linux-kernel - [PATCH 0/1] Clarify huge_pte

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Date:   Tue, 25 Jul 2017 16:41:13 +0100
From:   Punit Agrawal <punit.agrawal@....com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Punit Agrawal <punit.agrawal@....com>,
        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, Michal Hocko <mhocko@...e.com>,
        Mike Kravetz <mike.kravetz@...cle.com>
Subject: [PATCH 0/1] Clarify huge_pte_offset() semantics

Hi,

The following patch is an attempt to make huge_pte_offset() consistent
when dealing with different levels of the page table and document the
expected semantics. Previously posting can be found at [0].

Changelog

RFC - v1
* Merge Patch 1 and 2 - preserve bisectability
* Drop RFC tag

Original cover letter follows...

The generic implementation of huge_pte_offset() has inconsistent
behaviour when looking up hugepage PUDs vs PMDs entries that are not
present (returning NULL vs pte_t*).

Similarly, it returns NULL when encountering swap entries although all
the callers have special checks to properly deal with swap entries.

Without clear semantics, it is difficult to determine if a change
breaks huge_pte_offset() without going through all the scenarios where
it is used.

I faced this recently when updating the arm64 implementation of
huge_pte_offset() to handle swap entries (related to enabling poisoned
memeory)[1]. And will come across again when I update it for
contiguous hugepage support now that core changes have been merged.

To address these issues, this following patch -

* makes huge_pte_offset() consistent between PUD and PMDs
* and, documents the expected behaviour of huge_pte_offset()

All feedback welcome.

Thanks,
Punit

[0] https://lkml.org/lkml/2017/7/24/514
[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=f02ab08afbe76ee7b0b2a34a9970e7dd200d8b01

Punit Agrawal (1):
  mm/hugetlb: Make huge_pte_offset() consistent and document behaviour

 mm/hugetlb.c | 22 +++++++++++++++++++---
 1 file changed, 19 insertions(+), 3 deletions(-)

-- 
2.11.0