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 <hu...@google.com>
Cc: Seth Jennings <sjenn...@linux.vnet.ibm.com>
Cc: Nitin Gupta <ngu...@vflare.org>
Cc: Konrad Rzeszutek Wilk <kon...@darnok.org>
Cc: Shaohua Li <s...@kernel.org>
Signed-off-by: Dan Magenheimer <dan.magenhei...@oracle.com>
Signed-off-by: Minchan Kim <minc...@kernel.org>
---
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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