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

Hugh Dickins
2008-11-25T21:44:34Z

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>
---
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;