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

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Message-ID: <20240228052906.GI11972@google.com>
Date: Wed, 28 Feb 2024 14:29:06 +0900
From: Sergey Senozhatsky <senozhatsky@...omium.org>
To: Chengming Zhou <chengming.zhou@...ux.dev>
Cc: Sergey Senozhatsky <senozhatsky@...omium.org>,
	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 (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.

[..]

> > 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.