Message-ID: <20240213172205.GA6184@frogsfrogsfrogs>
Date: Tue, 13 Feb 2024 09:22:05 -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 3/6] fs: xfs: Support FS_XFLAG_ATOMICWRITES for rtvol

On Mon, Feb 05, 2024 at 12:51:07PM +0000, John Garry wrote:
> On 02/02/2024 17:52, Darrick J. Wong wrote:
> > On Wed, Jan 24, 2024 at 02:26:42PM +0000, John Garry wrote:
> > > Add initial support for FS_XFLAG_ATOMICWRITES in rtvol.
> > > 
> > > Current kernel support for atomic writes is based on HW support (for atomic
> > > writes). As such, it is required to ensure extent alignment with
> > > atomic_write_unit_max so that an atomic write can result in a single
> > > HW-compliant IO operation.
> > > 
> > > rtvol already guarantees extent alignment, so initially add support there.
> > > 
> > > Signed-off-by: John Garry <john.g.garry@...cle.com>
> > > ---
> > >   fs/xfs/libxfs/xfs_format.h |  8 ++++++--
> > >   fs/xfs/libxfs/xfs_sb.c     |  2 ++
> > >   fs/xfs/xfs_inode.c         | 22 ++++++++++++++++++++++
> > >   fs/xfs/xfs_inode.h         |  7 +++++++
> > >   fs/xfs/xfs_ioctl.c         | 19 +++++++++++++++++--
> > >   fs/xfs/xfs_mount.h         |  2 ++
> > >   fs/xfs/xfs_super.c         |  4 ++++
> > >   7 files changed, 60 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/fs/xfs/libxfs/xfs_format.h b/fs/xfs/libxfs/xfs_format.h
> > > index 382ab1e71c0b..79fb0d4adeda 100644
> > > --- a/fs/xfs/libxfs/xfs_format.h
> > > +++ b/fs/xfs/libxfs/xfs_format.h
> > > @@ -353,11 +353,13 @@ xfs_sb_has_compat_feature(
> > >   #define XFS_SB_FEAT_RO_COMPAT_RMAPBT   (1 << 1)		/* reverse map btree */
> > >   #define XFS_SB_FEAT_RO_COMPAT_REFLINK  (1 << 2)		/* reflinked files */
> > >   #define XFS_SB_FEAT_RO_COMPAT_INOBTCNT (1 << 3)		/* inobt block counts */
> > > +#define XFS_SB_FEAT_RO_COMPAT_ATOMICWRITES (1 << 29)	/* aligned file data extents */
> > 
> > I thought FORCEALIGN was going to signal aligned file data extent
> > allocations being mandatory?
> 
> Right, I'll fix that comment
> 
> > 
> > This flag (AFAICT) simply marks the inode as something that gets
> > FMODE_CAN_ATOMIC_WRITES, right?
> 
> Correct
> 
> > 
> > >   #define XFS_SB_FEAT_RO_COMPAT_ALL \
> > >   		(XFS_SB_FEAT_RO_COMPAT_FINOBT | \
> > >   		 XFS_SB_FEAT_RO_COMPAT_RMAPBT | \
> > >   		 XFS_SB_FEAT_RO_COMPAT_REFLINK| \
> > > -		 XFS_SB_FEAT_RO_COMPAT_INOBTCNT)
> > > +		 XFS_SB_FEAT_RO_COMPAT_INOBTCNT | \
> > > +		 XFS_SB_FEAT_RO_COMPAT_ATOMICWRITES)
> > >   #define XFS_SB_FEAT_RO_COMPAT_UNKNOWN	~XFS_SB_FEAT_RO_COMPAT_ALL
> > >   static inline bool
> > >   xfs_sb_has_ro_compat_feature(
> > > @@ -1085,16 +1087,18 @@ static inline void xfs_dinode_put_rdev(struct xfs_dinode *dip, xfs_dev_t rdev)
> > >   #define XFS_DIFLAG2_COWEXTSIZE_BIT   2  /* copy on write extent size hint */
> > >   #define XFS_DIFLAG2_BIGTIME_BIT	3	/* big timestamps */
> > >   #define XFS_DIFLAG2_NREXT64_BIT 4	/* large extent counters */
> > > +#define XFS_DIFLAG2_ATOMICWRITES_BIT 6
> > 
> > Needs a comment here ("files flagged for atomic writes").
