linux-kernel - Re: [PATCH V2] mm/slab: add a leak decoder callback

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Date:	Fri, 18 Jan 2013 09:11:29 +0800
From:	Liu Bo <bo.li.liu@...cle.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	linux-btrfs@...r.kernel.org, zab@...bo.net, penberg@...nel.org
Subject: Re: [PATCH V2] mm/slab: add a leak decoder callback

On Wed, Jan 16, 2013 at 03:20:57PM +0000, Christoph Lameter wrote:
> On Wed, 16 Jan 2013, Liu Bo wrote:
> 
> > --- a/include/linux/slub_def.h
> > +++ b/include/linux/slub_def.h
> > @@ -93,6 +93,7 @@ struct kmem_cache {
> >  	gfp_t allocflags;	/* gfp flags to use on each alloc */
> >  	int refcount;		/* Refcount for slab cache destroy */
> >  	void (*ctor)(void *);
> > +	void (*decoder)(void *);
> 
> The field needs to be moved away from the first hot cachelines in
> kmem_cache.
> 
> 
> > index 3f3cd97..8c19bfd 100644
> > --- a/mm/slab_common.c
> > +++ b/mm/slab_common.c
> > @@ -193,6 +193,7 @@ kmem_cache_create_memcg(struct mem_cgroup *memcg, const char *name, size_t size,
> >  		s->object_size = s->size = size;
> >  		s->align = calculate_alignment(flags, align, size);
> >  		s->ctor = ctor;
> > +		s->decoder = NULL;
> >
> >  		if (memcg_register_cache(memcg, s, parent_cache)) {
> >  			kmem_cache_free(kmem_cache, s);
> 
> Not necessary since s is filled with zeros on allocation.
> 
> > @@ -248,7 +249,7 @@ kmem_cache_create(const char *name, size_t size, size_t align,
> >  }
> >  EXPORT_SYMBOL(kmem_cache_create);
> >
> > -void kmem_cache_destroy(struct kmem_cache *s)
> > +static void __kmem_cache_destroy(struct kmem_cache *s, void (*decoder)(void *))
> >  {
> >  	/* Destroy all the children caches if we aren't a memcg cache */
> >  	kmem_cache_destroy_memcg_children(s);
> > @@ -259,6 +260,9 @@ void kmem_cache_destroy(struct kmem_cache *s)
> >  	if (!s->refcount) {
> >  		list_del(&s->list);
> >
> > +		if (unlikely(decoder))
> > +			s->decoder = decoder;
> > +
> >  		if (!__kmem_cache_shutdown(s)) {
> >  			mutex_unlock(&slab_mutex);
> >  			if (s->flags & SLAB_DESTROY_BY_RCU)
> 
> Now that is a bit weird since __kmem_cache_destroy now sets a field in
> kmem_cache?
> 
> If a kmem_cache has a decoder field set then it is no longer mergeable.
> 
> It looks like the decoder field would have to be set on cache creation.
> 
> If we do that then the functionality could be more generic. I always
> wanted to have a function that checks the object integrity as well.
> 
> The cache validation could then go through all objects and in addition to
> checking the slab meta data integrity could also have the subsystem
> confirm the integrity of the object.

Hmm...right, seems that we have to set the decoder field on creation
part, it brings us lots of API update though.

> 
> > index ba2ca53..34b3b75 100644
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -3098,6 +3098,8 @@ static void list_slab_objects(struct kmem_cache *s, struct page *page,
> >  	for_each_object(p, s, addr, page->objects) {
> >
> >  		if (!test_bit(slab_index(p, s, addr), map)) {
> > +			if (unlikely(s->decoder))
> > +				s->decoder(p);
> >  			printk(KERN_ERR "INFO: Object 0x%p @offset=%tu\n",
> >  							p, p - addr);
> >  			print_tracking(s, p);
> >
> 
> Hmmm... The function is currently only used on kmem_cache_destroy but that
> may change.

Good point.

list_slab_objects(struct kmem_cache *, struct page *, const char *,
		  void (*)(void *))

what about this?  At least we can interpret the objects as we want.

thanks,
liubo
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