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SubjectRe: [PATCH] mmtom : add VM_BUG_ON in __get_free_pages
On Mon, Mar 02, 2009 at 02:27:57PM -0800, Andrew Morton wrote:
> On Mon, 2 Mar 2009 18:31:48 +0900
> MinChan Kim <minchan.kim@gmail.com> wrote:
>
> >
> > The __get_free_pages is used in many place.
> > Also, driver developers can use it freely due to export function.
> > Some developers might use it to allocate high pages by mistake.
> >
> > The __get_free_pages can allocate high page using alloc_pages,
> > but it can't return linear address for high page.
> >
> > Even worse, in this csse, caller can't free page which are there in high zone.
> > So, It would be better to add VM_BUG_ON.
> >
> > It's based on mmtom 2009-02-27-13-54.
> >
> > Signed-off-by: MinChan Kim <minchan.kim@gmail.com>
> > ---
> > mm/page_alloc.c | 7 +++++++
> > 1 files changed, 7 insertions(+), 0 deletions(-)
> >
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 8294107..381056b 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -1681,6 +1681,13 @@ EXPORT_SYMBOL(__alloc_pages_internal);
> > unsigned long __get_free_pages(gfp_t gfp_mask, unsigned int order)
> > {
> > struct page * page;
> > +
> > + /*
> > + * __get_free_pages() returns a 32-bit address, which cannot represent
> > + * a highmem page
> > + */
> > + VM_BUG_ON((gfp_mask & __GFP_HIGHMEM) != 0);
> > +
> > page = alloc_pages(gfp_mask, order);
> > if (!page)
> > return 0;
>
> If someone calls __get_free_pages(__GFP_HIGHMEM) then page_address()
> will reliably return NULL and the caller's code will oops.

It will allocate a page, fail to translate it to a virtual address,
return 0 and the caller will think allocation failed because it checks
for the return value.

But the highmem page is still allocated and now leaked, isn't it?

> Yes, there's a decent (and increasing) risk that the developer won't be
> testing the code on a highmem machine, but there are enough highmem
> machines out there that the bug should be discovered pretty quickly.

Another thing is that a device driver developer does not necessarily
has CONFIG_DEBUG_VM set. Can we expect him to?

> So I'm not sure that this test is worth the additional overhead to a
> fairly frequently called function?

Well, it's only done conditionally if you want to debug the thing
anyway. But as mentioned above, maybe this isn't the right condition.


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