> 
> ok
> 
> > Also not sure
> > why you skipped bit 5, though I'm guessing it's because the forcealign
> > series is/was using it?
> 
> Right, I'll fix that
> 
> > 
> > >   #define XFS_DIFLAG2_DAX		(1 << XFS_DIFLAG2_DAX_BIT)
> > >   #define XFS_DIFLAG2_REFLINK     (1 << XFS_DIFLAG2_REFLINK_BIT)
> > >   #define XFS_DIFLAG2_COWEXTSIZE  (1 << XFS_DIFLAG2_COWEXTSIZE_BIT)
> > >   #define XFS_DIFLAG2_BIGTIME	(1 << XFS_DIFLAG2_BIGTIME_BIT)
> > >   #define XFS_DIFLAG2_NREXT64	(1 << XFS_DIFLAG2_NREXT64_BIT)
> > > +#define XFS_DIFLAG2_ATOMICWRITES	(1 << XFS_DIFLAG2_ATOMICWRITES_BIT)
> > >   #define XFS_DIFLAG2_ANY \
> > >   	(XFS_DIFLAG2_DAX | XFS_DIFLAG2_REFLINK | XFS_DIFLAG2_COWEXTSIZE | \
> > > -	 XFS_DIFLAG2_BIGTIME | XFS_DIFLAG2_NREXT64)
> > > +	 XFS_DIFLAG2_BIGTIME | XFS_DIFLAG2_NREXT64 | XFS_DIFLAG2_ATOMICWRITES)
> > >   static inline bool xfs_dinode_has_bigtime(const struct xfs_dinode *dip)
> > >   {
> > > diff --git a/fs/xfs/libxfs/xfs_sb.c b/fs/xfs/libxfs/xfs_sb.c
> > > index 4a9e8588f4c9..28a98130a56d 100644
> > > --- a/fs/xfs/libxfs/xfs_sb.c
> > > +++ b/fs/xfs/libxfs/xfs_sb.c
> > > @@ -163,6 +163,8 @@ xfs_sb_version_to_features(
> > >   		features |= XFS_FEAT_REFLINK;
> > >   	if (sbp->sb_features_ro_compat & XFS_SB_FEAT_RO_COMPAT_INOBTCNT)
> > >   		features |= XFS_FEAT_INOBTCNT;
> > > +	if (sbp->sb_features_ro_compat & XFS_SB_FEAT_RO_COMPAT_ATOMICWRITES)
> > > +		features |= XFS_FEAT_ATOMICWRITES;
> > >   	if (sbp->sb_features_incompat & XFS_SB_FEAT_INCOMPAT_FTYPE)
> > >   		features |= XFS_FEAT_FTYPE;
> > >   	if (sbp->sb_features_incompat & XFS_SB_FEAT_INCOMPAT_SPINODES)
> > > diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
> > > index 1fd94958aa97..0b0f525fd043 100644
> > > --- a/fs/xfs/xfs_inode.c
> > > +++ b/fs/xfs/xfs_inode.c
> > > @@ -65,6 +65,26 @@ xfs_get_extsz_hint(
> > >   	return 0;
> > >   }
> > > +/*
> > > + * helper function to extract extent size
> > 
> > How does that differ from xfs_get_extsz_hint?
> 
> The idea of this function is to return the guaranteed extent alignment, and
> not just the hint
> 
> > 
> > > + */
> > > +xfs_extlen_t
> > > +xfs_get_extsz(
> > > +	struct xfs_inode	*ip)
> > > +{
> > > +	/*
> > > +	 * No point in aligning allocations if we need to COW to actually
> > > +	 * write to them.
> > 
> > What does alwayscow have to do with untorn writes?
> 
> Nothing at the moment, so I'll remove this.
> 
> > 
> > > +	 */
> > > +	if (xfs_is_always_cow_inode(ip))
> > > +		return 0;
> > > +
> > > +	if (XFS_IS_REALTIME_INODE(ip))
> > > +		return ip->i_mount->m_sb.sb_rextsize;
> > > +
> > > +	return 1;
> > > +}
> > 
> > Does this function exist to return the allocation unit for a given file?
> > https://git.kernel.org/pub/scm/linux/kernel/git/djwong/xfs-linux.git/commit/?h=djwong-wtf&id=b8ddcef3df8da02ed2c4aacbed1d811e60372006
> > 
> 
> Yes, something like xfs_inode_alloc_unitsize() there.
> 
> What's the upstream status for that change? I see it mentioned in linux-xfs
> lore and seems to be part of a mega patchset.

