Opportunistically move mlocked pages off the LRU

Add a new function try_to_mlock() that attempts to
move a page off the LRU and marks it mlocked.

This function can then be used in various code paths to move
pages off the LRU immediately. Early discovery will make NR_MLOCK
track the actual number of mlocked pages in the system more closely.

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

Index: current/mm/memory.c
===================================================================
--- current.orig/mm/memory.c	2007-02-05 12:00:30.000000000 -0800
+++ current/mm/memory.c	2007-02-05 12:01:52.000000000 -0800
@@ -919,6 +919,30 @@ static void add_anon_page(struct vm_area
 }
 
 /*
+ * Opportunistically move the page off the LRU
+ * if possible. If we do not succeed then the LRU
+ * scans will take the page off.
+ */
+void try_to_set_mlocked(struct page *page)
+{
+	struct zone *zone;
+	unsigned long flags;
+
+	if (!PageLRU(page) || PageMlocked(page))
+		return;
+
+	zone = page_zone(page);
+	if (spin_trylock_irqsave(&zone->lru_lock, flags)) {
+		if (PageLRU(page) && !PageMlocked(page)) {
+			ClearPageLRU(page);
+			list_del(&page->lru);
+			SetPageMlocked(page);
+			__inc_zone_page_state(page, NR_MLOCK);
+		}
+		spin_unlock_irqrestore(&zone->lru_lock, flags);
+	}
+}
+/*
  * Do a quick page-table lookup for a single page.
  */
 struct page *follow_page(struct vm_area_struct *vma, unsigned long address,
@@ -978,6 +1002,8 @@ struct page *follow_page(struct vm_area_
 			set_page_dirty(page);
 		mark_page_accessed(page);
 	}
+	if (vma->vm_flags & VM_LOCKED)
+		try_to_set_mlocked(page);
 unlock:
 	pte_unmap_unlock(ptep, ptl);
 out:
@@ -2271,6 +2297,8 @@ retry:
 		else {
 			inc_mm_counter(mm, file_rss);
 			page_add_file_rmap(new_page);
+			if (vma->vm_flags & VM_LOCKED)
+				try_to_set_mlocked(new_page);
 			if (write_access) {
 				dirty_page = new_page;
 				get_page(dirty_page);