linux-kernel - Re: fd3e45436660 ("ACPI / NUMA: ia64: Parse all entries of SRAT memory affinity table")

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Message-ID: <20180510082219.GH32366@dhcp22.suse.cz>
Date:   Thu, 10 May 2018 10:22:19 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Ganapatrao Kulkarni <gklkml16@...il.com>
Cc:     Ganapatrao Kulkarni <ganapatrao.kulkarni@...ium.com>,
        Tony Luck <tony.luck@...el.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        tiantao6@...wei.com, LKML <linux-kernel@...r.kernel.org>,
        linux-acpi@...r.kernel.org
Subject: Re: fd3e45436660 ("ACPI / NUMA: ia64: Parse all entries of SRAT
 memory affinity table")

On Thu 10-05-18 13:36:11, Ganapatrao Kulkarni wrote:
> On Thu, May 10, 2018 at 1:00 PM, Michal Hocko <mhocko@...nel.org> wrote:
> > On Thu 10-05-18 08:27:35, Ganapatrao Kulkarni wrote:
> >> On Wed, May 9, 2018 at 6:26 PM, Michal Hocko <mhocko@...nel.org> wrote:
> >> > On Wed 09-05-18 18:07:16, Ganapatrao Kulkarni wrote:
> >> >> Hi Michal
> >> >>
> >> >>
> >> >> On Wed, May 9, 2018 at 5:54 PM, Michal Hocko <mhocko@...nel.org> wrote:
> >> >> > On Wed 11-04-18 12:48:32, Michal Hocko wrote:
> >> >> >> Hi,
> >> >> >> my attention was brought to the %subj commit and either I am missing
> >> >> >> something or the patch is quite dubious. What is it actually trying to
> >> >> >> fix? If a BIOS/FW provides more memblocks than the limit then we would
> >> >> >> get misleading numa topology (numactl -H output) but is the situation
> >> >> >> much better with it applied? Numa init code will refuse to init more
> >> >> >> memblocks than the limit and falls back to dummy_numa_init (AFAICS)
> >> >> >> which will break the topology again and numactl -H will have a
> >> >> >> misleading output anyway.
> >> >>
> >> >> IIRC, the MEMBLOCK beyond max limit getting dropped from visible
> >> >> memory(partial drop from a node).
> >> >> this patch removed any upper limit on memblocks and allowed to parse
> >> >> all entries of SRAT.
> >> >
> >> > Yeah I've understood that much. My question is, however, why do we care
> >> > about parsing the NUMA topology when we fallback into a single NUMA node
> >> > anyway? Or do I misunderstand the code? I do not have any platform with
> >> > that many memblocks.
> >>
> >> IMHO, this fix is very much logical by removing the SRAT parsing restriction.
> >> below is the crash log which made us to debug and eventually fix with
> >> this patch.
> >
> > Ohh, I am not saying that the current code handles too many memblocks
> > correctly. I just think that your fix is not correct or incomplete at
> > least. Assuming that my understanding is correct which you haven't
> > disputed yet. So can we focus on the proper solution now? Do we actually
> > need the memblock restrictions? We do not need those for reserved
> > memblocks so I do not see any real reason to simply remove the
> > restriction altogether. Have you explored that option?
> 
> my logic was simple, when i added this patch, when the cap on max
> memblocks is arch specific, why to restrict SRAT parsing which is not
> arch specific.

Because there are other parts which are still restricted and that we
want to have all parts in sync.

> other way around argument is, why the restriction added in the first
> place itself!

Exactly. So have you explored that path?
-- 
Michal Hocko
SUSE Labs