[PATCH] Updates for page migration

This adds some additional comments in order to help others figure out how
exactly the code works.  And fix a variable name.

Also swap_page does need to ignore all reference bits when unmapping a
page.  Otherwise we may have to repeatedly unmap a frequently touched page.
So change the try_to_unmap parameter to 1.

Signed-off-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 5a61080..5db32fd 100644 (file)
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -632,7 +632,7 @@ static int swap_page(struct page *page)
        struct address_space *mapping = page_mapping(page);
 
        if (page_mapped(page) && mapping)
-               if (try_to_unmap(page, 0) != SWAP_SUCCESS)
+               if (try_to_unmap(page, 1) != SWAP_SUCCESS)
                        goto unlock_retry;
 
        if (PageDirty(page)) {
@@ -839,7 +839,7 @@ EXPORT_SYMBOL(migrate_page);
  * pages are swapped out.
  *
  * The function returns after 10 attempts or if no pages
- * are movable anymore because t has become empty
+ * are movable anymore because to has become empty
  * or no retryable pages exist anymore.
  *
  * Return: Number of pages not migrated when "to" ran empty.
@@ -928,12 +928,21 @@ redo:
                        goto unlock_both;
 
                if (mapping->a_ops->migratepage) {
+                       /*
+                        * Most pages have a mapping and most filesystems
+                        * should provide a migration function. Anonymous
+                        * pages are part of swap space which also has its
+                        * own migration function. This is the most common
+                        * path for page migration.
+                        */
                        rc = mapping->a_ops->migratepage(newpage, page);
                        goto unlock_both;
                 }
 
                /*
-                * Trigger writeout if page is dirty
+                * Default handling if a filesystem does not provide
+                * a migration function. We can only migrate clean
+                * pages so try to write out any dirty pages first.
                 */
                if (PageDirty(page)) {
                        switch (pageout(page, mapping)) {
@@ -949,9 +958,10 @@ redo:
                                ; /* try to migrate the page below */
                        }
                 }
+
                /*
-                * If we have no buffer or can release the buffer
-                * then do a simple migration.
+                * Buffers are managed in a filesystem specific way.
+                * We must have no buffers or drop them.
                 */
                if (!page_has_buffers(page) ||
                    try_to_release_page(page, GFP_KERNEL)) {
@@ -966,6 +976,11 @@ redo:
                 * swap them out.
                 */
                if (pass > 4) {
+                       /*
+                        * Persistently unable to drop buffers..... As a
+                        * measure of last resort we fall back to
+                        * swap_page().
+                        */
                        unlock_page(newpage);
                        newpage = NULL;
                        rc = swap_page(page);