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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <D4359A94-6E95-4D89-B9F3-7A6CDB50C0A1@oracle.com>
Date:   Tue, 15 May 2018 00:59:16 -0600
From:   William Kucharski <william.kucharski@...cle.com>
To:     Christopher Lameter <cl@...ux.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 May 14, 2018, at 9:19 AM, Christopher Lameter <cl@...ux.com> wrote:
> 
> 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 directly.

It isn't presently but certainly could be; for the prototype it tries to
map a large page when needed and, should that fail, it will fall through
to the normal fault around code.

I would think we would want a separate parameter, as I can see the usefulness
of more fine-grained control. Many users may want to try mapping a large page
if possible, but would prefer a smaller number of bytes to be read in fault
around should we need to fall back to using PAGESIZE pages.

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

I agree; perhaps I could use "fault_around_thp_bytes" or something similar.

>> 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.

That is my ultimate long-term goal for this project - full r/w support of large
THP pages; prototyping with read-only text pages seemed like the best first step
to get a sense of the possible benefits.

  -- Bill