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Date:   Tue, 24 Oct 2023 18:16:59 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     "Darrick J. Wong" <djwong@kernel.org>
Cc:     Christoph Hellwig <hch@lst.de>, Jens Axboe <axboe@kernel.dk>,
        Matthew Wilcox <willy@infradead.org>,
        Ilya Dryomov <idryomov@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-xfs@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH 3/3] xfs: respect the stable writes flag on the RT
 device
Message-ID: <20231024161659.GB20546@lst.de>
References: <20231024064416.897956-1-hch@lst.de> <20231024064416.897956-4-hch@lst.de> <20231024150904.GA3195650@frogsfrogsfrogs>
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On Tue, Oct 24, 2023 at 08:09:04AM -0700, Darrick J. Wong wrote:
> > +	/*
> > +	 * Make the stable writes flag match that of the device the inode
> > +	 * resides on when flipping the RT flag.
> > +	 */
> > +	if (S_ISREG(VFS_I(ip)->i_mode) &&
> > +	    XFS_IS_REALTIME_INODE(ip) != (fa->fsx_xflags & FS_XFLAG_REALTIME))
> > +		xfs_update_stable_writes(ip);
> 
> Hmm.  Won't the masking operation here result in the if test comparing 0
> or FS_XFLAG_REALTIME to 0 or 1?
> 
> Oh.  FS_XFLAG_REALTIME == 1, so that's not an issue in this one case.
> That's a bit subtle though, I'd have preferred
> 
> 	    XFS_IS_REALTIME_INODE(ip) != !!(fa->fsx_xflags & FS_XFLAG_REALTIME))
> 
> to make it more obvious that the if test isn't comparing apples to
> oranges.

This is all copy and pasted from a check a few lines above :)

I guess I could clean it up as well or even add a rt_flag_changed local
variable instead of duplicating the check.

> > +	/*
> > +	 * For real-time inodes update the stable write flags to that of the RT
> > +	 * device instead of the data device.
> > +	 */
> > +	if (S_ISREG(inode->i_mode) && XFS_IS_REALTIME_INODE(ip))
> > +		xfs_update_stable_writes(ip);
> 
> I wonder if xfs_update_stable_writes should become an empty function for
> the CONFIG_XFS_RT=n case, to avoid the atomic flags update?
> 
> (The extra code is probably not worth the microoptimization.)

The compiler already eliminates the code because XFS_IS_REALTIME_INODE(ip)
has a stub for CONFIG_XFS_RT=n that always returns 0.