linux-kernel - Re: [PATCH v2 06/14] fs: xfs: Do not free EOF blocks for forcealign

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Message-ID: <ZejbFuavNva4ut/3@dread.disaster.area>
Date: Thu, 7 Mar 2024 08:07:34 +1100
From: Dave Chinner <david@...morbit.com>
To: John Garry <john.g.garry@...cle.com>
Cc: djwong@...nel.org, hch@....de, viro@...iv.linux.org.uk,
	brauner@...nel.org, jack@...e.cz, chandan.babu@...cle.com,
	axboe@...nel.dk, martin.petersen@...cle.com,
	linux-kernel@...r.kernel.org, linux-xfs@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, tytso@....edu, jbongio@...gle.com,
	ojaswin@...ux.ibm.com, ritesh.list@...il.com,
	linux-block@...r.kernel.org
Subject: Re: [PATCH v2 06/14] fs: xfs: Do not free EOF blocks for forcealign

On Mon, Mar 04, 2024 at 01:04:20PM +0000, John Garry wrote:
> For when forcealign is enabled, we want the EOF to be aligned as well, so
> do not free EOF blocks.
> 
> Add helper function xfs_get_extsz() to get the guaranteed extent size
> alignment for forcealign enabled. Since this code is only relevant to
> forcealign and forcealign is not possible for RT yet, ignore RT.
> 
> Signed-off-by: John Garry <john.g.garry@...cle.com>
> ---
>  fs/xfs/xfs_bmap_util.c |  7 ++++++-
>  fs/xfs/xfs_inode.c     | 14 ++++++++++++++
>  fs/xfs/xfs_inode.h     |  1 +
>  3 files changed, 21 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/xfs/xfs_bmap_util.c b/fs/xfs/xfs_bmap_util.c
> index c2531c28905c..07bfb03c671a 100644
> --- a/fs/xfs/xfs_bmap_util.c
> +++ b/fs/xfs/xfs_bmap_util.c
> @@ -542,8 +542,13 @@ xfs_can_free_eofblocks(
>  	 * forever.
>  	 */
>  	end_fsb = XFS_B_TO_FSB(mp, (xfs_ufsize_t)XFS_ISIZE(ip));
> -	if (XFS_IS_REALTIME_INODE(ip) && mp->m_sb.sb_rextsize > 1)
> +
> +	/* Do not free blocks when forcing extent sizes */

That comment seems wrong - this just rounds up where we start
trimming from to be aligned...

> +	if (xfs_get_extsz(ip) > 1)
> +		end_fsb = roundup_64(end_fsb, xfs_get_extsz(ip));
> +	else if (XFS_IS_REALTIME_INODE(ip) && mp->m_sb.sb_rextsize > 1)
>  		end_fsb = xfs_rtb_roundup_rtx(mp, end_fsb);

I think this would be better written as:

	/*
	 * Forced extent alignment requires us to round up where we
	 * start trimming from so that result is correctly aligned.
	 */
	if (xfs_inode_forcealign(ip)) {
		if (ip->i_extsize > 1)
			end_fsb = roundup_64(end_fsb, ip->i_extsize);
		else if (XFS_IS_REALTIME_INODE(ip) && mp->m_sb.sb_rextsize > 1)
			end_fsb = xfs_rtb_roundup_rtx(mp, end_fsb);
	}

Because we only want end-fsb alignment when forced alignment is
required.

Which then makes me wonder: truncate needs this, too, doesn't it?
And the various fallocate() ops like hole punching and extent
shifting?

> diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
> index 2c439df8c47f..bbb8886f1d32 100644
> --- a/fs/xfs/xfs_inode.c
> +++ b/fs/xfs/xfs_inode.c
> @@ -65,6 +65,20 @@ xfs_get_extsz_hint(
>  	return 0;
>  }
>  
> +/*
> + * Helper function to extract extent size. It will return a power-of-2,
> + * as forcealign requires this.
> + */
> +xfs_extlen_t
> +xfs_get_extsz(
> +	struct xfs_inode	*ip)
> +{
> +	if (xfs_inode_forcealign(ip) && ip->i_extsize)
> +		return ip->i_extsize;
> +
> +	return 1;
> +}

This can go away - if it is needed elsewhere, then I think it would
be better open coded because it better documents what the code is
doing...

-Dave.

-- 
Dave Chinner
david@...morbit.com