linux-kernel - Re: [PATCH 4/6] fs: xfs: Support atomic write for statx

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Message-ID: <20240213173704.GB6184@frogsfrogsfrogs>
Date: Tue, 13 Feb 2024 09:37:04 -0800
From: "Darrick J. Wong" <djwong@...nel.org>
To: John Garry <john.g.garry@...cle.com>
Cc: hch@....de, viro@...iv.linux.org.uk, brauner@...nel.org,
	dchinner@...hat.com, jack@...e.cz, chandan.babu@...cle.com,
	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
Subject: Re: [PATCH 4/6] fs: xfs: Support atomic write for statx

On Mon, Feb 05, 2024 at 01:10:54PM +0000, John Garry wrote:
> On 02/02/2024 18:05, Darrick J. Wong wrote:
> > On Wed, Jan 24, 2024 at 02:26:43PM +0000, John Garry wrote:
> > > Support providing info on atomic write unit min and max for an inode.
> > > 
> > > For simplicity, currently we limit the min at the FS block size, but a
> > > lower limit could be supported in future.
> > > 
> > > The atomic write unit min and max is limited by the guaranteed extent
> > > alignment for the inode.
> > > 
> > > Signed-off-by: John Garry <john.g.garry@...cle.com>
> > > ---
> > >   fs/xfs/xfs_iops.c | 45 +++++++++++++++++++++++++++++++++++++++++++++
> > >   fs/xfs/xfs_iops.h |  4 ++++
> > >   2 files changed, 49 insertions(+)
> > > 
> > > diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
> > > index a0d77f5f512e..0890d2f70f4d 100644
> > > --- a/fs/xfs/xfs_iops.c
> > > +++ b/fs/xfs/xfs_iops.c
> > > @@ -546,6 +546,44 @@ xfs_stat_blksize(
> > >   	return PAGE_SIZE;
> > >   }
> > > +void xfs_get_atomic_write_attr(
> > 
> > static void?
> 
> We use this in the iomap and statx code
> 
> > 
> > > +	struct xfs_inode *ip,
> > > +	unsigned int *unit_min,
> > > +	unsigned int *unit_max)
> > 
> > Weird indenting here.
> 
> hmmm... I thought that this was the XFS style
> 
> Can you show how it should look?

The parameter declarations should line up with the local variables:

void
xfs_get_atomic_write_attr(
	struct xfs_inode	*ip,
	unsigned int		*unit_min,
	unsigned int		*unit_max)
{
	struct xfs_buftarg	*target = xfs_inode_buftarg(ip);
	struct block_device	*bdev = target->bt_bdev;
	struct request_queue	*q = bdev->bd_queue;
	struct xfs_mount	*mp = ip->i_mount;
	unsigned int		awu_min, awu_max, align;
	xfs_extlen_t		extsz = xfs_get_extsz(ip);

> > 
> > > +{
> > > +	xfs_extlen_t		extsz = xfs_get_extsz(ip);
> > > +	struct xfs_buftarg	*target = xfs_inode_buftarg(ip);
> > > +	struct block_device	*bdev = target->bt_bdev;
> > > +	unsigned int		awu_min, awu_max, align;
> > > +	struct request_queue	*q = bdev->bd_queue;
> > > +	struct xfs_mount	*mp = ip->i_mount;
> > > +
> > > +	/*
> > > +	 * Convert to multiples of the BLOCKSIZE (as we support a minimum
> > > +	 * atomic write unit of BLOCKSIZE).
> > > +	 */
> > > +	awu_min = queue_atomic_write_unit_min_bytes(q);
> > > +	awu_max = queue_atomic_write_unit_max_bytes(q);
> > > +
> > > +	awu_min &= ~mp->m_blockmask;
> > 
> > Why do you round /down/ the awu_min value here?
> 
> This is just to ensure that we returning *unit_min >= BLOCKSIZE
> 
> For example, if awu_min, max 1K, 64K from the bdev, we now have 0 and 64K.
> And below this gives us awu_min, max of 4k, 64k.
> 
> Maybe there is a more logical way of doing this.

	awu_min = roundup(queue_atomic_write_unit_min_bytes(q),
			  mp->m_sb.sb_blocksize);

?

> 
> > 
> > > +	awu_max &= ~mp->m_blockmask;
> > 
> > Actually -- since the atomic write units have to be powers of 2, why is
> > rounding needed here at all?
> 
> Sure, but the bdev can report a awu_min < BLOCKSIZE
> 
> > 
> > > +
> > > +	align = XFS_FSB_TO_B(mp, extsz);
> > > +
> > > +	if (!awu_max || !xfs_inode_atomicwrites(ip) || !align ||
> > > +	    !is_power_of_2(align)) {
> > 
> > ...and if you take my suggestion to make a common helper to validate the
> > atomic write unit parameters, this can collapse into:
> > 
> > 	alloc_unit_bytes = xfs_inode_alloc_unitsize(ip);
> > 	if (!xfs_inode_has_atomicwrites(ip) ||
> > 	    !bdev_validate_atomic_write(bdev, alloc_unit_bytes))  > 		/* not supported, return zeroes */
> > 		*unit_min = 0;
> > 		*unit_max = 0;
> > 		return;
> > 	}
> > 
> > 	*unit_min = max(alloc_unit_bytes, awu_min);
> > 	*unit_max = min(alloc_unit_bytes, awu_max);
> 
> Again, we need to ensure that *unit_min >= BLOCKSIZE

The file allocation unit and hence the return value of
xfs_inode_alloc_unitsize is always a multiple of sb_blocksize.

--D

> Thanks,
> John
> 
>