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 14:33:57 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Punit Agrawal <punit.agrawal@....com>
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

On Wed 26-07-17 13:11:46, Punit Agrawal wrote:
> 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. :)

This is a huge understatement. It is a source of nightmares.

> 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?

Unfortunatelly I don't. I just know there were many subtle assumptions
all over the place so I am rather careful to not touch the code unless
really necessary.

That being said, I am not opposing your patch.

-- 
Michal Hocko
SUSE Labs