linux-kernel - Re: [RFC] mm, THP: Map read-only text segments using large THP pages

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Message-ID: <010001635f3c42d3-ed92871f-4fba-47dc-9750-69a40dd07ab6-000000@email.amazonses.com>
Date:   Mon, 14 May 2018 15:19:34 +0000
From:   Christopher Lameter <cl@...ux.com>
To:     William Kucharski <william.kucharski@...cle.com>
cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC] mm, THP: Map read-only text segments using large THP
 pages

On Mon, 14 May 2018, William Kucharski wrote:

> The idea is that the kernel will attempt to allocate and map the range using a
> PMD sized THP page upon first fault; if the allocation is successful the page
> will be populated (at present using a call to kernel_read()) and the page will
> be mapped at the PMD level. If memory allocation fails, the page fault routines
> will drop through to the conventional PAGESIZE-oriented routines for mapping
> the faulting page.

Cool. This could be controlled by the faultaround logic right? If we get
fault_around_bytes up to huge page size then it is reasonable to use a
huge page direcly.

fault_around_bytes can be set via sysfs so there is a natural way to
control this feature there I think.


> Since this approach will map a PMD size block of the memory map at a time, we
> should see a slight uptick in time spent in disk I/O but a substantial drop in
> page faults as well as a reduction in iTLB misses as address ranges will be
> mapped with the larger page. Analysis of a test program that consists of a very
> large text area (483,138,032 bytes in size) that thrashes D$ and I$ shows this
> does occur and there is a slight reduction in program execution time.

I think we would also want such a feature for regular writable pages as
soon as possible.