linux-kernel - Re: [PATCH v2] zsmalloc: fix migrate_zspage-zs

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Date:	Sat, 16 Jan 2016 09:16:41 +0100
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:	Minchan Kim <minchan@...nel.org>,
	Junil Lee <junil0814.lee@....com>, ngupta@...are.org,
	sergey.senozhatsky.work@...il.com, akpm@...ux-foundation.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] zsmalloc: fix migrate_zspage-zs_free race condition

On 16.1.2016 9:06, Sergey Senozhatsky wrote:
> On (01/16/16 08:44), Vlastimil Babka wrote:
>> On 16.1.2016 5:09, Sergey Senozhatsky wrote:
>>> On (01/15/16 16:49), Vlastimil Babka wrote:
>>
>> Hmm but that's an unpin, not a pin? A mistake or I'm missing something?
> 
> I'm sure it's just a compose-in-mail-app typo.

BTW, couldn't the correct fix also just look like this?

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index 9f15bdd9163c..43f743175ede 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -1635,8 +1635,8 @@ static int migrate_zspage(struct zs_pool *pool, struct
size_class *class,
                free_obj = obj_malloc(d_page, class, handle);
                zs_object_copy(free_obj, used_obj, class);
                index++;
+               /* This also effectively unpins the handle */
                record_obj(handle, free_obj);
-               unpin_tag(handle);
                obj_free(pool, class, used_obj);
        }

But I'd still recommend WRITE_ONCE in record_obj(). And I'm not even sure it's
safe on all architectures to do a simple overwrite of a word against somebody
else trying to lock a bit there?

> 	-ss
> 
>> Anyway the compiler can do the same thing here without a WRITE_ONCE().
> 
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