Re: [PATCH] zram: revive swap_slot_free_notify

[Joonsoo Kim]

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@xxxxxxxxxx>
> ---
>  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.