linux-kernel - Re: [PATCH] Fix kunmap() argument in sg_miter

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Date:	Mon, 17 Nov 2008 09:25:18 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
cc:	Ingo Molnar <mingo@...e.hu>, Jens Axboe <jens.axboe@...cle.com>,
	Tejun Heo <htejun@...il.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Hugh Dickins <hugh@...itas.com>, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org
Subject: Re: [PATCH] Fix kunmap() argument in sg_miter_stop



On Mon, 17 Nov 2008, Jeremy Fitzhardinge wrote:

> Pass the struct page * to kunmap, not the vaddr of the mapping itself.
> 
> Pointed out by Jens Axboe <jens.axboe@...cle.com>
> 
> Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
> ---
> drivers/xen/balloon.c |    2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
> 
> ===================================================================
> --- a/drivers/xen/balloon.c
> +++ b/drivers/xen/balloon.c
> @@ -120,7 +120,7 @@
> 	if (PageHighMem(page)) {
> 		void *v = kmap(page);
> 		clear_page(v);
> -		kunmap(v);
> +		kunmap(page);
> 	} else {
> 		void *v = page_address(page);
> 		clear_page(v);

Well, quite frankly, the whole thing looks like crud.

First off, 'kmap/kunmap' work on regular pages too. So if you're highmem 
aware, you should just do

	void *v = kmap(page);
	clear_page(v);
	kunmap(page);

and be done with it. 

Secondly, we actually have a function called "clear_highpage()" that does 
this, except it uses kmap_atomic(page, KM_USER0). Which is _probably 
better anyway, but I didn't check if there is some magical reason why it 
wouldn't work.

			Linus
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