently the same as sector size).
> +		 */
> +		if (right_type == BTRFS_FILE_EXTENT_INLINE) {
> +			ret = 0;
> +			goto out;
> +		}
> +
>  		left_offset_fixed = left_offset;
>  		if (key.offset < ekey->offset) {
>  			/* Fix the right offset for 2a and 7. */
> -- 
> 2.7.0.rc3
> 
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