Re: slab-nomerge (was Re: [git pull] device mapper changes for 4.3)

From: Andrew Morton <akpm@linux-foundation.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Snitzer <snitzer@redhat.com>,
	Dave Chinner <dchinner@redhat.com>,
	Christoph Lameter <cl@linux.com>,
	Pekka Enberg <penberg@kernel.org>,
	David Rientjes <rientjes@google.com>,
	Joonsoo Kim <iamjoonsoo.kim@lge.com>,
	"dm-devel@redhat.com" <dm-devel@redhat.com>,
	Alasdair G Kergon <agk@redhat.com>, Joe Thornber <ejt@redhat.com>,
	Mikulas Patocka <mpatocka@redhat.com>,
	Vivek Goyal <vgoyal@redhat.com>,
	Sami Tolvanen <samitolvanen@google.com>,
	Viresh Kumar <viresh.kumar@linaro.org>,
	Heinz Mauelshagen <heinzm@redhat.com>,
	linux-mm <linux-mm@kvack.org>
Subject: Re: slab-nomerge (was Re: [git pull] device mapper changes for 4.3)
Date: Wed, 2 Sep 2015 17:48:15 -0700	[thread overview]
Message-ID: <20150902174815.199d51a480a01e9a754367e3@linux-foundation.org> (raw)
In-Reply-To: <CA+55aFyepmdpbg9U2Pvp+aHjKmmGCrTK2ywzqfmaOTMXQasYNw@mail.gmail.com>

On Wed, 2 Sep 2015 16:13:44 -0700 Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Wed, Sep 2, 2015 at 10:39 AM, Mike Snitzer <snitzer@redhat.com> wrote:
> >
> > - last but not least: add SLAB_NO_MERGE flag to mm/slab_common and
> >   disable slab merging for all of DM's slabs (XFS will also use
> >   SLAB_NO_MERGE once merged).
> 
> So I'm not at all convinced this is the right thing to do. In fact,
> I'm pretty convinced it shouldn't be done this way. Since those
> commits were at the top of your tree, I just didn't pull them, but
> took the rest..

I don't have problems with the patch itself, really.  It only affects
callers who use SLAB_NO_MERGE and those developers can make
their own decisions.

It is a bit sad to de-optimise dm for all users for all time in order
to make life a bit easier for dm's developers, but maybe that's a
decent tradeoff.

What I do have a problem with is that afaict the patch appeared on
linux-mm for the first time just yesterday.  Didn't cc slab developers,
it isn't in linux-next, didn't cc linux-kernel or linux-mm or slab/mm
developers on the pull request.  Bad!

I'd like the slab developers to have time to understand and review this
change, please.  Partly so they have a chance to provide feedback for
the usual reasons, but also to help them understand the effect their
design choice had on client subystems.

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