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

[Rik van Riel]

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On 02/10/2015 04:06 PM, Andrea Arcangeli wrote:
> On Tue, Feb 10, 2015 at 12:47:37AM +0200, Ebru Akagunduz 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.
>> 
>> The patch was tested with a program that allocates 800MB of 
>> memory, and performs interleaved reads and writes, in a pattern 
>> that causes some 2MB areas to first see read accesses, resulting 
>> in the zero pfn being mapped there.
>> 
>> To simulate memory fragmentation at allocation time, I modified 
>> do_huge_pmd_anonymous_page to return VM_FAULT_FALLBACK for read 
>> faults.
>> 
>> Without the patch, only %50 of the program was collapsed into THP
>> and the percentage did not increase over time.
>> 
>> With this patch after 10 minutes of waiting khugepaged had 
>> collapsed %89 of the program's memory.
> 
> This is very good idea, associating it with the sysctl is sensible 
> here as collapsing zeropages would affect the memory footprint in
> the same way as none ptes.
> 
> __collapse_huge_page_copy however is likely screwing with the 
> refcounts of the zero page. Did you have DEBUG_VM=y enabled? If
> yes you should get one warning that the zeropage refcount
> underflowed that could confirm my concern:

In __collapse_huge_page_copy, the zero pte takes the same path
as pte_none, so I believe that part of the code is correct.

> So in short I think __collapse_huge_page_copy and
> release_pte_pages needs an additional case that complements the
> already existing special

You are right that release_pte_pages needs a special case too,
in order to skip refcounting on the zero page.

Ebru?

- -- 
All rights reversed
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