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

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Message-ID: <20170726085325.GC2981@dhcp22.suse.cz>
Date:   Wed, 26 Jul 2017 10:53:25 +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 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?

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.
-- 
Michal Hocko
SUSE Labs