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

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Message-ID: <ZRuZymVoFI7EUjbS@dread.disaster.area>
Date:   Tue, 3 Oct 2023 15:34:18 +1100
From:   Dave Chinner <david@...morbit.com>
To:     "Darrick J. Wong" <djwong@...nel.org>
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 Mon, Oct 02, 2023 at 08:00:10PM -0700, Darrick J. Wong wrote:
> 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;

See the email I just wrote about not needing per-inode on-disk state
or even extent size hints for doing allocation for atomic IO. Atomic
write unit alignment is a device parameter (similar to stripe unit)
that applies to context specific allocation requests - it's not an
inode property as such....

Cheers,

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