linux-kernel - [PATCH 3/3] zsmalloc: do not take class lock in zs_pages_to

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Date:	Sat, 11 Jul 2015 18:45:32 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: [PATCH 3/3] zsmalloc: do not take class lock in zs_pages_to_compact()

We can avoid taking class ->lock around zs_can_compact() in
zs_pages_to_compact(), because the number that we return back
is outdated in general case, by design. We have different
source that are able to change class's state right after we
return from zs_can_compact() -- ongoing IO operations, manually
triggered compaction or automatic compaction, or all three
simultaneously.

We re-do this calculations during compaction on a per class basis
anyway.

zs_unregister_shrinker() will not return until we have an active
shrinker, so classes won't unexpectedly disappear while
zs_pages_to_compact(), invoked by zs_shrinker_count(), iterates
them.

When called from zram, we are protected by zram's ->init_lock,
so, again, classes will be there until zs_pages_to_compact()
iterates them.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
---
 mm/zsmalloc.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index b10a228..824c182 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -1811,9 +1811,7 @@ unsigned long zs_pages_to_compact(struct zs_pool *pool)
 		if (class->index != i)
 			continue;

-		spin_lock(&class->lock);
 		pages_to_free += zs_can_compact(class);
-		spin_unlock(&class->lock);
 	}

 	return pages_to_free;
-- 
2.4.5

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