On Thu, May 06, 2021 at 06:00:16PM +0200, Vlastimil Babka wrote: > > On 5/5/21 10:06 PM, Waiman Long wrote: > > There are currently two problems in the way the objcg pointer array > > (memcg_data) in the page structure is being allocated and freed. > > > > On its allocation, it is possible that the allocated objcg pointer > > array comes from the same slab that requires memory accounting. If this > > happens, the slab will never become empty again as there is at least > > one object left (the obj_cgroup array) in the slab. > > > > When it is freed, the objcg pointer array object may be the last one > > in its slab and hence causes 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. > > > > One way to solve this problem is to split the kmalloc-<n> caches > > (KMALLOC_NORMAL) into two separate sets - a new set of kmalloc-<n> > > (KMALLOC_NORMAL) caches for unaccounted objects only and a new set of > > kmalloc-cg-<n> (KMALLOC_CGROUP) caches for accounted objects only. All > > the other caches can still allow a mix of accounted and unaccounted > > objects. > > > > With this change, all the objcg pointer array objects will come from > > KMALLOC_NORMAL caches which won't have their objcg pointer arrays. So > > both the recursive kfree() problem and non-freeable slab problem are > > gone. > > > > Since both the KMALLOC_NORMAL and KMALLOC_CGROUP caches no longer have > > mixed accounted and unaccounted objects, this will slightly reduce the > > number of objcg pointer arrays that need to be allocated and save a bit > > of memory. On the other hand, creating a new set of kmalloc caches does > > have the effect of reducing cache utilization. So it is properly a wash. > > > > The new KMALLOC_CGROUP is added between KMALLOC_NORMAL and > > KMALLOC_RECLAIM so that the first for loop in create_kmalloc_caches() > > will include the newly added caches without change. > > > > Suggested-by: Vlastimil Babka <vbabka@xxxxxxx> > > Signed-off-by: Waiman Long <longman@xxxxxxxxxx> > > Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx> > > Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx> > > I still believe the cgroup.memory=nokmem parameter should be respected, > otherwise the caches are not only created, but also used. +1 > I offer this followup > for squashing into your patch if you and Andrew agree: > > ----8<---- > From c87378d437d9a59b8757033485431b4721c74173 Mon Sep 17 00:00:00 2001 > From: Vlastimil Babka <vbabka@xxxxxxx> > Date: Thu, 6 May 2021 17:53:21 +0200 > Subject: [PATCH] mm: memcg/slab: don't create kmalloc-cg caches with > cgroup.memory=nokmem > > The caches should not be created when kmemcg is disabled on boot, otherwise > they are also filled by kmalloc(__GFP_ACCOUNT) allocations. When booted with > cgroup.memory=nokmem, link the kmalloc_caches[KMALLOC_CGROUP] entries to > KMALLOC_NORMAL entries instead. > > Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx> Acked-by: Roman Gushchin <guro@xxxxxx> Thanks!
