linux-kernel - Re: [PATCH v1] mm/mempolicy.c: Fix get

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Date:   Thu, 12 Oct 2017 08:28:25 -0700
From:   Andi Kleen <ak@...ux.intel.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Luis Felipe Sandoval Castro 
        <luis.felipe.sandoval.castro@...el.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        vbabka@...e.cz, mingo@...nel.org, rientjes@...gle.com,
        n-horiguchi@...jp.nec.com, salls@...ucsb.edu,
        Cristopher Lameter <cl@...ux.com>
Subject: Re: [PATCH v1] mm/mempolicy.c: Fix get_nodes() off-by-one error.

On Thu, Oct 12, 2017 at 10:46:33AM +0200, Michal Hocko wrote:
> [CC Christoph who seems to be the author of the code]

Actually you can blame me. I did the mistake originally.
It was found many years ago, but then it was already too late
to change.

> Andi has voiced a concern about backward compatibility but I am not sure
> the risk is very high. The current behavior is simply broken unless you
> use a large maxnode anyway. What kind of breakage would you envision
> Andi?

libnuma uses the available number of nodes as max. 

So it would always lose the last one with your chance.

Your change would be catastrophic.

The only way to fix it really would be to define
a new syscall. But I don't think it is needed, 
the existing maxnode+1 interface works
(just should be properly documented)

-Andi