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

Andrew Morton
2015-09-03T00:48:17Z

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.