It's stuck in review along with the other ~1400 patches that I've been
grumbling about in our staff meetings for years now.

> > > +
> > >   /*
> > >    * Helper function to extract CoW extent size hint from inode.
> > >    * Between the extent size hint and the CoW extent size hint, we
> > > @@ -629,6 +649,8 @@ xfs_ip2xflags(
> > >   			flags |= FS_XFLAG_DAX;
> > >   		if (ip->i_diflags2 & XFS_DIFLAG2_COWEXTSIZE)
> > >   			flags |= FS_XFLAG_COWEXTSIZE;
> > > +		if (ip->i_diflags2 & XFS_DIFLAG2_ATOMICWRITES)
> > > +			flags |= FS_XFLAG_ATOMICWRITES;
> > >   	}
> > >   	if (xfs_inode_has_attr_fork(ip))
> > > diff --git a/fs/xfs/xfs_inode.h b/fs/xfs/xfs_inode.h
> > > index 97f63bacd4c2..0e0a21d9d30f 100644
> > > --- a/fs/xfs/xfs_inode.h
> > > +++ b/fs/xfs/xfs_inode.h
> > > @@ -305,6 +305,11 @@ static inline bool xfs_inode_has_large_extent_counts(struct xfs_inode *ip)
> > >   	return ip->i_diflags2 & XFS_DIFLAG2_NREXT64;
> > >   }
> > > +static inline bool xfs_inode_atomicwrites(struct xfs_inode *ip)
> > 
> > I think this predicate wants a verb in its name, the rest of them have
> > "is" or "has" somewhere:
> > 
> > "xfs_inode_has_atomicwrites"
> 
> ok, fine.
> 
> Note that I was copying xfs_inode_forcealign() in terms of naming.

Yeah, I could rename that xfs_inode_forces_alignment() or something.

Or just leave the condensed version where the verb and object are
smashed together.

xfs_inode_has_forcealign?

Yeah.  I'll go with that.

