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

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Date:   Tue, 8 Sep 2020 15:27:58 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Zi Yan <ziy@...dia.com>
Cc:     David Hildenbrand <david@...hat.com>, Roman Gushchin <guro@...com>,
        "Kirill A. Shutemov" <kirill@...temov.name>, linux-mm@...ck.org,
        Rik van Riel <riel@...riel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        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, Sep 08, 2020 at 10:05:11AM -0400, Zi Yan wrote:
> On 8 Sep 2020, at 7:57, David Hildenbrand wrote:
> > I have concerns if we would silently use 1~GB THPs in most scenarios
> > where be would have used 2~MB THP. I'd appreciate a trigger to
> > explicitly enable that - MADV_HUGEPAGE is not sufficient because some
> > applications relying on that assume that the THP size will be 2~MB
> > (especially, if you want sparse, large VMAs).
> 
> This patchset is not intended to silently use 1GB THP in place of 2MB THP.
> First of all, there is a knob /sys/kernel/mm/transparent_hugepage/enable_1GB
> to enable 1GB THP explicitly. Also, 1GB THP is allocated from a reserved CMA
> region (although I had alloc_contig_pages as a fallback, which can be removed
> in next version), so users need to add hugepage_cma=nG kernel parameter to
> enable 1GB THP allocation. If a finer control is necessary, we can add
> a new MADV_HUGEPAGE_1GB for 1GB THP.

I think we do need that flag.  Machines don't run a single workload
(arguably with VMs, we're getting closer to going back to the single
workload per machine, but that's a different matter).  So if there's
one app that wants 2MB pages and one that wants 1GB pages, we need to
be able to distinguish them.

I could also see there being an app which benefits from 1GB for
one mapping and prefers 2GB for a different mapping, so I think the
per-mapping madvise flag is best.

I'm a little wary of encoding the size of an x86 PUD in the Linux API
though.  Probably best to follow the example set in
include/uapi/asm-generic/hugetlb_encode.h, but I don't love it.  I
don't have a better suggestion though.