linux-kernel - Re: [PATCH v2 06/11] xfs: Reflink CoW-based atomic write support

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Message-ID: <20250225173750.GE6242@frogsfrogsfrogs>
Date: Tue, 25 Feb 2025 09:37:50 -0800
From: "Darrick J. Wong" <djwong@...nel.org>
To: John Garry <john.g.garry@...cle.com>
Cc: brauner@...nel.org, cem@...nel.org, dchinner@...hat.com, hch@....de,
	linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, ojaswin@...ux.ibm.com,
	ritesh.list@...il.com, martin.petersen@...cle.com, tytso@....edu,
	linux-ext4@...r.kernel.org
Subject: Re: [PATCH v2 06/11] xfs: Reflink CoW-based atomic write support

On Tue, Feb 25, 2025 at 10:58:56AM +0000, John Garry wrote:
> On 24/02/2025 20:32, Darrick J. Wong wrote:
> > On Thu, Feb 13, 2025 at 01:56:14PM +0000, John Garry wrote:
> > > For CoW-based atomic write support, always allocate a cow hole in
> > > xfs_reflink_allocate_cow() to write the new data.
> > > 
> > > The semantics is that if @atomic is set, we will be passed a CoW fork
> > > extent mapping for no error returned.
> > > 
> > > Signed-off-by: John Garry <john.g.garry@...cle.com>
> > > ---
> > >   fs/xfs/xfs_iomap.c   |  2 +-
> > >   fs/xfs/xfs_reflink.c | 12 +++++++-----
> > >   fs/xfs/xfs_reflink.h |  2 +-
> > >   3 files changed, 9 insertions(+), 7 deletions(-)
> > > 
> > > diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
> > > index d61460309a78..ab79f0080288 100644
> > > --- a/fs/xfs/xfs_iomap.c
> > > +++ b/fs/xfs/xfs_iomap.c
> > > @@ -865,7 +865,7 @@ xfs_direct_write_iomap_begin(
> > >   		/* may drop and re-acquire the ilock */
> > >   		error = xfs_reflink_allocate_cow(ip, &imap, &cmap, &shared,
> > >   				&lockmode,
> > > -				(flags & IOMAP_DIRECT) || IS_DAX(inode));
> > > +				(flags & IOMAP_DIRECT) || IS_DAX(inode), false);
> > 
> > Now I'm /really/ think it's time for some reflink allocation flags,
> > because the function signature now involves two booleans...
> 
> ok, but the @convert_now arg is passed to other functions from
> xfs_reflink_allocate_cow() - so would you prefer to create a bool
> @convert_now inside xfs_reflink_allocate_cow() and pass that bool as before?
> Or pass the flags all the way down to end users of @convert_now?
> 
> > 
> > >   		if (error)
> > >   			goto out_unlock;
> > >   		if (shared)
> > > diff --git a/fs/xfs/xfs_reflink.c b/fs/xfs/xfs_reflink.c
> > > index 8428f7b26ee6..3dab3ba900a3 100644
> > > --- a/fs/xfs/xfs_reflink.c
> > > +++ b/fs/xfs/xfs_reflink.c
> > > @@ -435,7 +435,8 @@ xfs_reflink_fill_cow_hole(
> > >   	struct xfs_bmbt_irec	*cmap,
> > >   	bool			*shared,
> > >   	uint			*lockmode,
> > > -	bool			convert_now)
> > > +	bool			convert_now,
> > > +	bool			atomic)
> > 
> > ...but this can come later.
> 
> Do you mean that this would just be a new flag to set?

Sorry, I meant that the double booleans -> flags conversion could be a
cleanup patch at the end of the series.  But first we'd have to figure
out where we want the flags boundaries to be -- do we just pass the
IOMAP_{DIRECT,DAX,ATOMIC_*} flags directly to the reflink code and let
it figure out what to do?  Or do we make the xfs_iomap.c code translate
that into XFS_REFLINK_ALLOC_* flags?

Either way, that is not something that needs to be done in this patch.

> > Also, is atomic==true only for the> ATOMIC_SW operation?
> 
> Right, so I think that the variable (or new flag) can be renamed for that.
> 
> > I think so, but that's the unfortunate thing about
> > booleans.
> > 
> > >   {
> > >   	struct xfs_mount	*mp = ip->i_mount;
> > >   	struct xfs_trans	*tp;
> > > @@ -466,7 +467,7 @@ xfs_reflink_fill_cow_hole(
> > >   	*lockmode = XFS_ILOCK_EXCL;
> > >   	error = xfs_find_trim_cow_extent(ip, imap, cmap, shared, &found);
> > > -	if (error || !*shared)
> > > +	if (error || (!*shared && !atomic))
> > >   		goto out_trans_cancel;
> > >   	if (found) {
> > > @@ -566,7 +567,8 @@ xfs_reflink_allocate_cow(
> > >   	struct xfs_bmbt_irec	*cmap,
> > >   	bool			*shared,
> > >   	uint			*lockmode,
> > > -	bool			convert_now)
> > > +	bool			convert_now,
> > > +	bool 			atomic)
> > 
> > Nit:        ^ space before tab.
> 
> ok
> 
> > 
> > If the answer to the question above is 'yes' then with that nit fixed,
> > Reviewed-by: "Darrick J. Wong" <djwong@...nel.org>
> 
> Thanks, but I will wait for your feedback to decide whether to pick that up.
> 
> John
>