linux-kernel - Re: [RFC PATCH v2 00/30] 1GB PUD THP support on x86

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Message-ID: <20200930115505.GT2277@dhcp22.suse.cz>
Date:   Wed, 30 Sep 2020 13:55:05 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Zi Yan <ziy@...dia.com>
Cc:     linux-mm@...ck.org,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Roman Gushchin <guro@...com>, Rik van Riel <riel@...riel.com>,
        Matthew Wilcox <willy@...radead.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Yang Shi <shy828301@...il.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        David Hildenbrand <david@...hat.com>,
        William Kucharski <william.kucharski@...cle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        David Nellans <dnellans@...dia.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v2 00/30] 1GB PUD THP support on x86_64

On Mon 28-09-20 13:53:58, Zi Yan wrote:
> From: Zi Yan <ziy@...dia.com>
> 
> Hi all,
> 
> This patchset adds support for 1GB PUD THP on x86_64. It is on top of
> v5.9-rc5-mmots-2020-09-18-21-23. It is also available at:
> https://github.com/x-y-z/linux-1gb-thp/tree/1gb_thp_v5.9-rc5-mmots-2020-09-18-21-23
> 
> Other than PUD THP, we had some discussion on generating THPs and contiguous
> physical memory via a synchronous system call [0]. I am planning to send out a
> separate patchset on it later, since I feel that it can be done independently of
> PUD THP support.

While the technical challenges for the kernel implementation can be
discussed before the user API is decided I believe we cannot simply add
something now and then decide about a proper interface. I have raised
few basic questions we should should find answers for before the any
interface is added. Let me copy them here for easier reference
- THP allocation time - #PF and/or madvise context
- lazy/sync instantiation
- huge page sizes controllable by the userspace?
- aggressiveness - how hard to try
- internal fragmentation - allow to create THPs on sparsely or unpopulated
  ranges
- do we need some sort of access control or privilege check as some THPs
  would be a really scarce (like those that require pre-reservation).
-- 
Michal Hocko
SUSE Labs