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?
> 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
confused.
> --- 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)
> iminor(bio->bi_bdev->bd_inode),
> (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;
> +
> SetPageUptodate(page);
> + 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);
> + }
> + }
> }
> +
> unlock_page(page);
> bio_put(bio);
The new code is wasted space if CONFIG_BLOCK=n, yes?
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.
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