Re: [PATCH 2/2] fs: proc/stat: use usual seq_file ops rather than single_open

From: Heiko Carstens <heiko.carstens@de.ibm.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Christoph Hellwig <hch@infradead.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
	Andrea Righi <andrea@betterlinux.com>,
	Eric Dumazet <eric.dumazet@gmail.com>,
	linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	Hendrik Brueckner <brueckner@linux.vnet.ibm.com>,
	Thorsten Diehl <thorsten.diehl@de.ibm.com>,
	Ian Kent <raven@themaw.net>,
	"Elliott, Robert (Server Storage)" <Elliott@hp.com>
Subject: Re: [PATCH 2/2] fs: proc/stat: use usual seq_file ops rather than single_open
Date: Fri, 30 May 2014 10:38:30 +0200	[thread overview]
Message-ID: <20140530083830.GA4732@osiris> (raw)
In-Reply-To: <20140528153704.b2a3f46dc39ebf8f681d528a@linux-foundation.org>

On Wed, May 28, 2014 at 03:37:04PM -0700, Andrew Morton wrote:
> On Wed, 28 May 2014 11:01:53 +0200 Heiko Carstens <heiko.carstens@de.ibm.com> wrote:
> > With this patch it should not happen anymore that reading /proc/stat
> > fails because of a failing high order memory allocation.
> 
> So this deletes the problematic allocation which [1/2] kind-of fixed,
> yes?

Yes.

> I agree with Ian - there's a hotplugging race.  And [1/2] doesn't do
> anything to address the worst-case allocation size.  So I think we may
> as well do this all in a single patch.

Fine with me. However the hotplugging race in 1/2 doesn't matter: if the
result doesn't fit into the preallocated buffer the seq_file infrastructure
would simply allocate a buffer twice as large as before and retry.

The point of patch 1/2 was to have a patch that probably solves the problem
almost always ;) , without having the problems you describe below.

> Without having looked closely at the code I worry a bit about the
> effects.  /proc/pid/stat is a complex thing and its contents will vary

It's /proc/stat not /proc/pid/stat.

> So..  can we take this up for 3.16-rc1?  See if we can get some careful
> review done then and test it for a couple of months?

Sure.

> Meanwhile, the changelog looks a bit hastily thrown together - some
> smoothing would be nice, and perhaps some work spent identifying
> possible behavioural changes.  Timing changes, locking canges, effects
> of concurrent fork/exit activity etc?

Well... I'll try to come up with something better. Even though I only
forward ported an existing patch to address a memory allocation failure.
Oh oh...