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

From: Rik van Riel
Date: Wed Feb 18 2015 - 19:08:40 EST

Hash: SHA1

On 02/18/2015 06:31 PM, Andrew Morton wrote:
> On Wed, 11 Feb 2015 23:03:55 +0200 Ebru Akagunduz
> <ebru.akagunduz@xxxxxxxxx> 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...

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