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

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Message-Id: <4D23D83F-190F-40B3-9EB9-C167642321B2@lca.pw>
Date:   Thu, 24 Oct 2019 01:33:01 -0400
From:   Qian Cai <cai@....pw>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Waiman Long <longman@...hat.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.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 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 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.