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Date: Tue, 22 Mar 2016 23:06:29 +0900
From: Minchan Kim <minchan@...nel.org>
To: Joonsoo Kim <js1304@...il.com>
CC: Joonsoo Kim <iamjoonsoo.kim@....com>,
Andrew Morton <akpm@...ux-foundation.org>,
LKML <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 05:20:08PM +0900, Joonsoo Kim wrote:
> 2016-03-22 17:00 GMT+09:00 Minchan Kim <minchan@...nel.org>:
> > 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.
>
> 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.
diff --git a/mm/page_io.c b/mm/page_io.c
index 18aac7819cc9..6592893d16ca 100644
--- a/mm/page_io.c
+++ b/mm/page_io.c
@@ -353,6 +353,7 @@ int swap_readpage(struct page *page)
ret = bdev_read_page(sis->bdev, swap_page_sector(page), page);
if (!ret) {
+ WARN_ON_ONCE(!PageUptodate(page));
swap_slot_free_notify(page);
count_vm_event(PSWPIN);
return 0;
>
> >>
> >> 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?
Hmm, I couldn't remember. Maybe, it's not zswap stuff but frontswap stuff
so I guess zswap user can enable it via frontswap interface if he want.
>
> Thanks.
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