linux-kernel - Re: [PATCH] proc: update i

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Message-ID: <20190225194627.GA14348@avx2>
Date:   Mon, 25 Feb 2019 22:47:30 +0300
From:   Alexey Dobriyan <adobriyan@...il.com>
To:     Jeremy Cline <jcline@...hat.com>
Cc:     David Both <dboth@...lennium-technology.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] proc: update i_atime when reading files

On Fri, Feb 22, 2019 at 10:00:20AM -0500, Jeremy Cline wrote:
> On Fri, Feb 22, 2019 at 08:37:42AM +0300, Alexey Dobriyan wrote:
> > On Thu, Feb 21, 2019 at 11:37:14AM -0500, Jeremy Cline wrote:
> > > Prior to commit 1da4d377f943 ("proc: revalidate misc dentries"), the
> > > access, modify, and change times of files in /proc were just the current
> > > time.
> > 
> > Ehh, actually no. Doing
> > 
> > 	$(which sleep) infinity </proc/foo &
> > 
> > will sabotage atime updates because dentry and inode will be pinned in
> > caches.
> > 
> > "revalidate misc denries" commit simply makes the effect (much) more
> > visible by making objects stay in caches for longer.
> 
> Indeed. It wasn't my intention to imply there's anything wrong with that
> commit, just that that's what caused this apparent change in behavior
> for users. In the "common" case when something hasn't pinned the dentry
> and inode what users saw was the current time.
> 
> > 
> > > Now the mtime and ctime values change mostly as a user would
> > > expect, but the atime isn't updated when the file read. This patch
> > > updates the access time of /proc files when they are read.
> > 
> > >  			rv = read(file, buf, count, ppos);
> > > +			if (rv >= 0)
> > > +				inode->i_atime = current_time(inode);
> > > +		}
> > 
> > Maybe it should be done given /proc is virtual so there are no concerns
> > about scheduling writes noone cares about to the filesystem.
> 
> Sorry, maybe I've not had enough coffee yet, but I don't understand this
> sentence.

I meant it should be harmless to enable atime unconditionally for /proc
because it is virtual filesystem.