Re: [PATCH] zram: revive swap_slot_free_notify

[Joonsoo Kim]

On Tue, Mar 22, 2016 at 11:06:29PM +0900, Minchan Kim wrote:
> On Tue, Mar 22, 2016 at 05:20:08PM +0900, Joonsoo Kim wrote:
> > 2016-03-22 17:00 GMT+09:00 Minchan Kim <minchan@xxxxxxxxxx>:
> > > 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.
> > 
> > Yes, I also looked up other rw_page users and found that
> > there is no async rw_page now.
> > 
> > > 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. :(
> > 
> > But, I think that we should add the check. If someone want it, how does
> > he/she know about it? Even, if someone makes zram to read/write
> > asynchronously, we can miss it easily. This is error-prone practice.
> 
> Okay, I don't have strong against it.
> If we really want to catch such case, let's add WARN_ON_ONCE.

I'm okay with it. But, please add code comment why WARN_ON_ONCE() is
added here.

Then,
Acked-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>

Thanks.