linux-kernel - Re: [PATCH] hugetlb: simplify hugetlb handling in follow_page

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Message-ID: <323fdb0f-c5a5-e0e5-1ff4-ab971bc295cc@redhat.com>
Date:   Fri, 2 Sep 2022 20:52:44 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        inuxppc-dev@...ts.ozlabs.org, linux-ia64@...r.kernel.org,
        Baolin Wang <baolin.wang@...ux.alibaba.com>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>,
        Naoya Horiguchi <naoya.horiguchi@...ux.dev>,
        Michael Ellerman <mpe@...erman.id.au>,
        Muchun Song <songmuchun@...edance.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christophe Leroy <christophe.leroy@...roup.eu>
Subject: Re: [PATCH] hugetlb: simplify hugetlb handling in follow_page_mask

>>> Adding Christophe on Cc:
>>>
>>> Christophe do you know if is_hugepd is true for all hugetlb entries, not
>>> just hugepd?
>>>
>>> On systems without hugepd entries, I guess ptdump skips all hugetlb entries.
>>> Sigh!
>>
>> IIUC, the idea of ptdump_walk_pgd() is to dump page tables even outside
>> VMAs (for debugging purposes?).
>>
>> I cannot convince myself that that's a good idea when only holding the
>> mmap lock in read mode, because we can just see page tables getting
>> freed concurrently e.g., during concurrent munmap() ... while holding
>> the mmap lock in read we may only walk inside VMA boundaries.
>>
>> That then raises the questions if we're only calling this on special MMs
>> (e.g., init_mm) whereby we cannot really see concurrent munmap() and
>> where we shouldn't have hugetlb mappings or hugepd entries.
>>
> 
> This is going to require a little more thought.
> 
> Since Baolin's patch for stable releases is moving forward, I want to
> get the cleanup provided by this patch in ASAP.  So, I am going to rebase
> this patch on Baolin's with the other fixups.
> 
> Will come back to this cleanup later.

Sure, no need to do it all at once (I was just bringing it up while
thinking about it).

-- 
Thanks,

David / dhildenb