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Message-ID: <83497a70-45c3-4386-8994-92207ab521bd@arm.com>
Date:   Mon, 4 Dec 2023 12:57:05 +0000
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Subject: Re: [PATCH v1 4/4] mm/mmu_gather: Store and process pages in contig
 ranges
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To:     David Hildenbrand <david@redhat.com>, Zi Yan <ziy@nvidia.com>,
        Matthew Wilcox <willy@infradead.org>
Cc:     Will Deacon <will@kernel.org>,
        "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Nick Piggin <npiggin@gmail.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Christian Borntraeger <borntraeger@linux.ibm.com>,
        Sven Schnelle <svens@linux.ibm.com>,
        Arnd Bergmann <arnd@arndb.de>, Yu Zhao <yuzhao@google.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Yin Fengwei <fengwei.yin@intel.com>,
        Yang Shi <shy828301@gmail.com>,
        "Huang, Ying" <ying.huang@intel.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
References: <20230810103332.3062143-1-ryan.roberts@arm.com>
 <20230810103332.3062143-5-ryan.roberts@arm.com>
 <800937DA-BAD0-4C60-B155-AECCA21E955E@nvidia.com>
 <1a0f5cb8-421c-4f28-a986-f3c381406e81@arm.com>
 <90EC4C0D-0254-4B93-AFD5-3C09580A77DE@nvidia.com>
 <6dd6164a-1dd5-46e7-bcf7-b62ff5c6e8ec@arm.com>
 <890f4455-bf1b-437b-a355-7895e4dd3085@redhat.com>
 <52b042b9-ec95-4db0-b38a-f7f1cea0b90c@arm.com>
 <635de797-1219-40b0-b4b2-7eba758749a5@redhat.com>
From:   Ryan Roberts <ryan.roberts@arm.com>
In-Reply-To: <635de797-1219-40b0-b4b2-7eba758749a5@redhat.com>
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On 04/12/2023 12:43, David Hildenbrand wrote:
> On 04.12.23 13:39, Ryan Roberts wrote:
>> On 04/12/2023 12:28, David Hildenbrand wrote:
>>> On 04.12.23 13:26, Ryan Roberts wrote:
>>>>>>>
>>>>>>> Also, struct page (memmap) might not be always contiguous, using struct page
>>>>>>> points to represent folio range might not give the result you want.
>>>>>>> See nth_page() and folio_page_idx() in include/linux/mm.h.
>>>>>>
>>>>>> Is that true for pages within the same folio too? Or are all pages in a folio
>>>>>> guarranteed contiguous? Perhaps I'm better off using pfn?
>>>>>
>>>>> folio_page_idx() says not all pages in a folio is guaranteed to be contiguous.
>>>>> PFN might be a better choice.
>>>>
>>>> Hi Zi, Matthew,
>>>>
>>>> Zi made this comment a couple of months back that it is incorrect to assume
>>>> that
>>>> `struct page`s within a folio are (virtually) contiguous. I'm not sure if
>>>> that's
>>>> really the case though? I see other sites in the source that do page++ when
>>>> iterating over a folio. e.g. smaps_account(), splice_folio_into_pipe(),
>>>> __collapse_huge_page_copy(), etc.
>>>>
>>>> Any chance someone could explain the rules?
>>>
>>> With the vmemmap, they are contiguous. Without a vmemmap, but with sparsemem, we
>>> might end up allocating one memmap chunk per memory section (e.g., 128 MiB).
>>>
>>> So, for example, a 1 GiB hugetlb page could cross multiple 128 MiB sections, and
>>> therefore, the memmap might not be virtually consecutive.
>>
>> OK, is a "memory section" always 128M or is it variable? If fixed, does that
>> mean that it's impossible for a THP to cross section boundaries? (because a THP
>> is always smaller than a section?)
> 
> Section size is variable (see SECTION_SIZE_BITS), but IIRC, buddy allocations
> will never cross them.
> 
>>
>> Trying to figure out why my original usage in this series was wrong, but
>> presumably the other places that I mentioned are safe.
> 
> If only dealing with buddy allocations, *currently* it might always fall into a
> single memory section.

OK that makes sense - thanks!

>