linux-kernel - Re: [PATCH 11/21] fs: xfs: Don't use low-space allocator for alignment

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Message-ID: <20231003030010.GE21298@frogsfrogsfrogs>
Date:   Mon, 2 Oct 2023 20:00:10 -0700
From:   "Darrick J. Wong" <djwong@...nel.org>
To:     Dave Chinner <david@...morbit.com>
Cc:     John Garry <john.g.garry@...cle.com>, axboe@...nel.dk,
        kbusch@...nel.org, hch@....de, sagi@...mberg.me,
        jejb@...ux.ibm.com, martin.petersen@...cle.com,
        viro@...iv.linux.org.uk, brauner@...nel.org,
        chandan.babu@...cle.com, dchinner@...hat.com,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-nvme@...ts.infradead.org, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, tytso@....edu, jbongio@...gle.com,
        linux-api@...r.kernel.org
Subject: Re: [PATCH 11/21] fs: xfs: Don't use low-space allocator for
 alignment > 1

On Tue, Oct 03, 2023 at 12:16:26PM +1100, Dave Chinner wrote:
> On Fri, Sep 29, 2023 at 10:27:16AM +0000, John Garry wrote:
> > The low-space allocator doesn't honour the alignment requirement, so don't
> > attempt to even use it (when we have an alignment requirement).
> > 
> > Signed-off-by: John Garry <john.g.garry@...cle.com>
> > ---
> >  fs/xfs/libxfs/xfs_bmap.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> > 
> > diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
> > index 30c931b38853..328134c22104 100644
> > --- a/fs/xfs/libxfs/xfs_bmap.c
> > +++ b/fs/xfs/libxfs/xfs_bmap.c
> > @@ -3569,6 +3569,10 @@ xfs_bmap_btalloc_low_space(
> >  {
> >  	int			error;
> >  
> > +	/* The allocator doesn't honour args->alignment */
> > +	if (args->alignment > 1)
> > +		return 0;
> > +
> 
> How does this happen?
> 
> The earlier failing aligned allocations will clear alignment before
> we get here....

I was thinking the predicate should be xfs_inode_force_align(ip) to save
me/us from thinking about all the other weird ways args->alignment could
end up 1.

	/* forced-alignment means we don't use low mode */
	if (xfs_inode_force_align(ip))
		return -ENOSPC;

--D

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