linux-kernel - Re: [PATCH 3/4] mm: Add free()

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Date:   Fri, 23 Mar 2018 18:49:53 +0300
From:   Kirill Tkhai <ktkhai@...tuozzo.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-mm@...ck.org, Matthew Wilcox <mawilcox@...rosoft.com>,
        linux-kernel@...r.kernel.org,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH 3/4] mm: Add free()

On 23.03.2018 18:14, Matthew Wilcox wrote:
> On Fri, Mar 23, 2018 at 04:33:24PM +0300, Kirill Tkhai wrote:
>>> +	page = virt_to_head_page(ptr);
>>> +	if (likely(PageSlab(page)))
>>> +		return kmem_cache_free(page->slab_cache, (void *)ptr);
>>
>> It seems slab_cache is not generic for all types of slabs. SLOB does not care about it:
> 
> Oof.  I was sure I checked that.  You're quite right that it doesn't ...
> this should fix that problem:
> 
> diff --git a/mm/slob.c b/mm/slob.c
> index 623e8a5c46ce..96339420c6fc 100644
> --- a/mm/slob.c
> +++ b/mm/slob.c
> @@ -266,7 +266,7 @@ static void *slob_page_alloc(struct page *sp, size_t size, int align)
>  /*
>   * slob_alloc: entry point into the slob allocator.
>   */
> -static void *slob_alloc(size_t size, gfp_t gfp, int align, int node)
> +static void *slob_alloc(size_t size, gfp_t gfp, int align, int node, void *c)
>  {
>  	struct page *sp;
>  	struct list_head *prev;
> @@ -324,6 +324,7 @@ static void *slob_alloc(size_t size, gfp_t gfp, int align, int node)
>  		sp->units = SLOB_UNITS(PAGE_SIZE);
>  		sp->freelist = b;
>  		INIT_LIST_HEAD(&sp->lru);
> +		sp->slab_cache = c;
>  		set_slob(b, SLOB_UNITS(PAGE_SIZE), b + SLOB_UNITS(PAGE_SIZE));
>  		set_slob_page_free(sp, slob_list);
>  		b = slob_page_alloc(sp, size, align);
> @@ -440,7 +441,7 @@ __do_kmalloc_node(size_t size, gfp_t gfp, int node, unsigned long caller)
>  		if (!size)
>  			return ZERO_SIZE_PTR;
>  
> -		m = slob_alloc(size + align, gfp, align, node);
> +		m = slob_alloc(size + align, gfp, align, node, NULL);
>  
>  		if (!m)
>  			return NULL;
> @@ -544,7 +545,7 @@ static void *slob_alloc_node(struct kmem_cache *c, gfp_t flags, int node)
>  	fs_reclaim_release(flags);
>  
>  	if (c->size < PAGE_SIZE) {
> -		b = slob_alloc(c->size, flags, c->align, node);
> +		b = slob_alloc(c->size, flags, c->align, node, c);
>  		trace_kmem_cache_alloc_node(_RET_IP_, b, c->object_size,
>  					    SLOB_UNITS(c->size) * SLOB_UNIT,
>  					    flags, node);
> @@ -600,6 +601,8 @@ static void kmem_rcu_free(struct rcu_head *head)
>  
>  void kmem_cache_free(struct kmem_cache *c, void *b)
>  {
> +	if (!c)
> +		return kfree(b);
>  	kmemleak_free_recursive(b, c->flags);
>  	if (unlikely(c->flags & SLAB_TYPESAFE_BY_RCU)) {
>  		struct slob_rcu *slob_rcu;
> 
>> Also, using kmem_cache_free() for kmalloc()'ed memory will connect them hardly,
>> and this may be difficult to maintain in the future.
> 
> I think the win from being able to delete all the little RCU callbacks
> that just do a kmem_cache_free() is big enough to outweigh the
> disadvantage of forcing slab allocators to support kmem_cache_free()
> working on kmalloced memory.
> 
>> One more thing, there is
>> some kasan checks on the main way of kfree(), and there is no guarantee they
>> reflected in kmem_cache_free() identical.
> 
> Which function are you talking about here?
> 
> slub calls slab_free() for both kfree() and kmem_cache_free().
> slab calls __cache_free() for both kfree() and kmem_cache_free().
> Each of them do their kasan handling in the called function.

Maybe not KASAN, I never dived deeply into sl[*]b. But they look like
just three different functions, doing different actions...

Kirill