linux-kernel - Re: [PATCH mmotm/next] mm: memcontrol: rewrite charge API: fix shmem

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Date:	Mon, 30 Jun 2014 17:10:54 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	Hugh Dickins <hughd@...gle.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH mmotm/next] mm: memcontrol: rewrite charge API: fix
 shmem_unuse

On Mon, 30 Jun 2014, Andrew Morton wrote:
> On Mon, 30 Jun 2014 15:48:39 -0700 (PDT) Hugh Dickins <hughd@...gle.com> wrote:
> > -		return 0;
> > +		return -EAGAIN;
> 
> Maybe it's time to document the shmem_unuse_inode() return values.

Oh dear.  I had hoped they would look after themselves.  This one is a
private matter between shmem_unuse_inode and its one caller, just below.

> > +	if (error) {
> > +		if (error != -ENOMEM)
> > +			error = 0;
> >  		mem_cgroup_cancel_charge(page, memcg);
> >  	} else
> >  		mem_cgroup_commit_charge(page, memcg, true);
> 
> If I'm reading this correctly, shmem_unuse() can now return -EAGAIN and
> that can get all the way back to userspace.  `man 2 swapoff' doesn't
> know this...

if (error) {
	if (error != -ENOMEM)
		error = 0;
...
	return error;

So the only values returned from shmem_unuse_inode() to its caller
try_to_unuse() are 0 and -ENOMEM.  Those may get passed back to the
user, but -EAGAIN was just an internal shmem.c detail.

Hugh
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