From: KAMEZAWA Hiroyuki [mailto:kamezawa.hiroyu@jp.fujitsu.com]
Subject: Re: Subject: [PATCH V7 3/4] mm: frontswap: add swap hooks and extend try_to_unuse

Thanks again for the great review!  I think the only change required
for V8 is the addition of a comment in find_next_to_unuse (see below).
After reading all of my replies, please let me know if you disagree.

Dan

quoted

From: Dan Magenheimer <redacted>

This third patch of four in the frontswap series adds hooks in the swap
subsystem and extends try_to_unuse so that frontswap_shrink can do a
"partial swapoff".  Also, declarations for the extern-ified swap variables
in the first patch are declared.

Note that failed frontswap_map allocation is safe... failure is noted
by lack of "FS" in the subsequent printk.

[v7: rebase to 3.0-rc3]
[v7: JBeulich@novell.com: use new static inlines, no-ops if not config'd]
[v6: rebase to 3.1-rc1]
[v6: lliubbo@gmail.com: use vzalloc]
[v5: accidentally posted stale code for v4 that failed to compile :-(]
[v4: rebase to 2.6.39]
Signed-off-by: Dan Magenheimer <redacted>
Reviewed-by: Konrad Wilk <redacted>
Acked-by: Jan Beulich <redacted>
Acked-by: Seth Jennings <redacted>
Cc: Jeremy Fitzhardinge <redacted>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Nitin Gupta <redacted>
Cc: Matthew Wilcox <redacted>
Cc: Chris Mason <redacted>
Cc: Rik Riel <redacted>
Cc: Andrew Morton <akpm@linux-foundation.org>

--- linux/mm/swapfile.c	2011-08-08 08:19:26.336684746 -0600
+++ frontswap/mm/swapfile.c	2011-08-23 08:21:15.301998803 -0600

@@ -32,6 +32,8 @@
 #include <linux/memcontrol.h>
 #include <linux/poll.h>
 #include <linux/oom.h>
+#include <linux/frontswap.h>
+#include <linux/swapfile.h>

 #include <asm/pgtable.h>
 #include <asm/tlbflush.h>

@@ -43,7 +45,7 @@ static bool swap_count_continued(struct
 static void free_swap_count_continuations(struct swap_info_struct *);
 static sector_t map_swap_entry(swp_entry_t, struct block_device**);

-static DEFINE_SPINLOCK(swap_lock);
+DEFINE_SPINLOCK(swap_lock);
 static unsigned int nr_swapfiles;
 long nr_swap_pages;
 long total_swap_pages;

@@ -54,9 +56,9 @@ static const char Unused_file[] = "Unuse
 static const char Bad_offset[] = "Bad swap offset entry ";
 static const char Unused_offset[] = "Unused swap offset entry ";

-static struct swap_list_t swap_list = {-1, -1};
+struct swap_list_t swap_list = {-1, -1};

-static struct swap_info_struct *swap_info[MAX_SWAPFILES];
+struct swap_info_struct *swap_info[MAX_SWAPFILES];

 static DEFINE_MUTEX(swapon_mutex);

@@ -557,6 +559,7 @@ static unsigned char swap_entry_free(str
 			swap_list.next = p->type;
 		nr_swap_pages++;
 		p->inuse_pages--;
+		frontswap_flush_page(p->type, offset);
 		if ((p->flags & SWP_BLKDEV) &&
 				disk->fops->swap_slot_free_notify)
 			disk->fops->swap_slot_free_notify(p->bdev, offset);

@@ -1022,7 +1025,7 @@ static int unuse_mm(struct mm_struct *mm
  * Recycle to start on reaching the end, returning 0 when empty.
  */
 static unsigned int find_next_to_unuse(struct swap_info_struct *si,
-					unsigned int prev)
+					unsigned int prev, bool frontswap)
 {
 	unsigned int max = si->max;
 	unsigned int i = prev;

@@ -1048,6 +1051,12 @@ static unsigned int find_next_to_unuse(s
 			prev = 0;
 			i = 1;
 		}

quoted

+		if (frontswap) {
+			if (frontswap_test(si, i))
+				break;
+			else
+				continue;
+		}

Could you add comment ? If frontswap==true, only scan frontswap ?

Yes, thank you, this is a good comment to add.

quoted

@@ -1059,8 +1068,12 @@ static unsigned int find_next_to_unuse(s
  * We completely avoid races by reading each swap page in advance,
  * and then search for the process using it.  All the necessary
  * page table adjustments can then be made atomically.
+ *
+ * if the boolean frontswap is true, only unuse pages_to_unuse pages;
+ * pages_to_unuse==0 means all pages; ignored if frontswap is false
  */
-static int try_to_unuse(unsigned int type)
+int try_to_unuse(unsigned int type, bool frontswap,
+		 unsigned long pages_to_unuse)
 {
 	struct swap_info_struct *si = swap_info[type];
 	struct mm_struct *start_mm;

@@ -1093,7 +1106,7 @@ static int try_to_unuse(unsigned int typ
 	 * one pass through swap_map is enough, but not necessarily:
 	 * there are races when an instance of an entry might be missed.
 	 */
-	while ((i = find_next_to_unuse(si, i)) != 0) {
+	while ((i = find_next_to_unuse(si, i, frontswap)) != 0) {
 		if (signal_pending(current)) {
 			retval = -EINTR;
 			break;

@@ -1260,6 +1273,10 @@ static int try_to_unuse(unsigned int typ
 		 * interactive performance.
 		 */
 		cond_resched();
+		if (frontswap && pages_to_unuse > 0) {
+			if (!--pages_to_unuse)
+				break;
+		}
 	}

Is this a best-effort function and doesn't need to return condition
of pages_to_unuse ?
Caller of try_to_unuse(si, true....) is frontswap_shrink(). Right ?

Right.  This function is best-effort with frontswap or without frontswap.
In a non-frontswap system, a swapoff command may fail because try_to_unuse
wasn't able to swap in all pages.  The same is true of a "partial swapoff"
when frontswap_shrink() is called.  Since this behavior didn't change,
I didn't add a comment for that.  A return condition isn't needed because
frontswap_curr_pages can be queried.

Thanks!
Dan

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