[RFC][PATCH 07/26] mm: Handle misplaced anon pages

[Peter Zijlstra]

From: Lee Schermerhorn <Lee.Schermerhorn@xxxxxx>

This patch simply hooks the anon page fault handler [do_swap_page()]
to check for and migrate misplaced pages if enabled and page won't
be "COWed".

This introduces can_reuse_swap_page() since reuse_swap_page() does
delete_from_swap_cache() which messes our migration path (since that
assumes its still a swapcache page).

Signed-off-by: Lee Schermerhorn <lee.schermerhorn@xxxxxx>
[ removed the retry loops after lock_page on a swapcache which tried
  to fixup the wreckage caused by ignoring the page count on migate;
  added can_reuse_swap_page(); moved the migrate-on-fault enabled
  test into check_migrate_misplaced_page() ]
Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
---
 include/linux/swap.h |    4 +++-
 mm/memory.c          |   17 +++++++++++++++++
 mm/swapfile.c        |   13 +++++++++++++
 3 files changed, 33 insertions(+), 1 deletion(-)
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -342,6 +342,7 @@ extern unsigned int count_swap_pages(int
 extern sector_t map_swap_page(struct page *, struct block_device **);
 extern sector_t swapdev_block(int, pgoff_t);
 extern int reuse_swap_page(struct page *);
+extern int can_reuse_swap_page(struct page *);
 extern int try_to_free_swap(struct page *);
 struct backing_dev_info;
 
@@ -459,7 +460,8 @@ static inline void delete_from_swap_cach
 {
 }
 
-#define reuse_swap_page(page)	(page_mapcount(page) == 1)
+#define reuse_swap_page(page)		(page_mapcount(page) == 1)
+#define can_reuse_swap_page(page)	(page_mapcount(page) == 1)
 
 static inline int try_to_free_swap(struct page *page)
 {
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -57,6 +57,7 @@
 #include <linux/swapops.h>
 #include <linux/elf.h>
 #include <linux/gfp.h>
+#include <linux/mempolicy.h>	/* check_migrate_misplaced_page() */
 
 #include <asm/io.h>
 #include <asm/pgalloc.h>
@@ -2962,6 +2963,22 @@ static int do_swap_page(struct mm_struct
 	}
 
 	/*
+	 * No sense in migrating a page that will be "COWed" as the new
+	 * new page will be allocated according to effective mempolicy.
+	 */
+	if ((flags & FAULT_FLAG_WRITE) && can_reuse_swap_page(page)) {
+		/*
+		 * check for misplacement and migrate, if necessary/possible,
+		 * here and now.  Note that if we're racing with another thread,
+		 * we may end up discarding the migrated page after locking
+		 * the page table and checking the pte below.  However, we
+		 * don't want to hold the page table locked over migration, so
+		 * we'll live with that [unlikely, one hopes] possibility.
+		 */
+		page = check_migrate_misplaced_page(page, vma, address);
+	}
+
+	/*
 	 * Back out if somebody else already faulted in this pte.
 	 */
 	page_table = pte_offset_map_lock(mm, pmd, address, &ptl);
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -640,6 +640,19 @@ int reuse_swap_page(struct page *page)
 	return count <= 1;
 }
 
+int can_reuse_swap_page(struct page *page)
+{
+	int count;
+
+	VM_BUG_ON(!PageLocked(page));
+	if (unlikely(PageKsm(page)))
+		return 0;
+	count = page_mapcount(page);
+	if (count <= 1 && PageSwapCache(page))
+		count += page_swapcount(page);
+	return count <= 1;
+}
+
 /*
  * If swap is getting full, or if there are no more mappings of this page,
  * then try_to_free_swap is called to free its swap space.


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