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

[Minchan Kim <minchan@kernel.org>]

Hi Andrew,

On Mon, Feb 06, 2017 at 12:16:44AM +0900, Minchan Kim wrote:
> From: zhouxianrong <zhouxianrong@huawei.com>
> 
> 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@huawei.com
> Signed-off-by: zhouxianrong <zhouxianrong@huawei.com>
> Signed-off-by: Minchan Kim <minchan@kernel.org>

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.


>From e5a2a1dac4783f29d028170724578c0d11c80975 Mon Sep 17 00:00:00 2001
From: Minchan Kim <minchan@kernel.org>
Date: Tue, 7 Feb 2017 12:00:05 +0900
Subject: [PATCH] zram: do not free same element pages in zram_meta_free

zhouxianrong pointed out that we shouldn't free same element pages
in zram_meta_free. Otherwise, it will encounter oops due to invalid
handle value.

Signed-off-by: Minchan Kim <minchan@kernel.org>
---
Andrew,

Please fold it to zram-extend-zero-pages-to-same-element-pages.patch.

Thanks.

 drivers/block/zram/zram_drv.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index c20b05a84f21..a25d34a8af19 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -425,8 +425,11 @@ static void zram_meta_free(struct zram_meta *meta, u64 disksize)
 	/* Free all pages that are still in this zram device */
 	for (index = 0; index < num_pages; index++) {
 		unsigned long handle = meta->table[index].handle;
-
-		if (!handle)
+		/*
+		 * No memory is allocated for same element filled pages.
+		 * Simply clear same page flag.
+		 */
+		if (!handle || zram_test_flag(meta, index, ZRAM_SAME))
 			continue;
 
 		zs_free(meta->mem_pool, handle);
-- 
2.7.4