linux-kernel - Re: [PATCH RFC v3 12/21] xfs: Only free full extents for forcealign

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Date: Wed, 1 May 2024 16:53:10 -0700
From: "Darrick J. Wong" <djwong@...nel.org>
To: Dave Chinner <david@...morbit.com>
Cc: John Garry <john.g.garry@...cle.com>, hch@....de,
	viro@...iv.linux.org.uk, brauner@...nel.org, jack@...e.cz,
	chandan.babu@...cle.com, willy@...radead.org, axboe@...nel.dk,
	martin.petersen@...cle.com, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, tytso@....edu, jbongio@...gle.com,
	ojaswin@...ux.ibm.com, ritesh.list@...il.com, mcgrof@...nel.org,
	p.raghav@...sung.com, linux-xfs@...r.kernel.org,
	catherine.hoang@...cle.com
Subject: Re: [PATCH RFC v3 12/21] xfs: Only free full extents for forcealign

On Wed, May 01, 2024 at 10:53:28AM +1000, Dave Chinner wrote:
> On Mon, Apr 29, 2024 at 05:47:37PM +0000, John Garry wrote:
> > Like we already do for rtvol, only free full extents for forcealign in
> > xfs_free_file_space().
> > 
> > Signed-off-by: John Garry <john.g.garry@...cle.com>
> > ---
> >  fs/xfs/xfs_bmap_util.c | 7 +++++--
> >  1 file changed, 5 insertions(+), 2 deletions(-)
> > 
> > diff --git a/fs/xfs/xfs_bmap_util.c b/fs/xfs/xfs_bmap_util.c
> > index f26d1570b9bd..1dd45dfb2811 100644
> > --- a/fs/xfs/xfs_bmap_util.c
> > +++ b/fs/xfs/xfs_bmap_util.c
> > @@ -847,8 +847,11 @@ xfs_free_file_space(
> >  	startoffset_fsb = XFS_B_TO_FSB(mp, offset);
> >  	endoffset_fsb = XFS_B_TO_FSBT(mp, offset + len);
> >  
> > -	/* We can only free complete realtime extents. */
> > -	if (XFS_IS_REALTIME_INODE(ip) && mp->m_sb.sb_rextsize > 1) {
> > +	/* Free only complete extents. */
> > +	if (xfs_inode_has_forcealign(ip) && ip->i_extsize > 1) {
> > +		startoffset_fsb = roundup_64(startoffset_fsb, ip->i_extsize);
> > +		endoffset_fsb = rounddown_64(endoffset_fsb, ip->i_extsize);
> > +	} else if (XFS_IS_REALTIME_INODE(ip) && mp->m_sb.sb_rextsize > 1) {
> >  		startoffset_fsb = xfs_rtb_roundup_rtx(mp, startoffset_fsb);
> >  		endoffset_fsb = xfs_rtb_rounddown_rtx(mp, endoffset_fsb);
> >  	}
> 
> When you look at xfs_rtb_roundup_rtx() you'll find it's just a one
> line wrapper around roundup_64().

I added this a couple of cycles ago to get ready for realtime
modernization.  That will create a bunch *more* churn in my tree just to
convert everything *back*.

Where the hell were you when that was being reviewed?!!!

NO!  This is pointless busywork!

--D

> So lets get rid of the obfuscation that the one line RT wrapper
> introduces, and it turns into this:
> 
> 	rounding = 1;
> 	if (xfs_inode_has_forcealign(ip)
> 		rounding = ip->i_extsize;
> 	else if (XFS_IS_REALTIME_INODE(ip))
> 		rounding = mp->m_sb.sb_rextsize;
> 
> 	if (rounding > 1) {
> 		startoffset_fsb = roundup_64(startoffset_fsb, rounding);
> 		endoffset_fsb = rounddown_64(endoffset_fsb, rounding);
> 	}
> 
> What this points out is that the prep steps for fallocate operations
> also need to handle both forced alignment and rtextsize rounding,
> and it does neither right now.  xfs_flush_unmap_range() is the main
> offender here, but xfs_prepare_shift() also needs fixing.
> 
> Hence:
> 
> static inline xfs_extlen_t
> xfs_extent_alignment(
> 	struct xfs_inode	*ip)
> {
> 	if (xfs_inode_has_forcealign(ip))
> 		return ip->i_extsize;
> 	if (XFS_IS_REALTIME_INODE(ip))
> 		return mp->m_sb.sb_rextsize;
> 	return 1;
> }
> 
> 
> In xfs_flush_unmap_range():
> 
> 	/*
> 	 * Make sure we extend the flush out to extent alignment
> 	 * boundaries so any extent range overlapping the start/end
> 	 * of the modification we are about to do is clean and idle.
> 	 */
> 	rounding = XFS_FSB_TO_B(mp, xfs_extent_alignment(ip));
> 	rounding = max(rounding, PAGE_SIZE);
> 	...
> 
> in xfs_free_file_space()
> 
> 	/*
> 	 * Round the range we are going to free inwards to extent
> 	 * alignment boundaries so we don't free blocks outside the
> 	 * range requested.
> 	 */
> 	rounding = xfs_extent_alignment(ip);
> 	if (rounding > 1 ) {
> 		startoffset_fsb = roundup_64(startoffset_fsb, rounding);
> 		endoffset_fsb = rounddown_64(endoffset_fsb, rounding);
> 	}
> 
> and in xfs_prepare_shift()
> 
> 	/*
> 	 * Shift operations must stabilize the start block offset boundary along
> 	 * with the full range of the operation. If we don't, a COW writeback
> 	 * completion could race with an insert, front merge with the start
> 	 * extent (after split) during the shift and corrupt the file. Start
> 	 * with the aligned block just prior to the start to stabilize the boundary.
> 	 */
> 	rounding = XFS_FSB_TO_B(mp, xfs_extent_alignment(ip));
> 	offset = round_down(offset, rounding);
> 	if (offset)
> 		offset -= rounding;
> 
> Also, I think that the changes I suggested earlier to 
> xfs_is_falloc_aligned() could use this xfs_extent_alignment()
> helper...
> 
> Overall this makes the code a whole lot easier to read and it also
> allows forced alignment to work correctly on RT devices...
> 
> -Dave.
> -- 
> Dave Chinner
> david@...morbit.com
>