Re: [PATCH 07/16] slab: overloading the RCU head over the LRU forRCU free

[Joonsoo Kim]

Hello,

On Tue, Aug 27, 2013 at 04:06:04PM -0600, Jonathan Corbet wrote:
> On Thu, 22 Aug 2013 17:44:16 +0900
> Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> wrote:
> 
> > With build-time size checking, we can overload the RCU head over the LRU
> > of struct page to free pages of a slab in rcu context. This really help to
> > implement to overload the struct slab over the struct page and this
> > eventually reduce memory usage and cache footprint of the SLAB.
> 
> So I'm taking a look at this, trying to figure out what's actually in
> struct page while this stuff is going on without my head exploding.  A
> couple of questions come to mind.
> 
> >  static void kmem_rcu_free(struct rcu_head *head)
> >  {
> > -	struct slab_rcu *slab_rcu = (struct slab_rcu *)head;
> > -	struct kmem_cache *cachep = slab_rcu->page->slab_cache;
> > +	struct kmem_cache *cachep;
> > +	struct page *page;
> >  
> > -	kmem_freepages(cachep, slab_rcu->page);
> > +	page = container_of((struct list_head *)head, struct page, lru);
> > +	cachep = page->slab_cache;
> > +
> > +	kmem_freepages(cachep, page);
> >  }
> 
> Is there a reason why you don't add the rcu_head structure as another field
> in that union alongside lru rather than playing casting games here?  This
> stuff is hard enough to follow as it is without adding that into the mix.

One reason is that the SLUB is already playing this games :)
And the struct page shouldn't be enlarged unintentionally when the size of
the rcu_head is changed.

> 
> The other question I had is: this field also overlays slab_page.  I guess
> that, by the time RCU comes into play, there will be no further use of
> slab_page?  It might be nice to document that somewhere if it's the case.

Ah..... I did a mistake in previous patch (06/16). We should leave an object
on slab_page until rcu finish the work since rcu_head is overloaded over it.

If I remove that patch, this patch has a problem you mentioned. But I think
that a fix is simple. Moving the slab_page to another union field in the
struct slab prio to this patch solves the problem you mentioned.

Thanks for pointing that!

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