swap: make swap discard async

swap can do cluster discard for SSD, which is good, but there are some problems here:
1. swap do the discard just before page reclaim gets a swap entry and writes the disk sectors. This is useless for high end SSD, because an overwrite to a sector implies a discard to original nand flash too. A discard + overwrite == overwrite.
2. the purpose of doing discard is to improve SSD firmware garbage collection. Doing discard just before write doesn't help, because the interval between discard and write is too short. Doing discard async and just after a swap entry is freed can make the interval longer, so SSD firmware has more time to do gc.
3. block discard is a sync API, which will delay scan_swap_map() significantly.
4. Write and discard command can be executed parallel in PCIe SSD. Making swap discard async can make execution more efficiently.

This patch makes swap discard async, and move discard to where swap entry is freed. Idealy we should do discard for any freed sectors, but some SSD discard is very slow. This patch still does discard for a whole cluster.

My test does a several round of 'mmap, write, unmap', which will trigger a lot of swap discard. In a fusionio card, with this patch, the test runtime is reduced to 18% of the time without it, so around 5.5x faster.

---
 include/linux/swap.h |    5 -
 mm/swapfile.c        |  157 ++++++++++++++++++++++++++-------------------------
 2 files changed, 86 insertions(+), 76 deletions(-)

Index: linux/include/linux/swap.h
===================================================================
@@ -194,8 +194,6 @@ struct swap_info_struct {
 	unsigned int inuse_pages;	/* number of those currently in use */
 	unsigned int cluster_next;	/* likely index for next allocation */
 	unsigned int cluster_nr;	/* countdown to next cluster search */
-	unsigned int lowest_alloc;	/* while preparing discard cluster */
-	unsigned int highest_alloc;	/* while preparing discard cluster */
 	struct swap_extent *curr_swap_extent;
 	struct swap_extent first_swap_extent;
 	struct block_device *bdev;	/* swap device or bdev of swap file */

@@ -205,6 +203,9 @@ struct swap_info_struct {
 	unsigned long *frontswap_map;	/* frontswap in-use, one bit per page */
 	atomic_t frontswap_pages;	/* frontswap pages in-use counter */
 #endif
+	struct work_struct discard_work;
+	unsigned int discard_cluster_head;
+	unsigned int discard_cluster_tail;
 };
 
 struct swap_list_t {

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