Re: [PATCH -mm v2 1/3] slub: never fail to shrink cache

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 28 Jan 2015 19:22:49 +0300 Vladimir Davydov <vdavydov@parallels.com> wrote:

> SLUB's version of __kmem_cache_shrink() not only removes empty slabs,
> but also tries to rearrange the partial lists to place slabs filled up
> most to the head to cope with fragmentation. To achieve that, it
> allocates a temporary array of lists used to sort slabs by the number of
> objects in use. If the allocation fails, the whole procedure is aborted.
> 
> This is unacceptable for the kernel memory accounting extension of the
> memory cgroup, where we want to make sure that kmem_cache_shrink()
> successfully discarded empty slabs. Although the allocation failure is
> utterly unlikely with the current page allocator implementation, which
> retries GFP_KERNEL allocations of order <= 2 infinitely, it is better
> not to rely on that.
> 
> This patch therefore makes __kmem_cache_shrink() allocate the array on
> stack instead of calling kmalloc, which may fail. The array size is
> chosen to be equal to 32, because most SLUB caches store not more than
> 32 objects per slab page. Slab pages with <= 32 free objects are sorted
> using the array by the number of objects in use and promoted to the head
> of the partial list, while slab pages with > 32 free objects are left in
> the end of the list without any ordering imposed on them.
> 
> ...
>
> @@ -3375,51 +3376,56 @@ int __kmem_cache_shrink(struct kmem_cache *s)
>  	struct kmem_cache_node *n;
>  	struct page *page;
>  	struct page *t;
> -	int objects = oo_objects(s->max);
> -	struct list_head *slabs_by_inuse =
> -		kmalloc(sizeof(struct list_head) * objects, GFP_KERNEL);
> +	LIST_HEAD(discard);
> +	struct list_head promote[SHRINK_PROMOTE_MAX];

512 bytes of stack.  The call paths leading to __kmem_cache_shrink()
are many and twisty.  How do we know this isn't a problem?

The logic behind choosing "32" sounds rather rubbery.  What goes wrong
if we use, say, "4"?

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