linux-kernel - Re: [PATCH v2] mm: incorporate zero pages into transparent huge 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]

Date:	Wed, 18 Feb 2015 15:31:19 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Ebru Akagunduz <ebru.akagunduz@...il.com>
Cc:	linux-mm@...ck.org, kirill@...temov.name, mhocko@...e.cz,
	mgorman@...e.de, rientjes@...gle.com, sasha.levin@...cle.com,
	hughd@...gle.com, hannes@...xchg.org, vbabka@...e.cz,
	linux-kernel@...r.kernel.org, riel@...hat.com, aarcange@...hat.com
Subject: Re: [PATCH v2] mm: incorporate zero pages into transparent huge
 pages

On Wed, 11 Feb 2015 23:03:55 +0200 Ebru Akagunduz <ebru.akagunduz@...il.com> wrote:

> This patch improves THP collapse rates, by allowing zero pages.
> 
> Currently THP can collapse 4kB pages into a THP when there
> are up to khugepaged_max_ptes_none pte_none ptes in a 2MB
> range.  This patch counts pte none and mapped zero pages
> with the same variable.

So if I'm understanding this correctly, with the default value of
khugepaged_max_ptes_none (HPAGE_PMD_NR-1), if an application creates a
2MB area which contains 511 mappings of the zero page and one real
page, the kernel will proceed to turn that area into a real, physical
huge page.  So it consumes 2MB of memory which would not have
previously been allocated?

If so, this might be rather undesirable behaviour in some situations
(and ditto the current behaviour for pte_none ptes)?

This can be tuned by adjusting khugepaged_max_ptes_none, but not many
people are likely to do that because we didn't document the damn thing.
 At all.  Can we please rectify this, and update it for the is_zero_pfn
feature?  The documentation should include an explanation telling
people how to decide what setting to use, how to observe its effects,
etc.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/