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Subject: Re: [PATCH v2 1/2] mm/memcg: rename mem_cgroup_split_huge_fixup to
 split_page_memcg
To:     Hugh Dickins <hughd@google.com>
Cc:     Michal Hocko <mhocko@suse.com>,
        Zhou Guanghui <zhouguanghui1@huawei.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        akpm@linux-foundation.org, hannes@cmpxchg.org,
        kirill.shutemov@linux.intel.com, npiggin@gmail.com, ziy@nvidia.com,
        wangkefeng.wang@huawei.com, guohanjun@huawei.com,
        dingtianhong@huawei.com, chenweilong@huawei.com,
        rui.xiang@huawei.com
References: <20210304074053.65527-1-zhouguanghui1@huawei.com>
 <20210304074053.65527-2-zhouguanghui1@huawei.com>
 <3ad50842-881f-eb88-a14d-0b6c70ce33d2@gmail.com>
 <YEcxqTUp88sl/cq3@dhcp22.suse.cz>
 <d20d5dbe-74cd-fe90-8e43-ebbc5a3b4403@gmail.com>
 <alpine.LSU.2.11.2103101353240.1783@eggly.anvils>
From:   "Singh, Balbir" <bsingharora@gmail.com>
Message-ID: <f276a028-e2d2-f098-fdf9-6ee505f32e33@gmail.com>
Date:   Thu, 11 Mar 2021 10:50:31 +1100
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On 11/3/21 9:00 am, Hugh Dickins wrote:
> On Thu, 11 Mar 2021, Singh, Balbir wrote:
>> On 9/3/21 7:28 pm, Michal Hocko wrote:
>>> On Tue 09-03-21 09:37:29, Balbir Singh wrote:
>>>> On 4/3/21 6:40 pm, Zhou Guanghui wrote:
>>> [...]
>>>>> -#ifdef CONFIG_TRANSPARENT_HUGEPAGE
>>>>>  /*
>>>>> - * Because page_memcg(head) is not set on compound tails, set it now.
>>>>> + * Because page_memcg(head) is not set on tails, set it now.
>>>>>   */
>>>>> -void mem_cgroup_split_huge_fixup(struct page *head)
>>>>> +void split_page_memcg(struct page *head, unsigned int nr)
>>>>>  {
>>>>
>>>> Do we need input validation on nr? Can nr be aribtrary or can we enforce
>>>>
>>>> VM_BUG_ON(!is_power_of_2(nr));
>>>
>>> In practice this will be power of 2 but why should we bother to sanitze
>>> that? 
>>>
>>
>> Just when DEBUG_VM is enabled to ensure the contract is valid, given that
>> nr is now variable, we could end up with subtle bugs unless we can audit
>> all callers. Even the power of 2 check does not catch the fact that nr
>> is indeed what we expect, but it still checks a large range of invalid
>> inputs.
> 
> I think you imagine this is something it's not.
> 
> "all callers" are __split_huge_page() and split_page() (maybe Matthew
> will have a third caller, maybe not).  It is not something drivers will
> be calling directly themselves, and it won't ever get EXPORTed to them.
> 

Don't feel strongly about it if that is the case.

Thanks,
Balbir Singh