[PATCH v2] mm: remove compressed copy from zram in-memory

[Minchan Kim]

Swap subsystem does lazy swap slot free with expecting the page
would be swapped out again so we can avoid unnecessary write.

But the problem in in-memory swap(ex, zram) 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 makes swap subsystem free swap slot as soon as swap-read
is completed and make the swapcache page dirty so the page should
be written out the swap device to reclaim it.
It means we never lose it.

I tested this patch with kernel compile workload.

1. before

compile time : 9882.42
zram max wasted space by fragmentation: 13471881 byte
memory space consumed by zram: 174227456 byte
the number of slot free notify: 206684

2. after

compile time : 9653.90
zram max wasted space by fragmentation: 11805932 byte
memory space consumed by zram: 154001408 byte
the number of slot free notify: 426972

* changelog from v1
  * Add more comment

Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Seth Jennings <sjenning@xxxxxxxxxxxxxxxxxx>
Cc: Nitin Gupta <ngupta@xxxxxxxxxx>
Cc: Konrad Rzeszutek Wilk <konrad@xxxxxxxxxx>
Cc: Shaohua Li <shli@xxxxxxxxxx>
Signed-off-by: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
---
 mm/page_io.c | 35 +++++++++++++++++++++++++++++++++++
 1 file changed, 35 insertions(+)

diff --git a/mm/page_io.c b/mm/page_io.c
index 50bb6ef..b09d40a 100644
--- a/mm/page_io.c
+++ b/mm/page_io.c
@@ -21,6 +21,7 @@
 #include <linux/writeback.h>
 #include <linux/frontswap.h>
 #include <linux/aio.h>
+#include <linux/blkdev.h>
 #include <asm/pgtable.h>
 
 static struct bio *get_swap_bio(gfp_t gfp_flags,
@@ -82,8 +83,42 @@ void end_swap_bio_read(struct bio *bio, int err, struct batch_complete *batch)
 				iminor(bio->bi_bdev->bd_inode),
 				(unsigned long long)bio->bi_sector);
 	} else {
+		struct swap_info_struct *sis;
+
 		SetPageUptodate(page);
+		sis = page_swap_info(page);
+		if (sis->flags & SWP_BLKDEV) {
+			/*
+			 * Swap subsystem does lazy swap slot free with
+			 * expecting the page would be swapped out again
+			 * so we can avoid unnecessary write if the page
+			 * isn't redirty.
+			 * It's good for real swap storage  because we can
+			 * reduce unnecessary I/O and enhance wear-leveling
+			 * if you use SSD as swap device.
+			 * But if you use in-memory swap device(ex, zram),
+			 * it causes 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 swap out somewhere again if
+			 * we want to reclaim it, again.
+			 */
+			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);
+			}
+		}
 	}
+
 	unlock_page(page);
 	bio_put(bio);
 }
-- 
1.8.2.1

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