linux-ext4 - Re: [PATCH 1/2] writeback: avoid double-writing the inode on a lazytime expiration

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Date:   Wed, 25 Mar 2020 11:21:13 -0400
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Ext4 Developers List <linux-ext4@...r.kernel.org>,
        linux-f2fs-devel@...ts.sourceforge.net, linux-xfs@...r.kernel.org,
        Eric Biggers <ebiggers@...nel.org>,
        Richard Weinberger <richard@....at>
Subject: Re: [PATCH 1/2] writeback: avoid double-writing the inode on a
 lazytime expiration

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?

					- Ted