[RFC] mm: remove swapcache page early

[Minchan Kim <minchan@kernel.org>]

Swap subsystem does lazy swap slot free with expecting the page
would be swapped out again so we can't avoid unnecessary write.

But the problem in in-memory swap is that it consumes memory space
until vm_swap_full(ie, used half of all of swap device) condition
meet. It could be bad if we use multiple swap device, small in-memory swap
and big storage swap or in-memory swap alone.

This patch changes vm_swap_full logic slightly so it could free
swap slot early if the backed device is really fast.
For it, I used SWP_SOLIDSTATE but It might be controversial.
So let's add Ccing Shaohua and Hugh.
If it's a problem for SSD, I'd like to create new type SWP_INMEMORY
or something for z* family.

Other problem is zram is block device so that it can set SWP_INMEMORY
or SWP_SOLIDSTATE easily(ie, actually, zram is already done) but
I have no idea to use it for frontswap.

Any idea?

Other optimize point is we remove it unconditionally when we
found it's exclusive when swap in happen.
It could help frontswap family, too.
What do you think about it?

Cc: Hugh Dickins <hughd@google.com>
Cc: Dan Magenheimer <dan.magenheimer@oracle.com>
Cc: Seth Jennings <sjenning@linux.vnet.ibm.com>
Cc: Nitin Gupta <ngupta@vflare.org>
Cc: Konrad Rzeszutek Wilk <konrad@darnok.org>
Cc: Shaohua Li <shli@kernel.org>
Signed-off-by: Minchan Kim <minchan@kernel.org>
---
 include/linux/swap.h | 11 ++++++++---
 mm/memory.c          |  3 ++-
 mm/swapfile.c        | 11 +++++++----
 mm/vmscan.c          |  2 +-
 4 files changed, 18 insertions(+), 9 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 2818a12..1f4df66 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -359,9 +359,14 @@ extern struct page *swapin_readahead(swp_entry_t, gfp_t,
 extern atomic_long_t nr_swap_pages;
 extern long total_swap_pages;
 
-/* Swap 50% full? Release swapcache more aggressively.. */
-static inline bool vm_swap_full(void)
+/*
+ * Swap 50% full or fast backed device?
+ * Release swapcache more aggressively.
+ */
+static inline bool vm_swap_full(struct swap_info_struct *si)
 {
+	if (si->flags & SWP_SOLIDSTATE)
+		return true;
 	return atomic_long_read(&nr_swap_pages) * 2 < total_swap_pages;
 }
 
@@ -405,7 +410,7 @@ mem_cgroup_uncharge_swapcache(struct page *page, swp_entry_t ent, bool swapout)
 #define get_nr_swap_pages()			0L
 #define total_swap_pages			0L
 #define total_swapcache_pages()			0UL
-#define vm_swap_full()				0
+#define vm_swap_full(si)			0
 
 #define si_swapinfo(val) \
 	do { (val)->freeswap = (val)->totalswap = 0; } while (0)
diff --git a/mm/memory.c b/mm/memory.c
index 705473a..1ca21a9 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -3084,7 +3084,8 @@ 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 (likely(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 (page != swapcache) {
diff --git a/mm/swapfile.c b/mm/swapfile.c
index 1bee6fa..f9cc701 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -293,7 +293,7 @@ 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(&si->lock);
 		swap_was_freed = __try_to_reclaim_swap(si, offset);
@@ -382,7 +382,8 @@ scan:
 			spin_lock(&si->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(&si->lock);
 			goto checks;
 		}
@@ -397,7 +398,8 @@ scan:
 			spin_lock(&si->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(&si->lock);
 			goto checks;
 		}
@@ -763,7 +765,8 @@ 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);
 		}
diff --git a/mm/vmscan.c b/mm/vmscan.c
index df78d17..145c59c 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -933,7 +933,7 @@ 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);
-- 
1.8.2

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