linux-kernel - Re: [PATCH v1 4/4] mm/mmu_gather: Store and process pages in contig ranges

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <635de797-1219-40b0-b4b2-7eba758749a5@redhat.com>
Date:   Mon, 4 Dec 2023 13:43:54 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Ryan Roberts <ryan.roberts@....com>, Zi Yan <ziy@...dia.com>,
        Matthew Wilcox <willy@...radead.org>
Cc:     Will Deacon <will@...nel.org>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Nick Piggin <npiggin@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Sven Schnelle <svens@...ux.ibm.com>,
        Arnd Bergmann <arnd@...db.de>, Yu Zhao <yuzhao@...gle.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Yin Fengwei <fengwei.yin@...el.com>,
        Yang Shi <shy828301@...il.com>,
        "Huang, Ying" <ying.huang@...el.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 4/4] mm/mmu_gather: Store and process pages in contig
 ranges

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.

-- 
Cheers,

David / dhildenb