mm: swap: don't delay swap free for fast swap devices

There are couple of issues with swapcache usage when ZRAM is used
as swap device.
1) Kernel does a swap readahead which can be around 6 to 8 pages
depending on total ram, which is not required for zram since
accesses are fast.
2) Kernel delays the freeing up of swapcache expecting a later hit,
which again is useless in the case of zram.
3) This is not related to swapcache, but zram usage itself.
As mentioned in (2) kernel delays freeing of swapcache, but along with
that it delays zram compressed page free also. i.e. there can be 2 copies,
though one is compressed.

This patch addresses these issues using two new flags
QUEUE_FLAG_FAST and SWP_FAST, to indicate that accesses to the device
will be fast and cheap, and instructs the swap layer to free up
swap space agressively, and not to do read ahead.

Change-Id: I5d2d5176a5f9420300bb2f843f6ecbdb25ea80e4
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>
Signed-off-by: D. Andrei Măceș <dmaces@nd.edu>

Conflicts:
	include/linux/blkdev.h
	include/linux/swap.h
	mm/swap_state.c
	mm/swapfile.c

Conflicts:
	include/linux/blkdev.h

3 files changed, 32 insertions, 6 deletions

diff --git a/mm/memory.c b/mm/memory.c
index ac8d3a1..682fca1 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3055,7 +3055,8 @@ static int do_swap_page(struct mm_struct *mm, struct vm_area_struct *vma,
 	mem_cgroup_commit_charge_swapin(page, ptr);
 
 	swap_free(entry);
-	if (vm_swap_full() || (vma->vm_flags & VM_LOCKED) || PageMlocked(page))
+	if ((PageSwapCache(page) && vm_swap_full(page_swap_info(page))) ||
+		(vma->vm_flags & VM_LOCKED) || PageMlocked(page))
 		try_to_free_swap(page);
 	unlock_page(page);
 	if (swapcache) {
diff --git a/mm/swapfile.c b/mm/swapfile.c
index c8f4338..7197864 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -69,6 +69,26 @@ static inline unsigned char swap_count(unsigned char ent)
 	return ent & ~SWAP_HAS_CACHE;	/* may include SWAP_HAS_CONT flag */
 }
 
+bool is_swap_fast(swp_entry_t entry)
+{
+	struct swap_info_struct *p;
+	unsigned long type;
+
+	if (non_swap_entry(entry))
+		return false;
+
+	type = swp_type(entry);
+	if (type >= nr_swapfiles)
+		return false;
+
+	p = swap_info[type];
+
+	if (p->flags & SWP_FAST)
+		return true;
+
+	return false;
+}
+
 /* returns 1 if swap entry is freed */
 static int
 __try_to_reclaim_swap(struct swap_info_struct *si, unsigned long offset)
@@ -289,7 +309,7 @@ checks:
 		scan_base = offset = si->lowest_bit;
 
 	/* reuse swap entry of cache-only swap if not busy. */
-	if (vm_swap_full() && si->swap_map[offset] == SWAP_HAS_CACHE) {
+	if (vm_swap_full(si) && si->swap_map[offset] == SWAP_HAS_CACHE) {
 		int swap_was_freed;
 		spin_unlock(&swap_lock);
 		swap_was_freed = __try_to_reclaim_swap(si, offset);
@@ -378,7 +398,8 @@ scan:
 			spin_lock(&swap_lock);
 			goto checks;
 		}
-		if (vm_swap_full() && si->swap_map[offset] == SWAP_HAS_CACHE) {
+		if (vm_swap_full(si) &&
+			si->swap_map[offset] == SWAP_HAS_CACHE) {
 			spin_lock(&swap_lock);
 			goto checks;
 		}
@@ -393,7 +414,8 @@ scan:
 			spin_lock(&swap_lock);
 			goto checks;
 		}
-		if (vm_swap_full() && si->swap_map[offset] == SWAP_HAS_CACHE) {
+		if (vm_swap_full(si) &&
+			si->swap_map[offset] == SWAP_HAS_CACHE) {
 			spin_lock(&swap_lock);
 			goto checks;
 		}
@@ -708,7 +730,8 @@ int free_swap_and_cache(swp_entry_t entry)
 		 * Also recheck PageSwapCache now page is locked (above).
 		 */
 		if (PageSwapCache(page) && !PageWriteback(page) &&
-				(!page_mapped(page) || vm_swap_full())) {
+				(!page_mapped(page) ||
+				vm_swap_full(page_swap_info(page)))) {
 			delete_from_swap_cache(page);
 			SetPageDirty(page);
 		}
@@ -2114,6 +2137,8 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
 		}
 		if (discard_swap(p) == 0 && (swap_flags & SWAP_FLAG_DISCARD))
 			p->flags |= SWP_DISCARDABLE;
+		if (blk_queue_fast(bdev_get_queue(p->bdev)))
+			p->flags |= SWP_FAST;
 	}
 
 	mutex_lock(&swapon_mutex);
diff --git a/mm/vmscan.c b/mm/vmscan.c
index a648d8e..5bd0dcb 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -984,7 +984,7 @@ cull_mlocked:
 
 activate_locked:
 		/* Not a candidate for swapping, so reclaim swap space. */
-		if (PageSwapCache(page) && vm_swap_full())
+		if (PageSwapCache(page) && vm_swap_full(page_swap_info(page)))
 			try_to_free_swap(page);
 		VM_BUG_ON(PageActive(page));
 		SetPageActive(page);