Re: [PATCH] zram: revive swap_slot_free_notify

Minchan Kim
2016-03-28T14:29:25Z

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