Commit b430e9d1 authored Jul 03, 2013 by Minchan Kim Committed by Linus Torvalds Jul 03, 2013

mm: remove compressed copy from zram in-memory



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

[akpm@linux-foundation.org: tweak comment text]
[artem.savkov@gmail.com: fix BUG due to non-swapcache pages in end_swap_bio_read()]
[akpm@linux-foundation.org: invert unlikely() test, augment comment, 80-col cleanup]
Signed-off-by: Dan Magenheimer <dan.magenheimer@oracle.com>
Signed-off-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Artem Savkov <artem.savkov@gmail.com>
Cc: Hugh Dickins <hughd@google.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: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent ffbdccf5

mm/page_io.c

+48 −2

Original line number	Diff line number	Diff line
		@@ -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,
		@@ -80,9 +81,54 @@ void end_swap_bio_read(struct bio *bio, int err)
		imajor(bio->bi_bdev->bd_inode),
		iminor(bio->bi_bdev->bd_inode),
		(unsigned long long)bio->bi_sector);
		} else {
		goto out;
		}

		SetPageUptodate(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);
		}