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

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Message-ID: <6dd6164a-1dd5-46e7-bcf7-b62ff5c6e8ec@arm.com>
Date:   Mon, 4 Dec 2023 12:26:27 +0000
From:   Ryan Roberts <ryan.roberts@....com>
To:     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>,
        David Hildenbrand <david@...hat.com>,
        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

>>>
>>> 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?

Thanks,
Ryan