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

Jens Axboe
2008-11-17T18:09:34Z

On Mon, Nov 17 2008, Linus Torvalds wrote:
>
>
> 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.

Indeed, scrub_page() should just be eliminated.

Any opinions on the kunmap/kunmap_atomic pointer checking? It's a bit
ugly that we have to enforce a void * rule for kunmap_atomic(), but
it'll definitely catch bugs. So I think it's worth it, I'd like to hear
your opinion before I queue it for 2.6.29 though.

--
Jens Axboe