linux-kernel - Re: [PATCH v3 3/5] mm,memory_hotplug: Add kernel boot option to enable memmap_on

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Message-ID: <20210308154603.GA23272@linux>
Date:   Mon, 8 Mar 2021 16:46:07 +0100
From:   Oscar Salvador <osalvador@...e.de>
To:     David Hildenbrand <david@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Pavel Tatashin <pasha.tatashin@...een.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 3/5] mm,memory_hotplug: Add kernel boot option to
 enable memmap_on_memory

On Fri, Mar 05, 2021 at 04:35:08PM +0100, David Hildenbrand wrote:
> On 04.03.21 11:00, Oscar Salvador wrote:
> > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > index 04545725f187..e626dab39c60 100644
> > --- a/Documentation/admin-guide/kernel-parameters.txt
> > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > @@ -2794,6 +2794,20 @@
> >   			seconds.  Use this parameter to check at some
> >   			other rate.  0 disables periodic checking.
> > +	memory_hotplug.memmap_on_memory
> > +			[KNL,X86,ARM] Boolean flag to enable this feature.
> 
> Right now it can be set on any arch with memory hotplug, right? It's simply
> not effective.

Right.

> > +			Format: {on | off (default)}
> > +			When enabled, memory to build the pages tables for the
> > +			memmap array describing the hot-added range will be taken
> > +			from the range itself, so the memmap page tables will be
> > +			self-hosted.
> > +			Since only single memory device ranges are supported at
> > +			the moment, this option is disabled by default because
> > +			it might have an impact on workloads that needs large
> > +			contiguous memory chunks.
> > +			The state of the flag can be read in
> > +			/sys/module/memory_hotplug/parameters/memmap_on_memory.
> 
> Maybe want to add that even if enabled, there are cases where it is not
> effective?

Sure, I already added a hint.

> > +static bool memmap_on_memory __ro_after_init;
> > +module_param(memmap_on_memory, bool, 0444);
> > +MODULE_PARM_DESC(memmap_on_memory, "Enable memmap on memory for memory hotplug");
> 
> Wondering if this makes sense getting wrapped in
> 
> #ifdef CONFIG MHP_MEMMAP_ON_MEMORY
> 
> just a thought.

Definitely, will wrapp it with MHP_MEMMAP_ON_MEMORY.

> 
> LGTM
> 
> Reviewed-by: David Hildenbrand <david@...hat.com>

Thanks!


-- 
Oscar Salvador
SUSE L3