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From:   Waiman Long <llong@redhat.com>
Subject: Re: [PATCH v3 2/2] mm: memcg/slab: Create a new set of kmalloc-cg-<n>
 caches
To:     Roman Gushchin <guro@fb.com>
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Christoph Lameter <cl@linux.com>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Shakeel Butt <shakeelb@google.com>,
        linux-kernel@vger.kernel.org, cgroups@vger.kernel.org,
        linux-mm@kvack.org
References: <20210505154613.17214-1-longman@redhat.com>
 <20210505154613.17214-3-longman@redhat.com>
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Message-ID: <5d1683d2-1023-1d1c-91e7-b68549debe1d@redhat.com>
Date:   Wed, 5 May 2021 14:54:00 -0400
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On 5/5/21 2:11 PM, Waiman Long wrote:
> On 5/5/21 1:30 PM, Roman Gushchin wrote:
>
>>
>> Btw, I wonder if we also need a change in the slab caches merging 
>> procedure?
>> KMALLOC_NORMAL caches should not be merged with caches which can 
>> potentially
>> include accounted objects.
>
> Thank for catching this omission.
>
> I will take a look and modify the merging procedure in a new patch. 
> Accounting is usually specified at kmem_cache_create() time. Though, I 
> did find one instance of setting ACCOUNT flag in kmem_cache_alloc(), I 
> will ignore this case and merge accounted, but unreclaimable caches to 
> KMALLOC_CGROUP. 

In mm/slab_common.c:

#define SLAB_MERGE_SAME (SLAB_RECLAIM_ACCOUNT | SLAB_CACHE_DMA | \
                          SLAB_CACHE_DMA32 | SLAB_ACCOUNT)

struct kmem_cache *find_mergeable(unsigned int size, unsigned int align,
   :
                 if ((flags & SLAB_MERGE_SAME) != (s->flags & 
SLAB_MERGE_SAME))
                         continue;

By making sure kmalloc-cg-* has SLAB_ACCOUNT bit set, a kmemcache 
created with with SLAB_ACCOUNT may merge with kmalloc-cg-* whereas one 
without SLAB_ACCOUNT may merge with kmalloc-* for now. So the current 
code should work fine for most cases. Though, if the ACCOUNT flag is set 
at kmem_cache_alloc() and the cache happens to be merged into kmalloc-*, 
we will have the rare case that an objcg pointer array may have to be 
added to a kmalloc-* cache. However, this is not a common practice, and 
the three cases (not one, sorry) that I found so far is in

arch/x86/kvm/x86.c:     ctxt = kmem_cache_zalloc(x86_emulator_cache, 
GFP_KERNEL_ACCOUNT);
fs/hostfs/hostfs_kern.c:        hi = 
kmem_cache_alloc(hostfs_inode_cache, GFP_KERNEL_ACCOUNT);
virt/kvm/kvm_main.c:    vcpu = kmem_cache_zalloc(kvm_vcpu_cache, 
GFP_KERNEL_ACCOUNT);

We will have to advise against doing that.

Cheers,
Longman