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

From: Ric Mason
Date: Tue Apr 09 2013 - 01:37:28 EST

Hi Minchan,
On 04/09/2013 09:02 AM, Minchan Kim wrote:
Hi Andrew,

On Mon, Apr 08, 2013 at 02:17:10PM -0700, Andrew Morton wrote:
On Mon, 8 Apr 2013 15:01:02 +0900 Minchan Kim <minchan@xxxxxxxxxx> wrote:

Swap subsystem does lazy swap slot free with expecting the page
would be swapped out again so we can avoid unnecessary write.
Is that correct? How can it save a write?

The add_to_swap makes the page dirty and we must pageout only if the page is
dirty. If a anon page is already charged into swapcache, we skip writeout
the page in shrink_page_list, then just remove the page from swapcache and
free it by __remove_mapping.

I did received same question multiple time so it would be good idea to
write down it in vmscan.c somewhere.

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.
>From my reading of the patch, that isn't how it works? It changed
end_swap_bio_read() to call zram_slot_free_notify(), which appears to
free the underlying compressed page. I have a feeling I'm hopelessly
You understand right totally.
Selecting swap slot in my description was totally miss.
Need to rewrite the description.

free the swap slot and free compress page is the same, isn't it?

--- a/mm/page_io.c
+++ b/mm/page_io.c
@@ -20,6 +20,7 @@
#include <linux/buffer_head.h>
#include <linux/writeback.h>
#include <linux/frontswap.h>
+#include <linux/blkdev.h>
#include <asm/pgtable.h>
static struct bio *get_swap_bio(gfp_t gfp_flags,
@@ -81,8 +82,30 @@ void end_swap_bio_read(struct bio *bio, int err)
(unsigned long long)bio->bi_sector);
} else {
+ /*
+ * There is no reason to keep both uncompressed data and
+ * compressed data in memory.
+ */
+ struct swap_info_struct *sis;
+ sis = page_swap_info(page);
+ if (sis->flags & SWP_BLKDEV) {
+ 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);
+ }
+ }
The new code is wasted space if CONFIG_BLOCK=n, yes?
CONFIG_SWAP is already dependent on CONFIG_BLOCK.

Also, what's up with the SWP_BLKDEV test? zram doesn't support
SWP_FILE? Why on earth not?

Putting swap_slot_free_notify() into block_device_operations seems
rather wrong. It precludes zram-over-swapfiles for all time and means
that other subsystems cannot get notifications for swap slot freeing
for swapfile-backed swap.
Zram is just pseudo-block device so anyone can format it with any FSes
and swapon a file. In such case, he can't get a benefit from
swap_slot_free_notify. But I think it's not a severe problem because
there is no reason to use a file-swap on zram. If anyone want to use it,
I'd like to know the reason. If it's reasonable, we have to rethink a
wheel and it's another story, IMHO.

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