zram: revive swap_slot_free_notify

Commit b430e9d1c6 ("remove compressed copy from zram in-memory")
applied swap_slot_free_notify call in *end_swap_bio_read* to remove
duplicated memory between zram and memory.

However, with the introduction of rw_page in zram: 8c7f01025f ("zram:
implement rw_page operation of zram"), it became void because rw_page
doesn't need bio.

Memory footprint is really important in embedded platforms which have
small memory, for example, 512M) recently because it could start to kill
processes if memory footprint exceeds some threshold by LMK or some
similar memory management modules.

This patch restores the function for rw_page, thereby eliminating this
duplication.

Signed-off-by: Minchan Kim <minchan@kernel.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: karam.lee <karam.lee@lge.com>
Cc: <sangseok.lee@lge.com>
Cc: Chan Jeong <chan.jeong@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_io.c

@@ -66,6 +66,54 @@ void end_swap_bio_write(struct bio *bio)
 	bio_put(bio);
 }
 
+static void swap_slot_free_notify(struct page *page)
+{
+	struct swap_info_struct *sis;
+	struct gendisk *disk;
+
+	/*
+	 * There is no guarantee that the page is in swap cache - the software
+	 * suspend code (at least) uses end_swap_bio_read() against a non-
+	 * swapcache page.  So we must check PG_swapcache before proceeding with
+	 * this optimization.
+	 */
+	if (unlikely(!PageSwapCache(page)))
+		return;
+
+	sis = page_swap_info(page);
+	if (!(sis->flags & SWP_BLKDEV))
+		return;
+
+	/*
+	 * The swap subsystem performs lazy swap slot freeing,
+	 * expecting that the page will be swapped out again.
+	 * So we can avoid an unnecessary write if the page
+	 * isn't redirtied.
+	 * This is good for real swap storage because we can
+	 * reduce unnecessary I/O and enhance wear-leveling
+	 * if an SSD is used as the as swap device.
+	 * But if in-memory swap device (eg zram) is used,
+	 * this causes a duplicated copy between uncompressed
+	 * data in VM-owned memory and compressed data in
+	 * zram-owned memory.  So let's free zram-owned memory
+	 * and make the VM-owned decompressed page *dirty*,
+	 * so the page should be swapped out somewhere again if
+	 * we again wish to reclaim it.
+	 */
+	disk = sis->bdev->bd_disk;
+	if (disk->fops->swap_slot_free_notify) {
+		swp_entry_t entry;
+		unsigned long offset;
+
+		entry.val = page_private(page);
+		offset = swp_offset(entry);
+
+		SetPageDirty(page);
+		disk->fops->swap_slot_free_notify(sis->bdev,
+				offset);
+	}
+}
+
 static void end_swap_bio_read(struct bio *bio)
 {
 	struct page *page = bio->bi_io_vec[0].bv_page;
@@ -81,49 +129,7 @@ static void end_swap_bio_read(struct bio *bio)
 	}
 
 	SetPageUptodate(page);
-
+	swap_slot_free_notify(page);
-	/*
-	 * There is no guarantee that the page is in swap cache - the software
-	 * suspend code (at least) uses end_swap_bio_read() against a non-
-	 * swapcache page.  So we must check PG_swapcache before proceeding with
-	 * this optimization.
-	 */
-	if (likely(PageSwapCache(page))) {
-		struct swap_info_struct *sis;
-
-		sis = page_swap_info(page);
-		if (sis->flags & SWP_BLKDEV) {
-			/*
-			 * The swap subsystem performs lazy swap slot freeing,
-			 * expecting that the page will be swapped out again.
-			 * So we can avoid an unnecessary write if the page
-			 * isn't redirtied.
-			 * This is good for real swap storage because we can
-			 * reduce unnecessary I/O and enhance wear-leveling
-			 * if an SSD is used as the as swap device.
-			 * But if in-memory swap device (eg zram) is used,
-			 * this causes a duplicated copy between uncompressed
-			 * data in VM-owned memory and compressed data in
-			 * zram-owned memory.  So let's free zram-owned memory
-			 * and make the VM-owned decompressed page *dirty*,
-			 * so the page should be swapped out somewhere again if
-			 * we again wish to reclaim it.
-			 */
-			struct gendisk *disk = sis->bdev->bd_disk;
-			if (disk->fops->swap_slot_free_notify) {
-				swp_entry_t entry;
-				unsigned long offset;
-
-				entry.val = page_private(page);
-				offset = swp_offset(entry);
-
-				SetPageDirty(page);
-				disk->fops->swap_slot_free_notify(sis->bdev,
-						offset);
-			}
-		}
-	}
-
 out:
 	unlock_page(page);
 	bio_put(bio);
@@ -347,6 +353,7 @@ int swap_readpage(struct page *page)
 
 	ret = bdev_read_page(sis->bdev, swap_page_sector(page), page);
 	if (!ret) {
+		swap_slot_free_notify(page);
 		count_vm_event(PSWPIN);
 		return 0;
 	}