Re: [PATCH v4] zram: extend zero pages to same element pages

[Minchan Kim]

Hi Andrew,

On Mon, Feb 06, 2017 at 12:16:44AM +0900, Minchan Kim wrote:
> From: zhouxianrong <zhouxianrong@xxxxxxxxxx>
> 
> the idea is that without doing more calculations we extend zero pages
> to same element pages for zram. zero page is special case of
> same element page with zero element.
> 
> 1. the test is done under android 7.0
> 2. startup too many applications circularly
> 3. sample the zero pages, same pages (none-zero element)
>    and total pages in function page_zero_filled
> 
> the result is listed as below:
> 
> ZERO	SAME	TOTAL
> 36214	17842	598196
> 
> 		ZERO/TOTAL	 SAME/TOTAL	  (ZERO+SAME)/TOTAL ZERO/SAME
> AVERAGE	0.060631909	 0.024990816  0.085622726		2.663825038
> STDEV	0.00674612	 0.005887625  0.009707034		2.115881328
> MAX		0.069698422	 0.030046087  0.094975336		7.56043956
> MIN		0.03959586	 0.007332205  0.056055193		1.928985507
> 
> from above data, the benefit is about 2.5% and up to 3% of total
> swapout pages.
> 
> the defect of the patch is that when we recovery a page from
> non-zero element the operations are low efficient for partial
> read.
> 
> This patch extend zero_page to same_page so if there is any user to have
> monitored zero_pages, he will be surprised if the number is increased
> but it's no harmful, I believe.
> 
> Link: http://lkml.kernel.org/r/1483692145-75357-1-git-send-email-zhouxianrong@xxxxxxxxxx
> Signed-off-by: zhouxianrong <zhouxianrong@xxxxxxxxxx>
> Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>

zhouxianrong pointed out we shouldn't free same element pages in zram_meta_free.
Otherwise, it can encounter oops due to invalid handle passed into zram_free.
This patch fixes it. Please fold this patch into zram-extend-zero-pages-to-same-element-pages.patch.

Thanks.