From: Waiman Long <longman@xxxxxxxxxx> Subject: mm: memcg/slab: properly set up gfp flags for objcg pointer array Patch series "mm: memcg/slab: Fix objcg pointer array handling problem", v4. Since the merging of the new slab memory controller in v5.9, the page structure stores a pointer to objcg pointer array for slab pages. When the slab has no used objects, it can be freed in free_slab() which will call kfree() to free the objcg pointer array in memcg_alloc_page_obj_cgroups(). If it happens that the objcg pointer array is the last used object in its slab, that slab may then be freed which may caused kfree() to be called again. With the right workload, the slab cache may be set up in a way that allows the recursive kfree() calling loop to nest deep enough to cause a kernel stack overflow and panic the system. In fact, we have a reproducer that can cause kernel stack overflow on a s390 system involving kmalloc-rcl-256 and kmalloc-rcl-128 slabs with the following kfree() loop recursively called 74 times: [ 285.520739] [<000000000ec432fc>] kfree+0x4bc/0x560 [ 285.520740] [<000000000ec43466>] __free_slab+0xc6/0x228 [ 285.520741] [<000000000ec41fc2>] __slab_free+0x3c2/0x3e0 [ 285.520742] [<000000000ec432fc>] kfree+0x4bc/0x560 : While investigating this issue, I also found an issue on the allocation side. If the objcg pointer array happen to come from the same slab or a circular dependency linkage is formed with multiple slabs, those affected slabs can never be freed again. This patch series addresses these two issues by introducing a new set of kmalloc-cg-<n> caches split from kmalloc-<n> caches. The new set will only contain non-reclaimable and non-dma objects that are accounted in memory cgroups whereas the old set are now for unaccounted objects only. By making this split, all the objcg pointer arrays will come from the kmalloc-<n> caches, but those caches will never hold any objcg pointer array. As a result, deeply nested kfree() call and the unfreeable slab problems are now gone. This patch (of 4): Since the merging of the new slab memory controller in v5.9, the page structure may store a pointer to obj_cgroup pointer array for slab pages. Currently, only the __GFP_ACCOUNT bit is masked off. However, the array is not readily reclaimable and doesn't need to come from the DMA buffer. So those GFP bits should be masked off as well. Do the flag bit clearing at memcg_alloc_page_obj_cgroups() to make sure that it is consistently applied no matter where it is called. Link: https://lkml.kernel.org/r/20210505200610.13943-1-longman@xxxxxxxxxx Link: https://lkml.kernel.org/r/20210505200610.13943-2-longman@xxxxxxxxxx Fixes: 286e04b8ed7a ("mm: memcg/slab: allocate obj_cgroups for non-root slab pages") Signed-off-by: Waiman Long <longman@xxxxxxxxxx> Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx> Acked-by: Roman Gushchin <guro@xxxxxx> Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx> Cc: Johannes Weiner <hannes@xxxxxxxxxxx> Cc: Michal Hocko <mhocko@xxxxxxxxxx> Cc: Vladimir Davydov <vdavydov.dev@xxxxxxxxx> Cc: Christoph Lameter <cl@xxxxxxxxx> Cc: Pekka Enberg <penberg@xxxxxxxxxx> Cc: David Rientjes <rientjes@xxxxxxxxxx> Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- mm/memcontrol.c | 8 ++++++++ mm/slab.h | 1 - 2 files changed, 8 insertions(+), 1 deletion(-) --- a/mm/memcontrol.c~mm-memcg-slab-properly-set-up-gfp-flags-for-objcg-pointer-array +++ a/mm/memcontrol.c @@ -2803,6 +2803,13 @@ retry: } #ifdef CONFIG_MEMCG_KMEM +/* + * The allocated objcg pointers array is not accounted directly. + * Moreover, it should not come from DMA buffer and is not readily + * reclaimable. So those GFP bits should be masked off. + */ +#define OBJCGS_CLEAR_MASK (__GFP_DMA | __GFP_RECLAIMABLE | __GFP_ACCOUNT) + int memcg_alloc_page_obj_cgroups(struct page *page, struct kmem_cache *s, gfp_t gfp, bool new_page) { @@ -2810,6 +2817,7 @@ int memcg_alloc_page_obj_cgroups(struct unsigned long memcg_data; void *vec; + gfp &= ~OBJCGS_CLEAR_MASK; vec = kcalloc_node(objects, sizeof(struct obj_cgroup *), gfp, page_to_nid(page)); if (!vec) --- a/mm/slab.h~mm-memcg-slab-properly-set-up-gfp-flags-for-objcg-pointer-array +++ a/mm/slab.h @@ -298,7 +298,6 @@ static inline void memcg_slab_post_alloc if (!memcg_kmem_enabled() || !objcg) return; - flags &= ~__GFP_ACCOUNT; for (i = 0; i < size; i++) { if (likely(p[i])) { page = virt_to_head_page(p[i]); _
