Re: [PATCH v4 1/3] btrfs: defrag: don't try to merge regular extents with preallocated extents

[Filipe Manana <fdmanana@kernel.org>]

On Fri, Jan 28, 2022 at 03:21:20PM +0800, Qu Wenruo wrote:
> [BUG]
> With older kernels (before v5.16), btrfs will defrag preallocated extents.
> While with newer kernels (v5.16 and newer) btrfs will not defrag
> preallocated extents, but it will defrag the extent just before the
> preallocated extent, even it's just a single sector.
> 
> This can be exposed by the following small script:
> 
> 	mkfs.btrfs -f $dev > /dev/null
> 
> 	mount $dev $mnt
> 	xfs_io -f -c "pwrite 0 4k" -c sync -c "falloc 4k 16K" $mnt/file
> 	xfs_io -c "fiemap -v" $mnt/file
> 	btrfs fi defrag $mnt/file
> 	sync
> 	xfs_io -c "fiemap -v" $mnt/file
> 
> The output looks like this on older kernels:
> 
> /mnt/btrfs/file:
>  EXT: FILE-OFFSET      BLOCK-RANGE      TOTAL FLAGS
>    0: [0..7]:          26624..26631         8   0x0
>    1: [8..39]:         26632..26663        32 0x801
> /mnt/btrfs/file:
>  EXT: FILE-OFFSET      BLOCK-RANGE      TOTAL FLAGS
>    0: [0..39]:         26664..26703        40   0x1
> 
> Which defrags the single sector along with the preallocated extent, and
> replace them with an regular extent into a new location (caused by data
> COW).
> This wastes most of the data IO just for the preallocated range.
> 
> On the other hand, v5.16 is slightly better:
> 
> /mnt/btrfs/file:
>  EXT: FILE-OFFSET      BLOCK-RANGE      TOTAL FLAGS
>    0: [0..7]:          26624..26631         8   0x0
>    1: [8..39]:         26632..26663        32 0x801
> /mnt/btrfs/file:
>  EXT: FILE-OFFSET      BLOCK-RANGE      TOTAL FLAGS
>    0: [0..7]:          26664..26671         8   0x0
>    1: [8..39]:         26632..26663        32 0x801
> 
> The preallocated range is not defragged, but the sector before it still
> gets defragged, which has no need for it.
> 
> [CAUSE]
> One of the function reused by the old and new behavior is
> defrag_check_next_extent(), it will determine if we should defrag
> current extent by checking the next one.
> 
> It only checks if the next extent is a hole or inlined, but it doesn't
> check if it's preallocated.
> 
> On the other hand, out of the function, both old and new kernel will
> reject preallocated extents.
> 
> Such inconsistent behavior causes above behavior.
> 
> [FIX]
> - Also check if next extent is preallocated
>   If so, don't defrag current extent.
> 
> - Add comments for each branch why we reject the extent
> 
> This will reduce the IO caused by defrag ioctl and autodefrag.
> 
> Signed-off-by: Qu Wenruo <wqu@suse.com>

Reviewed-by: Filipe Manana <fdmanana@suse.com>

Thanks.

> ---
>  fs/btrfs/ioctl.c | 19 ++++++++++++-------
>  1 file changed, 12 insertions(+), 7 deletions(-)
> 
> diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
> index 91ba2efe9792..a622b1ac0fec 100644
> --- a/fs/btrfs/ioctl.c
> +++ b/fs/btrfs/ioctl.c
> @@ -1053,19 +1053,24 @@ static bool defrag_check_next_extent(struct inode *inode, struct extent_map *em,
>  				     bool locked)
>  {
>  	struct extent_map *next;
> -	bool ret = true;
> +	bool ret = false;
>  
>  	/* this is the last extent */
>  	if (em->start + em->len >= i_size_read(inode))
> -		return false;
> +		return ret;
>  
>  	next = defrag_lookup_extent(inode, em->start + em->len, locked);
> +	/* No more em or hole */
>  	if (!next || next->block_start >= EXTENT_MAP_LAST_BYTE)
> -		ret = false;
> -	else if ((em->block_start + em->block_len == next->block_start) &&
> -		 (em->block_len > SZ_128K && next->block_len > SZ_128K))
> -		ret = false;
> -
> +		goto out;
> +	if (test_bit(EXTENT_FLAG_PREALLOC, &next->flags))
> +		goto out;
> +	/* Physically adjacent and large enough */
> +	if ((em->block_start + em->block_len == next->block_start) &&
> +	    (em->block_len > SZ_128K && next->block_len > SZ_128K))
> +		goto out;
> +	ret = true;
> +out:
>  	free_extent_map(next);
>  	return ret;
>  }
> -- 
> 2.34.1
>