swapfile: swapon use discard (trim)

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@veritas.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Jens Axboe <jens.axboe@oracle.com>
Cc: Matthew Wilcox <matthew@wil.cx>
Cc: Joern Engel <joern@logfs.org>
Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
Cc: Donjun Shin <djshin90@gmail.com>
Cc: Tejun Heo <teheo@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 37 insertions, 2 deletions

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 4d9855f86e7d..fbeb4bb8eb50 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -84,6 +84,37 @@ void swap_unplug_io_fn(struct backing_dev_info *unused_bdi, struct page *page)
         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
 
@@ -1658,6 +1689,9 @@ asmlinkage long sys_swapon(const char __user * specialfile, int swap_flags)
                 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)
@@ -1671,9 +1705,10 @@ asmlinkage long sys_swapon(const char __user * specialfile, int swap_flags)
         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;