Re: [PATCH -next v5 7/8] xfs: speed up truncating down a big realtime inode

[Christoph Hellwig <hch@infradead.org>]

On Thu, Jun 13, 2024 at 05:00:32PM +0800, Zhang Yi wrote:
> From: Zhang Yi <yi.zhang@huawei.com>
> 
> If we truncate down a big realtime inode, zero out the entire aligned
> EOF extent could gets slow down as the rtextsize increases. Fortunately,
> __xfs_bunmapi() would align the unmapped range to rtextsize, split and
> convert the blocks beyond EOF to unwritten. So speed up this by
> adjusting the unitsize to the filesystem blocksize when truncating down
> a large realtime inode, let __xfs_bunmapi() convert the tail blocks to
> unwritten, this could improve the performance significantly.
> 
>  # mkfs.xfs -f -rrtdev=/dev/pmem1s -f -m reflink=0,rmapbt=0, \
>             -d rtinherit=1 -r extsize=$rtextsize /dev/pmem2s
>  # mount -ortdev=/dev/pmem1s /dev/pmem2s /mnt/scratch
>  # for i in {1..1000}; \
>    do dd if=/dev/zero of=/mnt/scratch/$i bs=$rtextsize count=1024; done
>  # sync
>  # time for i in {1..1000}; \
>    do xfs_io -c "truncate 4k" /mnt/scratch/$i; done
> 
>  rtextsize       8k      16k      32k      64k     256k     1024k
>  before:       9.601s  10.229s  11.153s  12.086s  12.259s  20.141s
>  after:        9.710s   9.642s   9.958s   9.441s  10.021s  10.526s
> 
> Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
> ---
>  fs/xfs/xfs_inode.c | 10 ++++++++--
>  fs/xfs/xfs_iops.c  |  9 +++++++++
>  2 files changed, 17 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
> index 92daa2279053..5e837ed093b0 100644
> --- a/fs/xfs/xfs_inode.c
> +++ b/fs/xfs/xfs_inode.c
> @@ -1487,6 +1487,7 @@ xfs_itruncate_extents_flags(
>  	struct xfs_trans	*tp = *tpp;
>  	xfs_fileoff_t		first_unmap_block;
>  	int			error = 0;
> +	unsigned int		unitsize = xfs_inode_alloc_unitsize(ip);
>  
>  	xfs_assert_ilocked(ip, XFS_ILOCK_EXCL);
>  	if (atomic_read(&VFS_I(ip)->i_count))
> @@ -1510,9 +1511,14 @@ xfs_itruncate_extents_flags(
>  	 *
>  	 * We have to free all the blocks to the bmbt maximum offset, even if
>  	 * the page cache can't scale that far.
> +	 *
> +	 * For big realtime inode, don't aligned to allocation unitsize,
> +	 * it'll split the extent and convert the tail blocks to unwritten.
>  	 */
> +	if (xfs_inode_has_bigrtalloc(ip))
> +		unitsize = i_blocksize(VFS_I(ip));
> +	first_unmap_block = XFS_B_TO_FSB(mp, roundup_64(new_size, unitsize));

If you expand what xfs_inode_alloc_unitsize and xfs_inode_has_bigrtalloc
this is looking a bit silly:

	unsigned int            blocks = 1;

	if (XFS_IS_REALTIME_INODE(ip))
		blocks = ip->i_mount->m_sb.sb_rextsize;

	unitsize = XFS_FSB_TO_B(ip->i_mount, blocks);
	if (XFS_IS_REALTIME_INODE(ip) && ip->i_mount->m_sb.sb_rextsize > 1)
		unsitsize = i_blocksize(inode);

So I think we can simply drop this part now that the variant that zeroes
the entire rtextent is gone.

> @@ -862,6 +862,15 @@ xfs_setattr_truncate_data(
>  	/* Truncate down */
>  	blocksize = xfs_inode_alloc_unitsize(ip);
>  
> +	/*
> +	 * If it's a big realtime inode, zero out the entire EOF extent could
> +	 * get slow down as the rtextsize increases, speed it up by adjusting
> +	 * the blocksize to the filesystem blocksize, let __xfs_bunmapi() to
> +	 * split the extent and convert the tail blocks to unwritten.
> +	 */
> +	if (xfs_inode_has_bigrtalloc(ip))
> +		blocksize = i_blocksize(inode);

Same here.  And with that probably also the passing of the block size
to the truncate_page helpers.