linux-kernel - Re: [PATCH v4] mm: incorporate read-only pages into transparent huge pages

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Date:	Fri, 30 Jan 2015 09:34:32 +0100
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Ebru Akagunduz <ebru.akagunduz@...il.com>, linux-mm@...ck.org
CC:	akpm@...ux-foundation.org, kirill@...temov.name, mhocko@...e.cz,
	mgorman@...e.de, rientjes@...gle.com, sasha.levin@...cle.com,
	hughd@...gle.com, hannes@...xchg.org, linux-kernel@...r.kernel.org,
	riel@...hat.com, aarcange@...hat.com, zhangyanfei.linux@...yun.com
Subject: Re: [PATCH v4] mm: incorporate read-only pages into transparent huge
 pages

On 01/29/2015 03:59 PM, Ebru Akagunduz wrote:
> This patch aims to improve THP collapse rates, by allowing
> THP collapse in the presence of read-only ptes, like those
> left in place by do_swap_page after a read fault.
> 
> 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 applies the same limit for
> read-only ptes.
> 
> The patch was tested with a test program that allocates
> 800MB of memory, writes to it, and then sleeps. I force
> the system to swap out all but 190MB of the program by
> touching other memory. Afterwards, the test program does
> a mix of reads and writes to its memory, and the memory
> gets swapped back in.
> 
> Without the patch, only the memory that did not get
> swapped out remained in THPs, which corresponds to 24% of
> the memory of the program. The percentage did not increase
> over time.
> 
> With this patch, after 5 minutes of waiting khugepaged had
> collapsed 60% of the program's memory back into THPs.
> 
> Signed-off-by: Ebru Akagunduz <ebru.akagunduz@...il.com>
> Signed-off-by: Ebru Akagunduz <ebru.akagunduz@...il.com>
> Reviewed-by: Rik van Riel <riel@...hat.com>
> Acked-by: Vlastimil Babka <vbabka@...e.cz>
> Acked-by: Zhang Yanfei <zhangyanfei@...fujitsu.com>
> ---
> Changes in v2:
>  - Remove extra code indent (Vlastimil Babka)
>  - Add comment line for check condition of page_count() (Vlastimil Babka)
>  - Add fast path optimistic check to
>    __collapse_huge_page_isolate() (Andrea Arcangeli)
>  - Move check condition of page_count() below to trylock_page() (Andrea Arcangeli)
> 
> Changes in v3:
>  - Add a at-least-one-writable-pte check (Zhang Yanfei)
>  - Debug page count (Vlastimil Babka, Andrea Arcangeli)
>  - Increase read-only pte counter if pte is none (Andrea Arcangeli)
> 
> Changes in v4:
>  - Remove read-only counter (Andrea Arcangeli)
>  - Remove debug page count  (Andrea Arcangeli)
>  - Change type of writable as bool (Andrea Arcangeli)
>  - Change type of referenced as bool (Vlastimil Babka)
>  - Change comment line (Vlastimil Babka, Zhang Yanfei)

Looks good, thanks!

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