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

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Date:   Wed, 9 Sep 2020 15:27:57 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Jason Gunthorpe <jgg@...pe.ca>,
        Matthew Wilcox <willy@...radead.org>
Cc:     Zi Yan <ziy@...dia.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 09.09.20 15:14, Jason Gunthorpe wrote:
> On Wed, Sep 09, 2020 at 01:32:44PM +0100, Matthew Wilcox wrote:
> 
>> But here's the thing ... we already allow
>> 	mmap(MAP_POPULATE | MAP_HUGETLB | MAP_HUGE_1GB)
>>
>> So if we're not doing THP, what's the point of this thread?
> 
> I wondered that too..
> 
>> An madvise flag is a different beast; that's just letting the kernel
>> know what the app thinks its behaviour will be.  The kernel can pay
> 
> But madvise is too late, the VMA already has an address, if it is not
> 1G aligned it cannot be 1G THP already.

That's why user space (like QEMU) is THP-aware and selects an address
that is aligned to the expected THP granularity (e.g., 2MB on x86_64).

-- 
Thanks,

David / dhildenb