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"? -- 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>