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

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Date:   Fri, 13 Oct 2017 10:04:03 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Andi Kleen <ak@...ux.intel.com>
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 12-10-17 08:28:25, Andi Kleen wrote:
> 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.

I must be missing something because libnuma does
if (set_mempolicy(policy, bmp->maskp, bmp->size + 1) < 0)

so it sets max as size + 1 which is exactly what the man page describes.

> Your change would be catastrophic.

I am not sure which change do you mean here. I wasn't proposing any
patch (yet). All I was saying is that the docuementation diagrees with
the in kernel implementation. The only applications that would break
would be those which do not comply to the documentation AFAICS, no?
-- 
Michal Hocko
SUSE Labs