linux-kernel - Re: [PATCH v2] mm/hugetlb.c: make huge_pte_offset() consistent and document behaviour

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Message-ID: <20170822101117.ilnys32tugytbbjc@armageddon.cambridge.arm.com>
Date:   Tue, 22 Aug 2017 11:11:18 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Punit Agrawal <punit.agrawal@....com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Steve Capper <steve.capper@....com>,
        Will Deacon <will.deacon@....com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Michal Hocko <mhocko@...e.com>,
        Mike Kravetz <mike.kravetz@...cle.com>
Subject: Re: [PATCH v2] mm/hugetlb.c: make huge_pte_offset() consistent and
 document behaviour

On Fri, Aug 18, 2017 at 03:54:15PM +0100, Punit Agrawal wrote:
> When walking the page tables to resolve an address that points to
> !p*d_present() entry, huge_pte_offset() returns inconsistent values
> depending on the level of page table (PUD or PMD).
> 
> It returns NULL in the case of a PUD entry while in the case of a PMD
> entry, it returns a pointer to the page table entry.
> 
> A similar inconsitency exists when handling swap entries - returns NULL
> for a PUD entry while a pointer to the pte_t is retured for the PMD entry.
> 
> Update huge_pte_offset() to make the behaviour consistent - return a
> pointer to the pte_t for hugepage or swap entries. Only return NULL in
> instances where we have a p*d_none() entry and the size parameter
> doesn't match the hugepage size at this level of the page table.
> 
> Document the behaviour to clarify the expected behaviour of this function.
> This is to set clear semantics for architecture specific implementations
> of huge_pte_offset().
> 
> Signed-off-by: Punit Agrawal <punit.agrawal@....com>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Naoya Horiguchi <n-horiguchi@...jp.nec.com>
> Cc: Steve Capper <steve.capper@....com>
> Cc: Will Deacon <will.deacon@....com>
> Cc: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> Cc: Michal Hocko <mhocko@...e.com>
> Cc: Mike Kravetz <mike.kravetz@...cle.com>

FWIW:

Reviewed-by: Catalin Marinas <catalin.marinas@....com>

Thanks.