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Date:   Wed, 10 Mar 2021 14:00:18 -0800 (PST)
From:   Hugh Dickins <hughd@google.com>
To:     "Singh, Balbir" <bsingharora@gmail.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, hughd@google.com,
        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
Subject: Re: [PATCH v2 1/2] mm/memcg: rename mem_cgroup_split_huge_fixup to
 split_page_memcg
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Message-ID: <alpine.LSU.2.11.2103101353240.1783@eggly.anvils>
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>
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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.

Hugh