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 16:43:50 +0100
From:   Punit Agrawal <punit.agrawal@....com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
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

"Kirill A. Shutemov" <kirill@...temov.name> writes:

> 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?

I came across the gup issues were found while investigating a failing
test from mce-tests.

I think the problem here is not due to the use of pmd_page() but because
page_cache_[add|get]_speculative() don't ensure they ref-count the head
page as is done in get_page().

Having said that, I had a quick look at the other uses of pmd_page() -

Quite a few of them are followed by an explicit BUG_ON() to check that
the page returned is a head page. All other instances seem to be dealing
with transparent hugepages where contiguous hugepages are not supported.

I don't see any call sites that ring alarm bells.

Did you have any particular part of the code in mind where pmd_page()
usage might be a problem?

Thanks,
Punit