linux-kernel - Re: [PATCH] fs: don't overwrite the nsec field if I_CTIME

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Message-ID: <20240809-zugkraft-ergehen-df138f9d564a@brauner>
Date: Fri, 9 Aug 2024 17:17:57 +0200
From: Christian Brauner <brauner@...nel.org>
To: Jeff Layton <jlayton@...nel.org>
Cc: Alexander Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.cz>, 
	Mateusz Guzik <mjguzik@...il.com>, linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs: don't overwrite the nsec field if I_CTIME_QUERIED is
 already set

On Fri, Aug 09, 2024 at 11:05:54AM GMT, Jeff Layton wrote:
> On Fri, 2024-08-09 at 16:55 +0200, Christian Brauner wrote:
> > On Fri, Aug 09, 2024 at 09:39:43AM GMT, Jeff Layton wrote:
> > > When fetching the ctime's nsec value for a stat-like operation, do
> > > a
> > > simple fetch first and avoid the atomic_fetch_or if the flag is
> > > already
> > > set.
> > > 
> > > Suggested-by: Mateusz Guzik <mjguzik@...il.com>
> > > Signed-off-by: Jeff Layton <jlayton@...nel.org>
> > > ---
> > > I'm running tests on this now, but I don't expect any problems.
> > > 
> > > This is based on top of Christian's vfs.mgtime branch. It may be
> > > best to
> > > squash this into 6feb43ecdd8e ("fs: add infrastructure for
> > > multigrain
> > > timestamps").
> > 
> > Squashed it. Can you double-check that things look correct?
> 
> One minor issue in fill_mg_cmtime:
> 
> -------------8<-----------------
>         if (!(stat->ctime.tv_nsec & I_CTIME_QUERIED))
>                 stat->ctime.tv_nsec = ((u32)atomic_fetch_or(I_CTIME_QUERIED, pcn));
>         trace_fill_mg_cmtime(inode, &stat->ctime, &stat->mtime);
>         stat->ctime.tv_nsec &= ~I_CTIME_QUERIED;
> }
> -------------8<-----------------
> 
> I'd swap the last two lines of the function. We print the ctime in the
> tracepoint as a timestamp, so if the QUERIED bit is present it's going
> to look funny. We _know_ that it's flagged after this function, so
> leaving it set is not terribly helpful.

Bah, fat-fingered on my end. I did intend to place it after. Thanks!