linux-kernel - Re: [PATCH 1/2] mm/zsmalloc: don't hold locks of all pages when free

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Message-ID: <913f1a04-8fa2-46b0-85dc-edd23477a1d6@linux.dev>
Date: Wed, 28 Feb 2024 13:42:47 +0800
From: Chengming Zhou <chengming.zhou@...ux.dev>
To: Sergey Senozhatsky <senozhatsky@...omium.org>
Cc: Chengming Zhou <zhouchengming@...edance.com>, yosryahmed@...gle.com,
 hannes@...xchg.org, nphamcs@...il.com,
 Andrew Morton <akpm@...ux-foundation.org>, Minchan Kim <minchan@...nel.org>,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 1/2] mm/zsmalloc: don't hold locks of all pages when
 free_zspage()

On 2024/2/28 13:29, Sergey Senozhatsky wrote:
> On (24/02/28 13:14), Chengming Zhou wrote:
>> On 2024/2/28 12:33, Sergey Senozhatsky wrote:
>>> On (24/02/27 03:02), Chengming Zhou wrote:
>>> [..]
>>>> @@ -978,10 +974,11 @@ static struct zspage *alloc_zspage(struct zs_pool *pool,
>>>>  		pages[i] = page;
>>>>  	}
>>>>  
>>>> -	create_page_chain(class, zspage, pages);
>>>>  	init_zspage(class, zspage);
>>>>  	zspage->pool = pool;
>>>>  	zspage->class = class->index;
>>>> +	/* RCU set_zspage() after zspage initialized. */
>>>> +	create_page_chain(class, zspage, pages);
>>>
>>> So this hasn't been tested, has it?
>> I have tested it in my test vm, but it hasn't KASAN enabled. I tested the
>> kernel build in tmpfs with zswap enabled using zsmalloc pool, not sure
>> why the kernel didn't crash then...
> 
> I hit the problem on non-kasan-enabled kernel.  KASAN was enabled
> later on.

Ok, It should be a problem with my process, sorry.

> 
> [..]
> 
>>> So when init_zspage() calls get_first_page() it gets NULL zspage->first_page
>>> which we then use in is_first_page(first_page)->PagePrivate(page). As far as
>>> I can tell.
>>
>> Thanks! I will fix it and test throughly before send an update.
> 
> I'm curious if we want to add RCU to the picture, given that zsmalloc
> is quite often run under memory pressure.

Yes, it's a reasonable point. But given struct zspage size has only 56 bytes,
it maybe not a problem to delay its free to RCU?

Thanks.