Re: Race in new page migration code?

[Christoph Lameter <clameter@engr.sgi.com>]

On Tue, 17 Jan 2006, Hugh Dickins wrote:

> Of course, page_mapcount may go up an instant later; but equally,
> another vma may get added to the prio_tree an instant later.

Right. The check does not have to be race free.

> It may be that, after much argument to and fro, the whole function will
> just reduce to checking "page_mapcount(page) == 1": if so, then I think
> you can go back to inlining it literally.

Oh. I love simplicity.... Sadly, one sometimes has these complicated 
notions in ones brain. Thanks.


Simplify migrate_page_add after feedback from Hugh.

Signed-off-by: Christoph Lameter <clameter@sgi.com>

Index: linux-2.6.15/mm/mempolicy.c
===================================================================
--- linux-2.6.15.orig/mm/mempolicy.c	2006-01-14 10:56:28.000000000 -0800
+++ linux-2.6.15/mm/mempolicy.c	2006-01-17 12:12:42.000000000 -0800
@@ -527,42 +527,13 @@ long do_get_mempolicy(int *policy, nodem
  * page migration
  */
 
-/* Check if we are the only process mapping the page in question */
-static inline int single_mm_mapping(struct mm_struct *mm,
-			struct address_space *mapping)
-{
-	struct vm_area_struct *vma;
-	struct prio_tree_iter iter;
-	int rc = 1;
-
-	spin_lock(&mapping->i_mmap_lock);
-	vma_prio_tree_foreach(vma, &iter, &mapping->i_mmap, 0, ULONG_MAX)
-		if (mm != vma->vm_mm) {
-			rc = 0;
-			goto out;
-		}
-	list_for_each_entry(vma, &mapping->i_mmap_nonlinear, shared.vm_set.list)
-		if (mm != vma->vm_mm) {
-			rc = 0;
-			goto out;
-		}
-out:
-	spin_unlock(&mapping->i_mmap_lock);
-	return rc;
-}
-
-/*
- * Add a page to be migrated to the pagelist
- */
 static void migrate_page_add(struct vm_area_struct *vma,
 	struct page *page, struct list_head *pagelist, unsigned long flags)
 {
 	/*
-	 * Avoid migrating a page that is shared by others and not writable.
+	 * Avoid migrating a page that is shared with others.
 	 */
-	if ((flags & MPOL_MF_MOVE_ALL) || !page->mapping || PageAnon(page) ||
-	    mapping_writably_mapped(page->mapping) ||
-	    single_mm_mapping(vma->vm_mm, page->mapping))
+	if ((flags & MPOL_MF_MOVE_ALL) || page_mapcount(page) ==1)
 		if (isolate_lru_page(page) == 1)
 			list_add(&page->lru, pagelist);
 }

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