linux-kernel - Re: [PATCH] mm/vmstat: Reduce zone lock hold time when reading /proc/pagetypeinfo

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Message-ID: <20191024074205.GQ17610@dhcp22.suse.cz>
Date:   Thu, 24 Oct 2019 09:42:05 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Qian Cai <cai@....pw>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Waiman Long <longman@...hat.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <guro@...com>, Vlastimil Babka <vbabka@...e.cz>,
        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>
Subject: Re: [PATCH] mm/vmstat: Reduce zone lock hold time when reading
 /proc/pagetypeinfo

On Thu 24-10-19 01:33:01, Qian Cai wrote:
> 
> 
> > On Oct 23, 2019, at 6:30 PM, Andrew Morton <akpm@...ux-foundation.org> wrote:
> > 
> > Yes, removing things is hard.  One approach is to add printk_once(this
> > is going away, please email us if you use it) then wait a few years. 
> > Backport that one-liner into -stable kernels to hopefully speed up the
> > process.
> 
> Although it still look like an overkill to me given,
> 
> 1) Mel given a green light to remove it.
> 2) Nobody justifies any sensible reason to keep it apart from it was
> probably only triggering by some testing tools blindly read procfs
> entries.

It's been useful for debugging memory fragmentation problems and we do
not have anything that would provide a similar information. Considering
that making it root only is trivial and reducing the lock hold times
likewise I do not really see any strong reason to dump it at this
moment.
 
> it is still better than wasting developers’ time to beating the “dead” horse.
> 
> > 
> > Meanwhile, we need to fix the DoS opportunity.  How about my suggestion
> > that we limit the count to 1024, see if anyone notices?  I bet they
> > don't!
> 
> The DoS is probably there since the file had been introduced almost 10
> years ago, so I suspect it is not that easily exploitable.

Yes you need _tons_ of memory. Reading the file on my 3TB system takes
sys     0m3.673s

The situation might be worse if the system is terribly fragmented which
is not the case here.
-- 
Michal Hocko
SUSE Labs