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

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Message-Id: <20140630173428.5ebeed18.akpm@linux-foundation.org>
Date:	Mon, 30 Jun 2014 17:34:28 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	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 17:10:54 -0700 (PDT) Hugh Dickins <hughd@...gle.com> wrote:

> 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.

Well, readers of shmem_unuse_inode() won't know that unless we tell them.


> > > +	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.

OK.
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