linux-kernel - Re: mmotm 2015-01-22-15-04: qemu failure due to 'mm: memcontrol: remove unnecessary soft limit tree node test'

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Message-ID: <20150126133712.GB9738@cmpxchg.org>
Date:	Mon, 26 Jan 2015 08:37:12 -0500
From:	Johannes Weiner <hannes@...xchg.org>
To:	Valdis.Kletnieks@...edu
Cc:	Christoph Lameter <cl@...ux.com>,
	Guenter Roeck <linux@...ck-us.net>, akpm@...ux-foundation.org,
	mm-commits@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
	linux-next@...r.kernel.org, sfr@...b.auug.org.au, mhocko@...e.cz
Subject: Re: mmotm 2015-01-22-15-04: qemu failure due to 'mm: memcontrol:
 remove unnecessary soft limit tree node test'

On Sun, Jan 25, 2015 at 04:36:28PM -0500, Valdis.Kletnieks@...edu wrote:
> On Sat, 24 Jan 2015 02:16:23 -0500, Johannes Weiner said:
> 
> > I would generally agree, but this code, which implements a userspace
> > interface, is already grotesquely inefficient and heavyhanded.  It's
> > also superseded in the next release, so we can just keep this simple
> > at this point.
> 
> Wait, what?  Userspace interface that's superceded in the next release?

The existing interface and its implementation are going to remain in
place, obviously, we can't break userspace.  But the semantics are
ill-defined and the implementation bad to a point where we decided to
fix both by adding a second interface and encouraging users to switch.

Now if a user were to report that these off-node allocations are
actually creating problems in real life I would fix it.  But I'm
fairly certain that remote access costs are overshadowed by the
reclaim stalls this mechanism creates.

So what I was trying to say above is that I don't see a point in
complicating the v1 implementation for a questionable minor
optimization when v2 is already being added to address much more
severe shortcomings in v1.
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