linux-kernel - Re: [RFC PATCH 00/16] 1GB THP support on x86

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Date:   Wed, 9 Sep 2020 09:04:45 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Rik van Riel <riel@...riel.com>
Cc:     Zi Yan <ziy@...dia.com>, David Hildenbrand <david@...hat.com>,
        Roman Gushchin <guro@...com>,
        "Kirill A. Shutemov" <kirill@...temov.name>, linux-mm@...ck.org,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        David Nellans <dnellans@...dia.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 00/16] 1GB THP support on x86_64

On Tue 08-09-20 10:41:10, Rik van Riel wrote:
> On Tue, 2020-09-08 at 16:35 +0200, Michal Hocko wrote:
> 
> > A global knob is insufficient. 1G pages will become a very precious
> > resource as it requires a pre-allocation (reservation). So it really
> > has
> > to be an opt-in and the question is whether there is also some sort
> > of
> > access control needed.
> 
> The 1GB pages do not require that much in the way of
> pre-allocation. The memory can be obtained through CMA,
> which means it can be used for movable 4kB and 2MB
> allocations when not
> being used for 1GB pages.

That CMA has to be pre-reserved, right? That requires a configuration.

> That makes it relatively easy to set aside
> some fraction
> of system memory in every system for 1GB and movable
> allocations, and use it for whatever way it is needed
> depending on what workload(s) end up running on a system.

I was not talking about how easy or hard it is. My main concern is that
this is effectively a pre-reserved pool and a global knob is a very
suboptimal way to control access to it. I (rather) strongly believe this
should be an explicit opt-in and ideally not 1GB specific but rather
something to allow large pages to be created as there is a fit. See
other subthread for more details.

-- 
Michal Hocko
SUSE Labs