Re: [PATCH] zram: revive swap_slot_free_notify

[Minchan Kim]

On Tue, Mar 22, 2016 at 02:08:59PM +0900, Joonsoo Kim wrote:
> 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,

Hey Joonsoo,

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

I considered it but decided not to add the check :(.
Because I couldn't justify what benfit we can have with the check.

The swap_slot_free_notify is tightly coupled with zram for several
years and zram have been worked synchronously. So if bdev_read_page
returns 0, it means we already have read the page successfully.
Even, when I looked up other rw_page user, it seems there is no async
rw_page users at the moment.

If there is someone want to use *async* rw_page && *swap_slot_free_noity*
in future, we could add the check easily. But I hope anyone never use
swap_slot_free_notify any more which is mess. :(

> 
> 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?

frontswap_tmem_exclusive_gets_enabled?

> 
> Thanks.