linux-kernel - Re: [PATCH v2] x86/mm: use max memory block size on bare metal

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Message-ID: <20200619120704.GD12177@dhcp22.suse.cz>
Date:   Fri, 19 Jun 2020 14:07:04 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Daniel Jordan <daniel.m.jordan@...cle.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        David Hildenbrand <david@...hat.com>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Sistare <steven.sistare@...cle.com>
Subject: Re: [PATCH v2] x86/mm: use max memory block size on bare metal

On Tue 09-06-20 18:54:51, Daniel Jordan wrote:
[...]
> @@ -1390,6 +1391,15 @@ static unsigned long probe_memory_block_size(void)
>  		goto done;
>  	}
>  
> +	/*
> +	 * Use max block size to minimize overhead on bare metal, where
> +	 * alignment for memory hotplug isn't a concern.

This really begs a clarification why this is not a concern. Bare metal
can see physical memory hotadd as well. I just suspect that you do not
consider that to be very common so it is not a big deal? And I would
tend to agree but still we are just going to wait until first user
stumbles over this.

Btw. memblock interface just doesn't scale and it is a terrible
interface for large machines and for the memory hotplug in general (just
look at ppc and their insanely small memblocks).

Most usecases I have seen simply want to either offline some portion of
memory without a strong requirement of the physical memory range as long
as it is from a particular node or simply offline and remove the full
node.

I believe that we should think about a future interface rather than
trying to ducktape the blocksize anytime it causes problems. I would be
even tempted to simply add a kernel command line option 
memory_hotplug=disable,legacy,new_shiny

for disable it would simply drop all the sysfs crud and speed up boot
for most users who simply do not care about memory hotplug. new_shiny
would ideally provide an interface that would either export logically
hotplugable memory ranges (e.g. DIMMs) or a query/action interface which
accepts physical ranges as input. Having gazillions of sysfs files is
simply unsustainable.
-- 
Michal Hocko
SUSE Labs