linux-kernel - Re: Commit 'fs: Do not check if there is a fsnotify watcher on pseudo inodes' breaks chromium here

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Date:   Mon, 29 Jun 2020 15:32:17 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Amir Goldstein <amir73il@...il.com>
Cc:     Jan Kara <jack@...e.cz>, Maxim Levitsky <mlevitsk@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: Commit 'fs: Do not check if there is a fsnotify watcher on
 pseudo inodes' breaks chromium here

On Mon, Jun 29, 2020 at 05:05:38PM +0300, Amir Goldstein wrote:
> > > The motivation for the patch "fs: Do not check if there is a fsnotify
> > > watcher on pseudo inodes"
> > > was performance, but actually, FS_CLOSE and FS_OPEN events probably do
> > > not impact performance as FS_MODIFY and FS_ACCESS.
> >
> > Correct.
> >
> > > Do you agree that dropping FS_MODIFY/FS_ACCESS events for FMODE_STREAM
> > > files as a general rule should be safe?
> >
> > Hum, so your patch drops FS_MODIFY/FS_ACCESS events also for named pipes
> > compared to the original patch AFAIU and for those fsnotify works fine
> > so far. So I'm not sure we won't regress someone else with this.
> >
> > I've also tested inotify on a sample pipe like: cat /dev/stdin | tee
> > and watched /proc/<tee pid>/fd/0 and it actually generated IN_MODIFY |
> > IN_ACCESS when data arrived to a pipe and tee(1) read it and then
> > IN_CLOSE_WRITE | IN_CLOSE_NOWRITE when the pipe got closed (I thought you
> > mentioned modify and access events didn't get properly generated?).
> 
> I don't think that I did (did I?)
> 

I didn't see them properly generated for fanotify_mark but that could
have been a failure. inotify-watch is able to generate the events.

> >
> > So as much as I agree that some fsnotify events on FMODE_STREAM files are
> > dubious, they could get used (possibly accidentally) and so after this
> > Chromium experience I think we just have to revert the change and live with
> > generating notification events for pipes to avoid userspace regressions.
> >
> > Thoughts?
> 
> I am fine with that.
> 
> Before I thought of trying out FMODE_STREAM I was considering to propose
> to set the new flag FMODE_NOIONOTIFY in alloc_file_pseudo() to narrow Mel's
> patch to dropping FS_MODIFY|FS_ACCESS.
> 
> But I guess the burden of proof is back on Mel.
> And besides, quoting Mel's patch:
> "A patch is pending that reduces, but does not eliminate, the overhead of
>     fsnotify but for files that cannot be looked up via a path, even that
>     small overhead is unnecessary"
> 
> So really, we are not even sacrificing much by reverting this patch.
> We down to "nano optimizations".
> 

It's too marginal to be worth the risk. A plain revert is safest when
multiple people are hitting this.

-- 
Mel Gorman
SUSE Labs