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SubjectRe: [PATCH] zram: revive swap_slot_free_notify
On Fri, Mar 18, 2016 at 04:58:31PM +0900, Minchan Kim wrote:
> <b430e9d1c6d4> "remove compressed copy from zram in-memory"
> applied swap_slot_free_notify call in *end_swap_bio_read* to
> remove duplicated memory between zram and memory.
>
> However, with introducing rw_page in zram <8c7f01025f7b>
> "zram: implement rw_page operation of zram", it became void
> because rw_page doesn't need bio.
>
> This patch restores the function for rw_page.
>
> Signed-off-by: Minchan Kim <minchan@kernel.org>
> ---
> mm/page_io.c | 93 ++++++++++++++++++++++++++++++++----------------------------
> 1 file changed, 50 insertions(+), 43 deletions(-)
>
> diff --git a/mm/page_io.c b/mm/page_io.c
> index ff74e512f029..18aac7819cc9 100644
> --- a/mm/page_io.c
> +++ b/mm/page_io.c
> @@ -66,6 +66,54 @@ void end_swap_bio_write(struct bio *bio)
> bio_put(bio);
> }
>
> +static void swap_slot_free_notify(struct page *page)
> +{
> + struct swap_info_struct *sis;
> + struct gendisk *disk;
> +
> + /*
> + * 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 (unlikely(!PageSwapCache(page)))
> + return;
> +
> + sis = page_swap_info(page);
> + if (!(sis->flags & SWP_BLKDEV))
> + return;
> +
> + /*
> + * 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.
> + */
> + 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);
> + }
> +}
> +
> static void end_swap_bio_read(struct bio *bio)
> {
> struct page *page = bio->bi_io_vec[0].bv_page;
> @@ -81,49 +129,7 @@ static void end_swap_bio_read(struct bio *bio)
> }
>
> 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);
> - }
> - }
> - }
> -
> + swap_slot_free_notify(page);
> out:
> unlock_page(page);
> bio_put(bio);
> @@ -347,6 +353,7 @@ int swap_readpage(struct page *page)
>
> ret = bdev_read_page(sis->bdev, swap_page_sector(page), page);
> if (!ret) {
> + swap_slot_free_notify(page);
> count_vm_event(PSWPIN);
> return 0;
> }

Hello,

You need to check PageUpdate() or something because bdev_read_page()
can be asynchronous.

BTW, something like as swap_slot_free_notify() which invalidate
backend of storage can also be possible for frontswap when
frontswap_load() succeed. Isn't it?

Thanks.

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