The pageout code takes a reference count to the page, which means
that remove_exclusive_swap_page (when called from the pageout code)
needs to take that extra refcount into account for mapped pages.
Introduces a remove_exclusive_swap_page_ref function to avoid
exposing too much magic to the rest of the VM.
Signed-off-by: Rik van Riel <riel@redhat.com>
Debugged-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
---
Daisuke: thank you for pointing out the problem. Does this patch look like
a reasonable fix to you?
Andrew: this patch is incremental to my patch
"[PATCH -mm 05/15] free swap space on swap-in/activation"
Index: linux-2.6.25-mm1/mm/vmscan.c
===================================================================
--- linux-2.6.25-mm1.orig/mm/vmscan.c 2008-05-13 11:59:34.000000000 -0400
+++ linux-2.6.25-mm1/mm/vmscan.c 2008-05-13 11:54:03.000000000 -0400
@@ -621,7 +621,7 @@ free_it:
activate_locked:
/* Not a candidate for swapping, so reclaim swap space. */
if (PageSwapCache(page) && vm_swap_full())
- remove_exclusive_swap_page(page);
+ remove_exclusive_swap_page_ref(page);
SetPageActive(page);
pgactivate++;
keep_locked:
Index: linux-2.6.25-mm1/mm/swap.c
===================================================================
--- linux-2.6.25-mm1.orig/mm/swap.c 2008-05-13 11:59:34.000000000 -0400
+++ linux-2.6.25-mm1/mm/swap.c 2008-05-13 11:53:47.000000000 -0400
@@ -455,7 +455,7 @@ void pagevec_swap_free(struct pagevec *p
if (PageSwapCache(page) && !TestSetPageLocked(page)) {
if (PageSwapCache(page))
- remove_exclusive_swap_page(page);
+ remove_exclusive_swap_page_ref(page);
unlock_page(page);
}
}
Index: linux-2.6.25-mm1/include/linux/swap.h
===================================================================
--- linux-2.6.25-mm1.orig/include/linux/swap.h 2008-05-13 11:20:46.000000000 -0400
+++ linux-2.6.25-mm1/include/linux/swap.h 2008-05-13 11:47:31.000000000 -0400
@@ -266,6 +266,7 @@ extern sector_t swapdev_block(int, pgoff
extern struct swap_info_struct *get_swap_info_struct(unsigned);
extern int can_share_swap_page(struct page *);
extern int remove_exclusive_swap_page(struct page *);
+extern int remove_exclusive_swap_page_ref(struct page *);
struct backing_dev_info;
extern spinlock_t swap_lock;
Index: linux-2.6.25-mm1/mm/swapfile.c
===================================================================
--- linux-2.6.25-mm1.orig/mm/swapfile.c 2008-04-22 10:33:45.000000000 -0400
+++ linux-2.6.25-mm1/mm/swapfile.c 2008-05-13 11:53:32.000000000 -0400
@@ -343,7 +343,7 @@ int can_share_swap_page(struct page *pag
* Work out if there are any other processes sharing this
* swap cache page. Free it if you can. Return success.
*/
-int remove_exclusive_swap_page(struct page *page)
+static int remove_exclusive_swap_page_count(struct page *page, int count)
{
int retval;
struct swap_info_struct * p;
@@ -356,7 +356,7 @@ int remove_exclusive_swap_page(struct pa
return 0;
if (PageWriteback(page))
return 0;
- if (page_count(page) != 2) /* 2: us + cache */
+ if (page_count(page) != count) /* 2: us + cache */
return 0;
entry.val = page_private(page);
@@ -369,7 +369,7 @@ int remove_exclusive_swap_page(struct pa
if (p->swap_map[swp_offset(entry)] == 1) {
/* Recheck the page count with the swapcache lock held.. */
write_lock_irq(&swapper_space.tree_lock);
- if ((page_count(page) == 2) && !PageWriteback(page)) {
+ if ((page_count(page) == count) && !PageWriteback(page)) {
__delete_from_swap_cache(page);
SetPageDirty(page);
retval = 1;
@@ -387,6 +387,25 @@ int remove_exclusive_swap_page(struct pa
}
/*
+ * Most of the time the page should have two references: one for the
+ * process and one for the swap cache.
+ */
+int remove_exclusive_swap_page(struct page *page)
+{
+ return remove_exclusive_swap_page_count(page, 2);
+}
+
+/*
+ * The pageout code holds an extra reference to the page. That raises
+ * the reference count to test for to 2 for a page that is only in the
+ * swap cache and 3 for a page that is mapped by a process.
+ */
+int remove_exclusive_swap_page_ref(struct page *page)
+{
+ return remove_exclusive_swap_page_count(page, 2 + page_mapped(page));
+}
+
+/*
* Free the swap entry like above, but also try to
* free the page cache entry if it is the last user.
*/
All Rights Reversed
--
