Re: [PATCH] zram: revive swap_slot_free_notify

[Joonsoo Kim]

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.

>>
>> 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?

Wow... yes. that's what I try to find.
Do you know the reason why zswap doesn't enable it?

Thanks.