linux-kernel - Re: [PATCH v3 2/6] mm, gup: Ensure real head page is ref-counted when using hugepages

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Date:   Tue, 23 May 2017 16:13:12 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Punit Agrawal <punit.agrawal@....com>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        catalin.marinas@....com, will.deacon@....com,
        n-horiguchi@...jp.nec.com, kirill.shutemov@...ux.intel.com,
        mike.kravetz@...cle.com, steve.capper@....com,
        mark.rutland@....com, hillf.zj@...baba-inc.com,
        linux-arch@...r.kernel.org, aneesh.kumar@...ux.vnet.ibm.com,
        Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH v3 2/6] mm, gup: Ensure real head page is ref-counted
 when using hugepages

On Mon, May 22, 2017 at 02:36:00PM +0100, Punit Agrawal wrote:
> When speculatively taking references to a hugepage using
> page_cache_add_speculative() in gup_huge_pmd(), it is assumed that the
> page returned by pmd_page() is the head page. Although normally true,
> this assumption doesn't hold when the hugepage comprises of successive
> page table entries such as when using contiguous bit on arm64 at PTE or
> PMD levels.
> 
> This can be addressed by ensuring that the page passed to
> page_cache_add_speculative() is the real head or by de-referencing the
> head page within the function.
> 
> We take the first approach to keep the usage pattern aligned with
> page_cache_get_speculative() where users already pass the appropriate
> page, i.e., the de-referenced head.
> 
> Apply the same logic to fix gup_huge_[pud|pgd]() as well.

Hm. Okay. But I'm kinda surprise that this is the only place that need to
be adjusted.

Have you validated all other pmd_page() use-cases?

-- 
 Kirill A. Shutemov