Re: [PATCH v4 2/3] mm: memcg/slab: Create a new set of kmalloc-cg-<n> caches

[Roman Gushchin <guro@fb.com>]

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@suse.cz>
> > Signed-off-by: Waiman Long <longman@redhat.com>
> > Reviewed-by: Shakeel Butt <shakeelb@google.com>
> 
> Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
> 
> 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@suse.cz>
> 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@suse.cz>

Acked-by: Roman Gushchin <guro@fb.com>

Thanks!