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SubjectRe: [patch][rfc] nommu: reverse mappings for nommu to solve get_user_pages problem
Nick Piggin wrote:

> int get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
> unsigned long start, int len, int write, int force,
> struct page **pages, struct vm_area_struct **vmas)
> {
> int i;
> - static struct vm_area_struct dummy_vma;
> + struct page *__page;
> + static struct vm_area_struct *__vma;
> + unsigned long addr = start;
>
> for (i = 0; i < len; i++) {
> + __vma = find_vma(mm, addr);
> + if (!__vma)
> + goto out_failed;
> +
> + __page = virt_to_page(addr);
> + if (!__page)
> + goto out_failed;
> +
> + BUG_ON(page_vma(__page) != __vma);
> +

Actually this check is leftover from a previous version. I think it
needs to be removed.

> if (pages) {
> - pages[i] = virt_to_page(start);
> - if (pages[i])
> - page_cache_get(pages[i]);
> + if (!__page->mapping) {
> + printk(KERN_INFO "get_user_pages on unaligned"
> + "anonymous page unsupported\n"); dump_stack();
> + goto out_failed;
> + }
> +

And this could trigger for file-backed pages that have been truncated meanwhile
I think. It wouldn't be a problem for a simple test-run, but does need to be
reworked slightly in order to be correct. Sub-page anonymous mappings cause a
lot of headaches :)

--
SUSE Labs, Novell Inc.
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