linux-kernel - Re: [PATCH 1/1] mm, compaction: correct the bounds of __fragmentation

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <CB73A16F-5B32-4681-86E3-00786C67ADEF@oracle.com>
Date:   Mon, 19 Feb 2018 12:26:39 +0000
From:   Robert Harris <robert.m.harris@...cle.com>
To:     Mel Gorman <mgorman@...e.de>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, Jonathan Corbet <corbet@....net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Kemi Wang <kemi.wang@...el.com>,
        David Rientjes <rientjes@...gle.com>,
        Yafang Shao <laoar.shao@...il.com>,
        Kangmin Park <l4stpr0gr4m@...il.com>,
        Yisheng Xie <xieyisheng1@...wei.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Huang Ying <ying.huang@...el.com>,
        Vinayak Menon <vinmenon@...eaurora.org>
Subject: Re: [PATCH 1/1] mm, compaction: correct the bounds of
 __fragmentation_index()



> On 19 Feb 2018, at 09:47, Mel Gorman <mgorman@...e.de> wrote:
> 
> On Sun, Feb 18, 2018 at 04:47:55PM +0000, robert.m.harris@...cle.com wrote:
>> From: "Robert M. Harris" <robert.m.harris@...cle.com>
>> 
>> __fragmentation_index() calculates a value used to determine whether
>> compaction should be favoured over page reclaim in the event of allocation
>> failure.  The calculation itself is opaque and, on inspection, does not
>> match its existing description.  The function purports to return a value
>> between 0 and 1000, representing units of 1/1000.  Barring the case of a
>> pathological shortfall of memory, the lower bound is instead 500.  This is
>> significant because it is the default value of sysctl_extfrag_threshold,
>> i.e. the value below which compaction should be avoided in favour of page
>> reclaim for costly pages.
>> 
>> This patch implements and documents a modified version of the original
>> expression that returns a value in the range 0 <= index < 1000.  It amends
>> the default value of sysctl_extfrag_threshold to preserve the existing
>> behaviour.
>> 
>> Signed-off-by: Robert M. Harris <robert.m.harris@...cle.com>
> 
> You have to update sysctl_extfrag_threshold as well for the new bounds.

This patch makes its default value zero.

> It effectively makes it a no-op but it was a no-op already and adjusting
> that default should be supported by data indicating it's safe.

Would it be acceptable to demonstrate using tracing that in both the
pre- and post-patch cases

  1. compaction is attempted regardless of fragmentation index,
     excepting that

  2. reclaim is preferred even for non-zero fragmentation during
     an extreme shortage of memory

?

Robert Harris