linux-kernel - Re: [PATCH 08/10] fs: add support for non-blocking timestamp updates

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Message-ID: <20251218061900.GB2775@lst.de>
Date: Thu, 18 Dec 2025 07:19:00 +0100
From: Christoph Hellwig <hch@....de>
To: Jan Kara <jack@...e.cz>
Cc: Christoph Hellwig <hch@....de>, Christian Brauner <brauner@...nel.org>,
	Al Viro <viro@...iv.linux.org.uk>, David Sterba <dsterba@...e.com>,
	Mike Marshall <hubcap@...ibond.com>,
	Martin Brandenburg <martin@...ibond.com>,
	Carlos Maiolino <cem@...nel.org>, Stefan Roesch <shr@...com>,
	Jeff Layton <jlayton@...nel.org>, linux-kernel@...r.kernel.org,
	linux-btrfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	gfs2@...ts.linux.dev, io-uring@...r.kernel.org,
	devel@...ts.orangefs.org, linux-unionfs@...r.kernel.org,
	linux-mtd@...ts.infradead.org, linux-xfs@...r.kernel.org,
	linux-nfs@...r.kernel.org
Subject: Re: [PATCH 08/10] fs: add support for non-blocking timestamp
 updates

On Wed, Dec 17, 2025 at 01:42:20PM +0100, Jan Kara wrote:
> > @@ -2110,12 +2110,26 @@ int inode_update_timestamps(struct inode *inode, int *flags)
> >  		now = inode_set_ctime_current(inode);
> >  		if (!timespec64_equal(&now, &ctime))
> >  			updated |= S_CTIME;
> > -		if (!timespec64_equal(&now, &mtime)) {
> > -			inode_set_mtime_to_ts(inode, now);
> > +		if (!timespec64_equal(&now, &mtime))
> >  			updated |= S_MTIME;
> > +
> > +		if (IS_I_VERSION(inode)) {
> > +			if (*flags & S_NOWAIT) {
> > +				/*
> > +				 * Error out if we'd need timestamp updates, as
> > +				 * the generally requires blocking to dirty the
> > +				 * inode in one form or another.
> > +				 */
> > +				if (updated && inode_iversion_need_inc(inode))
> > +					goto bail;
> 
> I'm confused here. What the code does is that if S_NOWAIT is set and
> i_version needs increment we bail. However the comment as well as the
> changelog speaks about timestamps needing update and not about i_version.
> And intuitively I agree that if any timestamp is updated, inode needs
> dirtying and thus we should bail regardless of whether i_version is updated
> as well or not. What am I missing?

With lazytime timestamp updates that don't require i_version updates
are performed in-memory only, and we'll only reach this with S_NOWAIT
set for those (later in the series, it can't be reached at all as
of this patch).

But yes, this needs to be documented much better.