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

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Date:	Wed, 18 Feb 2015 19:08:12 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	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, aarcange@...hat.com
Subject: Re: [PATCH v2] mm: incorporate zero pages into transparent huge pages

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On 02/18/2015 06:31 PM, Andrew Morton wrote:
> 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?

This is equivalent to an application doing a write fault
to a 2MB area that was previously untouched, going into
do_huge_pmd_anonymous_page() and receiving a 2MB page.

> 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,

The example of directly going into do_huge_pmd_anonymous_page()
is not influenced by the tunable.

It may indeed be undesirable in some situations, but I am
not sure how to detect those...

- -- 
All rights reversed
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