Re: [PATCH] zram: revive swap_slot_free_notify

[Minchan Kim <minchan@kernel.org>]

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@kernel.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;
>  	}
> -- 
> 1.9.1
> 

Kyeongdon, Could you try this patch?

>From 09497ba48f5621cd2737f72ac4d15e6b3e5e3d14 Mon Sep 17 00:00:00 2001
From: Minchan Kim <minchan@kernel.org>
Date: Mon, 28 Mar 2016 23:08:14 +0900
Subject: [PATCH] mm: call swap_slot_free_notify with holding page lock

Kyeongdon reported below error which is BUG_ON(!PageSwapCache(page))
in page_swap_info.
The reason is that page_endio in rw_page unlocks the page if read I/O
is completed so we need to hold a PG_lock again to check PageSwapCache.
Otherwise, the page can be removed from swapcache and trigger below
BUG_ON.

[27833.995833] ------------[ cut here ]------------
[27833.995853] Kernel BUG at c00f9040 [verbose debug info unavailable]
[27833.995865] Internal error: Oops - BUG: 0 [#1] PREEMPT SMP ARM
[27833.995876] Modules linked in:
[27833.995892] CPU: 4 PID: 13446 Comm: RenderThread Tainted: G        W
3.10.84-g9f14aec-dirty #73
[27833.995903] task: c3b73200 ti: dd192000 task.ti: dd192000
[27833.995922] PC is at page_swap_info+0x10/0x2c
[27833.995934] LR is at swap_slot_free_notify+0x18/0x6c
[27833.995946] pc : [<c00f9040>]    lr : [<c00f5560>]    psr: 400f0113
[27833.995946] sp : dd193d78  ip : c2deb1e4  fp : da015180
[27833.995959] r10: 00000000  r9 : 000200da  r8 : c120fe08
[27833.995968] r7 : 00000000  r6 : 00000000  r5 : c249a6c0  r4 : =
c249a6c0
[27833.995979] r3 : 00000000  r2 : 40080009  r1 : 200f0113  r0 : =
c249a6c0
..<snip>
[27833.996273] [<c00f9040>] (page_swap_info+0x10/0x2c) from [<c00f5560>]
(swap_slot_free_notify+0x18/0x6c)
[27833.996288] [<c00f5560>] (swap_slot_free_notify+0x18/0x6c) from
[<c00f5c5c>] (swap_readpage+0x90/0x11c)
[27833.996302] [<c00f5c5c>] (swap_readpage+0x90/0x11c) from [<c00f62dc>]
(read_swap_cache_async+0x134/0x1ac)
[27833.996317] [<c00f62dc>] (read_swap_cache_async+0x134/0x1ac) from
[<c00f63c4>] (swapin_readahead+0x70/0xb0)
[27833.996334] [<c00f63c4>] (swapin_readahead+0x70/0xb0) from =
[<c00e87e0>]
(handle_pte_fault+0x320/0x6fc)
[27833.996348] [<c00e87e0>] (handle_pte_fault+0x320/0x6fc) from
[<c00e8c7c>] (handle_mm_fault+0xc0/0xf0)
[27833.996363] [<c00e8c7c>] (handle_mm_fault+0xc0/0xf0) from =
[<c001ac18>]
(do_page_fault+0x11c/0x36c)
[27833.996378] [<c001ac18>] (do_page_fault+0x11c/0x36c) from =
[<c000838c>]
(do_DataAbort+0x34/0x118)
[27833.996392] [<c000838c>] (do_DataAbort+0x34/0x118) from [<c000d8b4>]
(__dabt_usr+0x34/0x40)

Reported-by: Kyeongdon Kim <kyeongdon.kim@lge.com>
Signed-off-by: Minchan Kim <minchan@kernel.org>
---
 mm/page_io.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/mm/page_io.c b/mm/page_io.c
index 18aac7819cc9..57e279b71695 100644
--- a/mm/page_io.c
+++ b/mm/page_io.c
@@ -353,7 +353,11 @@ int swap_readpage(struct page *page)
 
 	ret = bdev_read_page(sis->bdev, swap_page_sector(page), page);
 	if (!ret) {
-		swap_slot_free_notify(page);
+		if (trylock_page(page)) {
+			swap_slot_free_notify(page);
+			unlock_page(page);
+		}
+
 		count_vm_event(PSWPIN);
 		return 0;
 	}
-- 
1.9.1