Re: [patch][rfc] nommu: reverse mappings for nommu to solve get_user_pages problem

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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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