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SubjectRe: [uClinux-dev] bug in order>0 page allocations with !CONFIG_MMU
Hi David,

David Howells wrote:
> I've found that this:
>
> [mm/page_alloc.c]
> static inline void set_page_refs(struct page *page, int order)
> {
> #ifdef CONFIG_MMU
> set_page_count(page, 1);
> #else
> int i;
>
> /*
> * We need to reference all the pages for this order, otherwise if
> * anyone accesses one of the pages with (get/put) it will be freed.
> */
> for (i = 0; i < (1 << order); i++)
> set_page_count(page+i, 1);
> #endif /* CONFIG_MMU */
> }
>
> Causes problems if !CONFIG_MMU because __free_pages_ok()/free_pages_check()
> reports a bad page on the second page when it comes time to free it:
>
> Bad page state at __free_pages_ok (in process 'events/0', page c08132e0)
> flags:0x20000000 mapping:00000000 mapcount:0 count:1

That itself can be fixed by not checking the page_counts in
__free_pages_ok():

@@ -219,7 +219,9 @@
{
if ( page_mapped(page) ||
page->mapping != NULL ||
+#ifdef CONFIG_MMU
page_count(page) != 0 ||
+#endif
(page->flags & (
1 << PG_lru |
1 << PG_private |



> Why is doing this necessary at all? No one should be touching the individual
> pages of a block allocation. The kernel should defend itself against
> userspace trying to munmap part of a multipage mmap.

I don't recall right now why we had to do this originally.
It was absolutely neccessary once, but I don't think we need
this anymore. At least it runs fine on m68knommu targets now
without this.

Regards
Greg



------------------------------------------------------------------------
Greg Ungerer -- Chief Software Dude EMAIL: gerg@snapgear.com
SnapGear -- a CyberGuard Company PHONE: +61 7 3435 2888
825 Stanley St, FAX: +61 7 3891 3630
Woolloongabba, QLD, 4102, Australia WEB: http://www.SnapGear.com
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