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From:   Waiman Long <llong@redhat.com>
Subject: Re: [PATCH v2 1/2] mm: memcg/slab: Properly set up gfp flags for
 objcg pointer array
To:     Shakeel Butt <shakeelb@google.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>, Roman Gushchin <guro@fb.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Cgroups <cgroups@vger.kernel.org>, Linux MM <linux-mm@kvack.org>
References: <20210504132350.4693-1-longman@redhat.com>
 <20210504132350.4693-2-longman@redhat.com>
 <CALvZod438=YKZtV0qckoaMkdL1seu5PiLnvPPQyRzA0S60-TpQ@mail.gmail.com>
Message-ID: <267501a0-f416-4058-70d3-e32eeec3d6da@redhat.com>
Date:   Tue, 4 May 2021 16:02:14 -0400
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On 5/4/21 3:37 PM, Shakeel Butt wrote:
> On Tue, May 4, 2021 at 6:24 AM Waiman Long <longman@redhat.com> wrote:
>> 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.
>>
>> Fixes: 286e04b8ed7a ("mm: memcg/slab: allocate obj_cgroups for non-root slab pages")
>> Signed-off-by: Waiman Long <longman@redhat.com>
>> ---
>>   mm/memcontrol.c | 8 ++++++++
>>   mm/slab.h       | 1 -
>>   2 files changed, 8 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index c100265dc393..5e3b4f23b830 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -2863,6 +2863,13 @@ static struct mem_cgroup *get_mem_cgroup_from_objcg(struct obj_cgroup *objcg)
>>   }
>>
>>   #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)
> What about __GFP_DMA32? Does it matter? It seems like DMA32 requests
> go to normal caches.

I included __GFP_DMA32 in my first draft patch. However, __GFP_DMA32 is 
not considered in determining the right kmalloc_type() (patch 2), so I 
took it out to make it consistent. I can certainly add it back.

Cheers,
Longman