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Date: Tue, 22 Mar 2016 17:00:05 +0900
From: Minchan Kim <minchan@...nel.org>
To: Joonsoo Kim <iamjoonsoo.kim@....com>
CC: Andrew Morton <akpm@...ux-foundation.org>,
linux-kernel@...r.kernel.org,
Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
karam.lee@....com, sangseok.lee@....com, chan.jeong@....com
Subject: Re: [PATCH] zram: revive swap_slot_free_notify
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@...nel.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,
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.
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