> > 
> > > +{
> > > +	return ip->i_diflags2 & XFS_DIFLAG2_ATOMICWRITES;
> > > +}
> > > +
> > >   /*
> > >    * Return the buftarg used for data allocations on a given inode.
> > >    */
> > > @@ -542,7 +547,9 @@ void		xfs_lock_two_inodes(struct xfs_inode *ip0, uint ip0_mode,
> > >   				struct xfs_inode *ip1, uint ip1_mode);
> > >   xfs_extlen_t	xfs_get_extsz_hint(struct xfs_inode *ip);
> > > +xfs_extlen_t	xfs_get_extsz(struct xfs_inode *ip);
> > >   xfs_extlen_t	xfs_get_cowextsz_hint(struct xfs_inode *ip);
> > > +xfs_extlen_t	xfs_get_atomicwrites_size(struct xfs_inode *ip);
> > >   int xfs_init_new_inode(struct mnt_idmap *idmap, struct xfs_trans *tp,
> > >   		struct xfs_inode *pip, xfs_ino_t ino, umode_t mode,
> > > diff --git a/fs/xfs/xfs_ioctl.c b/fs/xfs/xfs_ioctl.c
> > > index f02b6e558af5..c380a3055be7 100644
> > > --- a/fs/xfs/xfs_ioctl.c
> > > +++ b/fs/xfs/xfs_ioctl.c
> > > @@ -1110,6 +1110,8 @@ xfs_flags2diflags2(
> > >   		di_flags2 |= XFS_DIFLAG2_DAX;
> > >   	if (xflags & FS_XFLAG_COWEXTSIZE)
> > >   		di_flags2 |= XFS_DIFLAG2_COWEXTSIZE;
> > > +	if (xflags & FS_XFLAG_ATOMICWRITES)
> > > +		di_flags2 |= XFS_DIFLAG2_ATOMICWRITES;
> > >   	return di_flags2;
> > >   }
> > > @@ -1122,10 +1124,12 @@ xfs_ioctl_setattr_xflags(
> > >   {
> > >   	struct xfs_mount	*mp = ip->i_mount;
> > >   	bool			rtflag = (fa->fsx_xflags & FS_XFLAG_REALTIME);
> > > +	bool			atomic_writes = fa->fsx_xflags & FS_XFLAG_ATOMICWRITES;
> > >   	uint64_t		i_flags2;
> > > -	if (rtflag != XFS_IS_REALTIME_INODE(ip)) {
> > > -		/* Can't change realtime flag if any extents are allocated. */
> > 
> > Please augment this comment ("Can't change realtime or atomicwrites
> > flags if any extents are allocated") instead of deleting it.
> 
> I wasn't supposed to delete that - will remedy.
> 
> >  This is
> > validation code, the requirements should be spelled out in English.
> > 
> > > +
> > > +	if (rtflag != XFS_IS_REALTIME_INODE(ip) ||
> > > +	    atomic_writes != xfs_inode_atomicwrites(ip)) {
> > >   		if (ip->i_df.if_nextents || ip->i_delayed_blks)
> > >   			return -EINVAL;
> > >   	}
> > > @@ -1146,6 +1150,17 @@ xfs_ioctl_setattr_xflags(
> > >   	if (i_flags2 && !xfs_has_v3inodes(mp))
> > >   		return -EINVAL;
> > > +	if (atomic_writes) {
> > > +		if (!xfs_has_atomicwrites(mp))
> > > +			return -EINVAL;
> > > +
> > > +		if (!rtflag)
> > > +			return -EINVAL;
> > > +
> > > +		if (!is_power_of_2(mp->m_sb.sb_rextsize))
> > > +			return -EINVAL;
> > 
> > Shouldn't we check sb_rextsize w.r.t. the actual block device queue
> > limits here?  I keep seeing similar validation logic open-coded
> > throughout both atomic write patchsets:
> > 
> > 	if (l < queue_atomic_write_unit_min_bytes())
> > 		/* fail */
> > 	if (l > queue_atomic_write_unit_max_bytes())
> > 		/* fail */
> > 	if (!is_power_of_2(l))
> > 		/* fail */
> > 	/* ok */
> > 
> > which really should be a common helper somewhere.
> 
> I think that it is a reasonable comment about duplication the atomic writes
> checks for the bdev and iomap write paths - I can try to improve that.
> 
> But the is_power_of_2(mp->m_sb.sb_rextsize) check is to ensure that the
> extent size is suitable for enabling atomic writes. I don't see a point in
> checking the bdev queue limits here.

Ok, skip the queue limits then.

> > 
> > 		/*
> > 		 * Don't set atomic write if the allocation unit doesn't
> > 		 * align with the device requirements.
> > 		 */
> > 		if (!bdev_validate_atomic_write(<target blockdev>,
> > 				XFS_FSB_TO_B(mp, mp->m_sb.sb_rextsize))
> > 			return -EINVAL;
> > 
> > Too bad we have to figure out the target blockdev and file allocation
> > unit based on the ioctl in-params and can't use the xfs_inode helpers
> > here.
> 
> I am not sure what bdev_validate_atomic_write() would even do. If
> sb_rextsize exceeded the bdev atomic write unit max, then we just cap
> reported atomic write unit max in statx to that which the bdev reports and
> vice-versa.
> 
> And didn't we previously have a concern that it is possible to change the
> geometry of the device?

The thing is, I don't want this logic:

	if (!is_power_of_2(mp->m_sb.sb_rextsize))
		/* fail */

to be open-coded inside xfs.  I'd rather have a standard bdev_* helper
that every filesystem can call, so we don't end up with more generic
code copy-pasted all over the codebase.

The awkward part (for me) is the naming, since filesystems usually don't
have to check with the block layer about their units of space allocation.

/*
 * Ensure that a file space allocation unit is congruent with the atomic
 * write unit capabilities of supported block devices.
 */
static inline bool bdev_validate_atomic_write_allocunit(unsigned au)
{
	return is_power_of_2(au);
}

	if (!bdev_validate_atomic_write_allocunit(mp->m-sb.sb_rextsize))
		return -EINVAL;

> If so, not much point in this check.

Yes, that is a disadvantage of me reading patchsets in reverse order. ;)

--D

> Thanks,
> John
> 
> 

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