linux-kernel - Re: [PATCH] mm/numa_balancing: Teach mpol_to

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Date: Fri, 28 Jun 2024 11:12:42 +0800
From: "Huang, Ying" <ying.huang@...el.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: Tvrtko Ursulin <tursulin@...lia.com>,  linux-mm@...ck.org,
  linux-kernel@...r.kernel.org,  kernel-dev@...lia.com,  Tvrtko Ursulin
 <tvrtko.ursulin@...lia.com>,  Mel Gorman <mgorman@...e.de>,  Peter
 Zijlstra <peterz@...radead.org>,  Ingo Molnar <mingo@...hat.com>,  Rik van
 Riel <riel@...riel.com>,  Johannes Weiner <hannes@...xchg.org>,  Dave
 Hansen <dave.hansen@...el.com>,  Andi Kleen <ak@...ux.intel.com>,  Michal
 Hocko <mhocko@...e.com>,  David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH] mm/numa_balancing: Teach mpol_to_str about the
 balancing mode

Hi, Matthew,

Matthew Wilcox <willy@...radead.org> writes:

> On Tue, Jun 25, 2024 at 02:26:05PM +0100, Tvrtko Ursulin wrote:
>>  		/*
>> -		 * Currently, the only defined flags are mutually exclusive
>> +		 * The below two flags are mutually exclusive:
>>  		 */
>>  		if (flags & MPOL_F_STATIC_NODES)
>>  			p += snprintf(p, buffer + maxlen - p, "static");
>>  		else if (flags & MPOL_F_RELATIVE_NODES)
>>  			p += snprintf(p, buffer + maxlen - p, "relative");
>> +
>> +		if (flags & MPOL_F_NUMA_BALANCING)
>> +			p += snprintf(p, buffer + maxlen - p, "balancing");
>>  	}
>
> So if MPOL_F_STATIC_NODES and MPOL_F_NUMA_BALANCING are set, then we
> get a string "staticbalancing"?  Is that intended?
>
> Or are these three all mutually exclusive and that should have been
> as "else if"?

Yes, this is an issue!

Dig the git history, in commit 2291990ab36b ("mempolicy: clean-up
mpol-to-str() mempolicy formatting"), the support for multiple flags are
removed.  I think that we need to restore it.

Done some basic testing.  It was found that when MPOL_F_NUMA_BALANCING
is set, /proc/PID/numa_maps always display "default".  That is wrong.

This make me think that this patch has never been tested!

The "default" displaying is introduced in commit 8790c71a18e5
("mm/mempolicy.c: fix mempolicy printing in numa_maps").  We need to fix
it firstly for MPOL_F_NUMA_BALANCING with more accurate filtering.  The
fix needs to be backported to -stable kernel.

--
Best Regards,
Huang, Ying