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

[Vlastimil Babka <vbabka@suse.cz>]

On 02/19/2015 01:08 AM, Rik van Riel wrote:
> On 02/18/2015 06:31 PM, Andrew Morton wrote:
>> On Wed, 11 Feb 2015 23:03:55 +0200 Ebru Akagunduz
>> <ebru.akagunduz@gmail.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...

Well, yeah. We seem to lack a setting to restrict page fault THP allocations to
e.g. madvise, while still letting khugepaged to collapse them later, taking
khugepaged_max_ptes_none into account.


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