linux-kernel - Re: [PATCH 1/2] mm, vmstat: hide /proc/pagetypeinfo from normal users

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Date:   Fri, 25 Oct 2019 14:58:26 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Michal Hocko <mhocko@...nel.org>, Mel Gorman <mgorman@...e.de>,
        Waiman Long <longman@...hat.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <guro@...com>,
        Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
        Jann Horn <jannh@...gle.com>, Song Liu <songliubraving@...com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Rafael Aquini <aquini@...hat.com>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>,
        Michal Hocko <mhocko@...e.com>,
        David Rientjes <rientjes@...gle.com>,
        stable <stable@...r.kernel.org>
Subject: Re: [PATCH 1/2] mm, vmstat: hide /proc/pagetypeinfo from normal
 users

On Fri, 25 Oct 2019 09:33:26 +0200 Vlastimil Babka <vbabka@...e.cz> wrote:

> On 10/25/19 9:26 AM, Michal Hocko wrote:
> > From: Michal Hocko <mhocko@...e.com>
> > 
> > /proc/pagetypeinfo is a debugging tool to examine internal page
> > allocator state wrt to fragmentation. It is not very useful for
> > any other use so normal users really do not need to read this file.
> > 
> > Waiman Long has noticed that reading this file can have negative side
> > effects because zone->lock is necessary for gathering data and that
> > a) interferes with the page allocator and its users and b) can lead to
> > hard lockups on large machines which have very long free_list.
> > 
> > Reduce both issues by simply not exporting the file to regular users.
> > 
> > Reported-by: Waiman Long <longman@...hat.com>
> > Cc: stable
> 
> Cc: <stable@...r.kernel.org>

As we don't really know how much damage this will cause, it would be
nice to let it bake in mainline for a month or three before committing
it to the -stable trees.  But we don't have a process for that, apart
from remembering to poke Greg at a suitable date.

Oh well.  I guess that if someone is truly harmed by this change, they
can just chmod /proc/pagetypeinfo back to 0444.