Date:   Fri, 7 May 2021 14:45:48 -0400
From:   Waiman Long <llong@...hat.com>
To:     Vlastimil Babka <vbabka@...e.cz>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Roman Gushchin <guro@...com>,
        Shakeel Butt <shakeelb@...gle.com>
Cc:     linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH v4 2/3] mm: memcg/slab: Create a new set of kmalloc-cg-<n>
 caches

On 5/6/21 12:00 PM, 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@...e.cz>
>> Signed-off-by: Waiman Long <longman@...hat.com>
>> Reviewed-by: Shakeel Butt <shakeelb@...gle.com>
> Reviewed-by: Vlastimil Babka <vbabka@...e.cz>
>
> I still believe the cgroup.memory=nokmem parameter should be respected,
> otherwise the caches are not only created, but also used. 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@...e.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@...e.cz>
> ---
>   mm/internal.h    | 5 +++++
>   mm/memcontrol.c  | 2 +-
>   mm/slab_common.c | 9 +++++++--
>   3 files changed, 13 insertions(+), 3 deletions(-)
>
> diff --git a/mm/internal.h b/mm/internal.h
> index ef5f336f59bd..b2d60b3403c7 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -135,6 +135,11 @@ extern void putback_lru_page(struct page *page);
>    */
>   extern pmd_t *mm_find_pmd(struct mm_struct *mm, unsigned long address);
>   
> +/*
> + * in mm/memcontrol.c:
> + */
> +extern bool cgroup_memory_nokmem;
> +
>   /*
>    * in mm/page_alloc.c
>    */
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 5e3b4f23b830..b9ec01f2b4f6 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -83,7 +83,7 @@ DEFINE_PER_CPU(struct mem_cgroup *, int_active_memcg);
>   static bool cgroup_memory_nosocket;
>   
>   /* Kernel memory accounting disabled? */
> -static bool cgroup_memory_nokmem;
> +bool cgroup_memory_nokmem;
>   
>   /* Whether the swap controller is active */
>   #ifdef CONFIG_MEMCG_SWAP
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index bbaf41a7c77e..363f90215401 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -832,10 +832,15 @@ void __init setup_kmalloc_cache_index_table(void)
>   static void __init
>   new_kmalloc_cache(int idx, enum kmalloc_cache_type type, slab_flags_t flags)
>   {
> -	if (type == KMALLOC_RECLAIM)
> +	if (type == KMALLOC_RECLAIM) {
>   		flags |= SLAB_RECLAIM_ACCOUNT;
> -	else if (IS_ENABLED(CONFIG_MEMCG_KMEM) && (type == KMALLOC_CGROUP))
> +	} else if (IS_ENABLED(CONFIG_MEMCG_KMEM) && (type == KMALLOC_CGROUP)) {
> +		if (cgroup_memory_nokmem) {
> +			kmalloc_caches[type][idx] = kmalloc_caches[KMALLOC_NORMAL][idx];
> +			return;
> +		}
>   		flags |= SLAB_ACCOUNT;
> +	}
>   
>   	kmalloc_caches[type][idx] = create_kmalloc_cache(
>   					kmalloc_info[idx].name[type],

Thanks, the patch looks good to me.

Acked-by: Waiman Long <longman@...hat.com>

Cheers,
Longman

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