Messages in this thread | | | Date | Tue, 3 Mar 2009 00:16:28 +0100 | From | Johannes Weiner <> | Subject | Re: [PATCH] mmtom : add VM_BUG_ON in __get_free_pages |
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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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