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

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Message-ID: <20190312184754.GB31407@tower.DHCP.thefacebook.com>
Date:   Tue, 12 Mar 2019 18:47:57 +0000
From:   Roman Gushchin <guro@...com>
To:     Christopher Lameter <cl@...ux.com>
CC:     "Tobin C. Harding" <tobin@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Pekka Enberg <penberg@...helsinki.fi>,
        "Matthew Wilcox" <willy@...radead.org>,
        Tycho Andersen <tycho@...ho.ws>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC 02/15] slub: Add isolate() and migrate() methods

On Tue, Mar 12, 2019 at 04:35:15AM +0000, Christopher Lameter wrote:
> 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.

Understood. I like the idea of an ops vector, but it can be done later,
agree.

> 
> > > +	/*
> > > +	 * 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.

Thank you for the explanation!