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Message-ID: <61751d01-2ba4-efc0-9cb8-eeeb3d70908d@nvidia.com>
Date:   Thu, 8 Dec 2022 13:54:27 -0800
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Subject: Re: [PATCH v2 10/10] mm/hugetlb: Document why page_vma_mapped_walk()
 is safe to walk
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To:     Peter Xu <peterx@redhat.com>
CC:     <linux-mm@kvack.org>, <linux-kernel@vger.kernel.org>,
        Jann Horn <jannh@google.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        James Houghton <jthoughton@google.com>,
        Rik van Riel <riel@surriel.com>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Nadav Amit <nadav.amit@gmail.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        David Hildenbrand <david@redhat.com>,
        "Andrew Morton" <akpm@linux-foundation.org>,
        Muchun Song <songmuchun@bytedance.com>
References: <20221207203034.650899-1-peterx@redhat.com>
 <20221207203158.651092-1-peterx@redhat.com>
 <6a970de6-e3da-f57d-14fd-55f65ddcb27d@nvidia.com> <Y5JRhU+9QqoUuSdR@x1n>
From:   John Hubbard <jhubbard@nvidia.com>
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On 12/8/22 13:05, Peter Xu wrote:
>>> +		/*
>>> +		 * NOTE: we don't need explicit lock here to walk the
>>> +		 * hugetlb pgtable because either (1) potential callers of
>>> +		 * hugetlb pvmw currently holds i_mmap_rwsem, or (2) the
>>> +		 * caller will not walk a hugetlb vma (e.g. ksm or uprobe).
>>> +		 * When one day this rule breaks, one will get a warning
>>> +		 * in hugetlb_walk(), and then we'll figure out what to do.
>>> +		 */
>>
>> Confused. Is this documentation actually intended to refer to hugetlb_walk()
>> itself, or just this call site? If the former, then let's move it over
>> to be right before hugetlb_walk().
> 
> It is for this specific code path not hugetlb_walk().
> 
> The "holds i_mmap_rwsem" here is a true statement (not requirement) because
> PVMW rmap walkers always have that.  That satisfies with hugetlb_walk()
> requirements already even without holding the vma lock.
> 

It's really hard to understand. Do you have a few extra words to explain it?
I can help with actual comment wording perhaps, but I am still a bit in
the dark as to the actual meaning. :)

thanks,
-- 
John Hubbard
NVIDIA