Re: [PATCH] Fix kunmap() argument in sg_miter_stop

Linus Torvalds
2008-11-17T17:26:17Z

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@oracle.com>
>
> Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.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