[PATCH 6/9] swapfile: swapon use discard (trim)

[Hugh Dickins]

When adding swap, all the old data on swap can be forgotten: sys_swapon()
discard all but the header page of the swap partition (or every extent
but the header of the swap file), to give a solidstate swap device the
opportunity to optimize its wear-levelling.

If that succeeds, note SWP_DISCARDABLE for later use, and report it
with a "D" at the right end of the kernel's "Adding ... swap" message.
Perhaps something should be shown in /proc/swaps (swapon -s), but we
have to be more cautious before making any addition to that format.

Signed-off-by: Hugh Dickins <hugh@xxxxxxxxxxx>
---
swapfile.c cleanup patches 0-5 just went to linux-mm: patches 6-9
may be of wider interest, so I'm extending the Cc list for them.

 include/linux/swap.h |    1 +
 mm/swapfile.c        |   39 +++++++++++++++++++++++++++++++++++++--
 2 files changed, 38 insertions(+), 2 deletions(-)

--- swapfile5/include/linux/swap.h	2008-11-25 12:41:31.000000000 +0000
+++ swapfile6/include/linux/swap.h	2008-11-25 12:41:34.000000000 +0000
@@ -120,6 +120,7 @@ struct swap_extent {
 enum {
 	SWP_USED	= (1 << 0),	/* is slot in swap_info[] used? */
 	SWP_WRITEOK	= (1 << 1),	/* ok to write to this swap?	*/
+	SWP_DISCARDABLE = (1 << 2),	/* blkdev supports discard */
 					/* add others here before... */
 	SWP_SCANNING	= (1 << 8),	/* refcount in scan_swap_map */
 };
--- swapfile5/mm/swapfile.c	2008-11-25 12:41:31.000000000 +0000
+++ swapfile6/mm/swapfile.c	2008-11-25 12:41:34.000000000 +0000
@@ -84,6 +84,37 @@ void swap_unplug_io_fn(struct backing_de
 	up_read(&swap_unplug_sem);
 }
 
+/*
+ * swapon tell device that all the old swap contents can be discarded,
+ * to allow the swap device to optimize its wear-levelling.
+ */
+static int discard_swap(struct swap_info_struct *si)
+{
+	struct swap_extent *se;
+	int err = 0;
+
+	list_for_each_entry(se, &si->extent_list, list) {
+		sector_t start_block = se->start_block << (PAGE_SHIFT - 9);
+		pgoff_t nr_blocks = se->nr_pages << (PAGE_SHIFT - 9);
+
+		if (se->start_page == 0) {
+			/* Do not discard the swap header page! */
+			start_block += 1 << (PAGE_SHIFT - 9);
+			nr_blocks -= 1 << (PAGE_SHIFT - 9);
+			if (!nr_blocks)
+				continue;
+		}
+
+		err = blkdev_issue_discard(si->bdev, start_block,
+						nr_blocks, GFP_KERNEL);
+		if (err)
+			break;
+
+		cond_resched();
+	}
+	return err;		/* That will often be -EOPNOTSUPP */
+}
+
 #define SWAPFILE_CLUSTER	256
 #define LATENCY_LIMIT		256
 
@@ -1649,6 +1680,9 @@ asmlinkage long sys_swapon(const char __
 		goto bad_swap;
 	}
 
+	if (discard_swap(p) == 0)
+		p->flags |= SWP_DISCARDABLE;
+
 	mutex_lock(&swapon_mutex);
 	spin_lock(&swap_lock);
 	if (swap_flags & SWAP_FLAG_PREFER)
@@ -1662,9 +1696,10 @@ asmlinkage long sys_swapon(const char __
 	total_swap_pages += nr_good_pages;
 
 	printk(KERN_INFO "Adding %uk swap on %s.  "
-			"Priority:%d extents:%d across:%lluk\n",
+			"Priority:%d extents:%d across:%lluk%s\n",
 		nr_good_pages<<(PAGE_SHIFT-10), name, p->prio,
-		nr_extents, (unsigned long long)span<<(PAGE_SHIFT-10));
+		nr_extents, (unsigned long long)span<<(PAGE_SHIFT-10),
+		(p->flags & SWP_DISCARDABLE) ? " D" : "");
 
 	/* insert swap space into swap_list: */
 	prev = -1;
--
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