linux-kernel - Re: [PATCH 1/1] mm/hugetlb: Make huge_pte_offset() consistent and document behaviour

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Date:   Wed, 26 Jul 2017 13:11:46 +0100
From:   Punit Agrawal <punit.agrawal@....com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        <linux-arch@...r.kernel.org>, <steve.capper@....com>,
        <will.deacon@....com>, <catalin.marinas@....com>,
        <kirill.shutemov@...ux.intel.com>,
        Mike Kravetz <mike.kravetz@...cle.com>
Subject: Re: [PATCH 1/1] mm/hugetlb: Make huge_pte_offset() consistent and document behaviour

Hi Michal,

Michal Hocko <mhocko@...nel.org> writes:

> On Wed 26-07-17 10:50:38, Michal Hocko wrote:
>> On Tue 25-07-17 16:41:14, 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 NULL
>> > in the case of p*d_none() and a pointer to the pte_t for hugepage or
>> > swap entries.
>> > 
>> > 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().
>> 
>> hugetlb pte semantic is a disaster and I agree it could see some
>> cleanup/clarifications but I am quite nervous to see a patchi like this.
>> How do we check that nothing will get silently broken by this change?

Glad I'm not the only one who finds the hugetlb semantics somewhat
confusing. :)

I've been running tests from mce-test suite and libhugetlbfs for similar
changes we did on arm64. There could be assumptions that were not
exercised but I'm not sure how to check for all the possible usages.

Do you have any other suggestions that can help improve confidence in
the patch?

>
> Forgot to add. Hugetlb have been special because of the pte sharing. I
> haven't looked into that code for quite some time but there might be a
> good reason why pud behave differently.

I checked the code and don't see anything that would explain (or
require) the difference in behaviour.