On Thu, Jan 29, 2015 at 3:27 AM, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: > On Wed, 28 Jan 2015 19:22:49 +0300 Vladimir Davydov <vdavydov@xxxxxxxxxxxxx> 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"? This much space in the stack may be fertile grounds for kernel stack overflow code execution :) Another perspective could be that there should be allocations that are not penalized to a particular cgroup (from an accounting perspective), should come from the reserved pool. Balbir Singh. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>