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

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Message-ID: <A42BA8431884844BBC20FACB734718294A319F85@FMSMSX106.amr.corp.intel.com>
Date:   Thu, 19 Oct 2017 03:48:09 +0000
From:   "Sandoval Castro, Luis Felipe" 
        <luis.felipe.sandoval.castro@...el.com>
To:     Michal Hocko <mhocko@...nel.org>, Andi Kleen <ak@...ux.intel.com>
CC:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "vbabka@...e.cz" <vbabka@...e.cz>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "rientjes@...gle.com" <rientjes@...gle.com>,
        "n-horiguchi@...jp.nec.com" <n-horiguchi@...jp.nec.com>,
        "salls@...ucsb.edu" <salls@...ucsb.edu>,
        Cristopher Lameter <cl@...ux.com>
Subject: RE: [PATCH v1] mm/mempolicy.c: Fix get_nodes() off-by-one error.

On Tue 18-10-17 10:42:34, Luis Felipe Sandoval Castro wrote:

Sorry for the delayed replay, from your feedback I don't think my
patch has any chances of being merged... I'm wondering though,
if a note in the man pages "range non inclusive" or something
like that would help to avoid confusions? Thanks

> 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

Best Regards,
Luis