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Date: Wed, 28 Aug 2013 15:36:06 +0900 From: Joonsoo Kim <iamjoonsoo.kim@....com> To: Jonathan Corbet <corbet@....net> Cc: Pekka Enberg <penberg@...nel.org>, Christoph Lameter <cl@...ux.com>, Andrew Morton <akpm@...ux-foundation.org>, David Rientjes <rientjes@...gle.com>, linux-mm@...ck.org, linux-kernel@...r.kernel.org Subject: Re: [PATCH 07/16] slab: overloading the RCU head over the LRU for RCU free 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@....com> 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! -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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