linux-kernel - Re: [PATCH 1/2] zsmalloc: do not remap dst page while prepare next src page

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Date:	Wed, 25 Mar 2015 14:05:33 +0900
From:	Heesub Shin <heesub.shin@...sung.com>
To:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Minchan Kim <minchan@...nel.org>
Cc:	Nitin Gupta <ngupta@...are.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	sunae.seo@...sung.com, cmlaika.kim@...sung.com
Subject: Re: [PATCH 1/2] zsmalloc: do not remap dst page while prepare next src
 page

Hello,

On 03/25/2015 12:24 AM, Sergey Senozhatsky wrote:
> object may belong to different pages. zs_object_copy() handles
> this case and maps a new source page (get_next_page() and
> kmap_atomic()) when object crosses boundaries of the current
> source page. But it also performs unnecessary kunmap/kmap_atomic
> of the destination page (it remains unchanged), which can be
> avoided.

No, it's not unnecessary. We should do kunmap_atomic() in the reverse
order of kmap_atomic(), so unfortunately it's inevitable to
kunmap_atomic() both on d_addr and s_addr.

> 
> 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 d920e8b..7af4456 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -1536,12 +1536,10 @@ static void zs_object_copy(unsigned long src, unsigned long dst,
>  			break;
>  
>  		if (s_off + size >= PAGE_SIZE) {
> -			kunmap_atomic(d_addr);
>  			kunmap_atomic(s_addr);

Removing kunmap_atomic(d_addr) here may cause BUG_ON() at __kunmap_atomic().

I tried yours to see it really happens:
> kernel BUG at arch/arm/mm/highmem.c:113!
> Internal error: Oops - BUG: 0 [#1] SMP ARM
> Modules linked in:
> CPU: 2 PID: 1774 Comm: bash Not tainted 4.0.0-rc2-mm1+ #105
> Hardware name: ARM-Versatile Express
> task: ee971300 ti: e8a26000 task.ti: e8a26000
> PC is at __kunmap_atomic+0x144/0x14c
> LR is at zs_object_copy+0x19c/0x2dc

regards
heesub
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