[PATCH] unpaged: ZERO_PAGE in VM_UNPAGED

It's strange enough to be looking out for anonymous pages in VM_UNPAGED areas,
let's not insert the ZERO_PAGE there - though whether it would matter will
depend on what we decide about ZERO_PAGE refcounting.

But whereas do_anonymous_page may (exceptionally) be called on a VM_UNPAGED
area, do_no_page should never be: just BUG_ON.
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

drivers/char/mem.c

@@ -591,7 +591,7 @@ static inline size_t read_zero_pagealigned(char __user * buf, size_t size)
 
 		if (vma->vm_start > addr || (vma->vm_flags & VM_WRITE) == 0)
 			goto out_up;
-		if (vma->vm_flags & (VM_SHARED | VM_HUGETLB))
+		if (vma->vm_flags & (VM_SHARED | VM_HUGETLB | VM_UNPAGED))
 			break;
 		count = vma->vm_end - addr;
 		if (count > size)

mm/memory.c

@@ -1812,7 +1812,16 @@ static int do_anonymous_page(struct mm_struct *mm, struct vm_area_struct *vma,
 	spinlock_t *ptl;
 	pte_t entry;
 
-	if (write_access) {
+	/*
+	 * A VM_UNPAGED vma will normally be filled with present ptes
+	 * by remap_pfn_range, and never arrive here; but it might have
+	 * holes, or if !VM_DONTEXPAND, mremap might have expanded it.
+	 * It's weird enough handling anon pages in unpaged vmas, we do
+	 * not want to worry about ZERO_PAGEs too (it may or may not
+	 * matter if their counts wrap): just give them anon pages.
+	 */
+
+	if (write_access || (vma->vm_flags & VM_UNPAGED)) {
 		/* Allocate our own private page. */
 		pte_unmap(page_table);
 
@@ -1887,6 +1896,7 @@ static int do_no_page(struct mm_struct *mm, struct vm_area_struct *vma,
 	int anon = 0;
 
 	pte_unmap(page_table);
+	BUG_ON(vma->vm_flags & VM_UNPAGED);
 
 	if (vma->vm_file) {
 		mapping = vma->vm_file->f_mapping;
@@ -1962,7 +1972,7 @@ retry:
 			inc_mm_counter(mm, anon_rss);
 			lru_cache_add_active(new_page);
 			page_add_anon_rmap(new_page, vma, address);
-		} else if (!(vma->vm_flags & VM_UNPAGED)) {
+		} else {
 			inc_mm_counter(mm, file_rss);
 			page_add_file_rmap(new_page);
 		}