Re: [RFC 02/15] slub: Add isolate() and migrate() methods

From: Christopher Lameter <cl@linux.com>
To: Roman Gushchin <guro@fb.com>
Cc: "Tobin C. Harding" <tobin@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Pekka Enberg <penberg@cs.helsinki.fi>,
	Matthew Wilcox <willy@infradead.org>,
	Tycho Andersen <tycho@tycho.ws>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [RFC 02/15] slub: Add isolate() and migrate() methods
Date: Tue, 12 Mar 2019 04:35:15 +0000	[thread overview]
Message-ID: <01000169702ee357-fe8b85e5-e601-41da-8ba2-25e8b7db52eb-000000@email.amazonses.com> (raw)
In-Reply-To: <20190311215106.GA7915@tower.DHCP.thefacebook.com>

On Mon, 11 Mar 2019, Roman Gushchin wrote:

> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -4325,6 +4325,34 @@ int __kmem_cache_create(struct kmem_cache *s, slab_flags_t flags)
> >  	return err;
> >  }
> >
> > +void kmem_cache_setup_mobility(struct kmem_cache *s,
> > +			       kmem_cache_isolate_func isolate,
> > +			       kmem_cache_migrate_func migrate)
> > +{
>
> I wonder if it's better to adapt kmem_cache_create() to take two additional
> argument? I suspect mobility is not a dynamic option, so it can be
> set on kmem_cache creation.

One other idea that prior versions of this patchset used was to change
kmem_cache_create() so that the ctor parameter becomes an ops vector.

However, in order to reduce the size of the patchset I dropped that. It
could be easily moved back to the way it was before.

> > +	/*
> > +	 * Sadly serialization requirements currently mean that we have
> > +	 * to disable fast cmpxchg based processing.
> > +	 */
>
> Can you, please, elaborate a bit more here?

cmpxchg based processing does not lock the struct page. SMO requires to
ensure that all changes on a slab page can be stopped. The page->lock will
accomplish that. I think we could avoid dealing with actually locking the
page with some more work.