Re: [PATCH 1/2] writeback: avoid double-writing the inode on a lazytime expiration

From: "Darrick J. Wong" <darrick.wong@oracle.com>
To: "Theodore Y. Ts'o" <tytso@mit.edu>
Cc: Christoph Hellwig <hch@infradead.org>,
	Ext4 Developers List <linux-ext4@vger.kernel.org>,
	linux-f2fs-devel@lists.sourceforge.net,
	linux-xfs@vger.kernel.org, Eric Biggers <ebiggers@kernel.org>,
	Richard Weinberger <richard@nod.at>
Subject: Re: [PATCH 1/2] writeback: avoid double-writing the inode on a lazytime expiration
Date: Wed, 25 Mar 2020 08:47:59 -0700	[thread overview]
Message-ID: <20200325154759.GY29339@magnolia> (raw)
In-Reply-To: <20200325152113.GK53396@mit.edu>

On Wed, Mar 25, 2020 at 11:21:13AM -0400, Theodore Y. Ts'o wrote:
> On Wed, Mar 25, 2020 at 02:20:57AM -0700, Christoph Hellwig wrote:
> > >  	spin_unlock(&inode->i_lock);
> > >  
> > > -	if (dirty & I_DIRTY_TIME)
> > > -		mark_inode_dirty_sync(inode);
> > > +	/* This was a lazytime expiration; we need to tell the file system */
> > > +	if (dirty & I_DIRTY_TIME_EXPIRED && inode->i_sb->s_op->dirty_inode)
> > > +		inode->i_sb->s_op->dirty_inode(inode, I_DIRTY_SYNC);
> > 
> > I think this needs a very clear comment explaining why we don't go
> > through __mark_inode_dirty.
> 
> I can take the explanation which is in the git commit description and
> move it into the comment.
> 
> > But as said before I'd rather have a new lazytime_expired operation that
> > makes it very clear what is happening.  We currenly have 4 file systems
> > (ext4, f2fs, ubifs and xfs) that support lazytime, so this won't really
> > be a major churn.
> 
> Again, I believe patch #2 does what you want; if it doesn't can you
> explain why passing I_DIRTY_TIME_EXPIRED to s_op->dirty_inode() isn't
> "a new lazytime expired operation that makes very clear what is
> happening"?
> 
> I separated out patch #1 and patch #2 because patch #1 preserves
> current behavior, and patch #2 modifies XFS code, which I don't want
> to push Linus without an XFS reviewed-by.
> 
> N.b.  None of the other file systems required a change for patch #2,
> so if you want, we can have the XFS tree carry patch #2, and/or
> combine that with whatever other simplifying changes that you want.
> Or I can combine patch #1 and patch #2, with an XFS Reviewed-by, and
> send it through the ext4 tree.
> 
> What's your pleasure?

TBH while I'm pretty sure this does actually maintain more or less the
same behavior on xfs, I prefer Christoph's explicit ->lazytime_expired
approach[1] over squinting at bitflag manipulations.

(It also took me a while to realize that this patch duo even existed, as
it was kinda buried in its parent thread...)

--D

[1] https://lore.kernel.org/linux-fsdevel/20200325122825.1086872-1-hch@lst.de/T/#t

> 
> 					- Ted
> 