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

[Andrea Arcangeli]

On Thu, Jan 29, 2015 at 04:59:07PM +0200, 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@xxxxxxxxx>
> Signed-off-by: Ebru Akagunduz <ebru.akagunduz@xxxxxxxxx>
> Reviewed-by: Rik van Riel <riel@xxxxxxxxxx>
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
> Acked-by: Zhang Yanfei <zhangyanfei@xxxxxxxxxxxxxx>

Reviewed-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
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