[PATCH AUTOSEL 4.20 282/304] zram: fix lockdep warning of free block handling

Sasha Levin
2019-01-28T15:58:12Z

From: Minchan Kim <minchan@kernel.org>

[ Upstream commit 3c9959e025472122a61faebb208525cf26b305d1 ]

Patch series "zram idle page writeback", v3.

Inherently, swap device has many idle pages which are rare touched since
it was allocated. It is never problem if we use storage device as swap.
However, it's just waste for zram-swap.

This patchset supports zram idle page writeback feature.

* Admin can define what is idle page "no access since X time ago"
* Admin can define when zram should writeback them
* Admin can define when zram should stop writeback to prevent wearout

Details are in each patch's description.

This patch (of 7):

================================
WARNING: inconsistent lock state
4.19.0+ #390 Not tainted
--------------------------------
inconsistent {SOFTIRQ-ON-W} -> {IN-SOFTIRQ-W} usage.
zram_verify/2095 [HC0[0]:SC1[1]:HE1:SE0] takes:
00000000b1828693 (&(&zram->bitmap_lock)->rlock){+.?.}, at: put_entry_bdev+0x1e/0x50
{SOFTIRQ-ON-W} state was registered at:
_raw_spin_lock+0x2c/0x40
zram_make_request+0x755/0xdc9
generic_make_request+0x373/0x6a0
submit_bio+0x6c/0x140
__swap_writepage+0x3a8/0x480
shrink_page_list+0x1102/0x1a60
shrink_inactive_list+0x21b/0x3f0
shrink_node_memcg.constprop.99+0x4f8/0x7e0
shrink_node+0x7d/0x2f0
do_try_to_free_pages+0xe0/0x300
try_to_free_pages+0x116/0x2b0
__alloc_pages_slowpath+0x3f4/0xf80
__alloc_pages_nodemask+0x2a2/0x2f0
__handle_mm_fault+0x42e/0xb50
handle_mm_fault+0x55/0xb0
__do_page_fault+0x235/0x4b0
page_fault+0x1e/0x30
irq event stamp: 228412
hardirqs last enabled at (228412): [<ffffffff98245846>] __slab_free+0x3e6/0x600
hardirqs last disabled at (228411): [<ffffffff98245625>] __slab_free+0x1c5/0x600
softirqs last enabled at (228396): [<ffffffff98e0031e>] __do_softirq+0x31e/0x427
softirqs last disabled at (228403): [<ffffffff98072051>] irq_exit+0xd1/0xe0

other info that might help us debug this:
Possible unsafe locking scenario:

CPU0
----
lock(&(&zram->bitmap_lock)->rlock);
<Interrupt>
lock(&(&zram->bitmap_lock)->rlock);

*** DEADLOCK ***

no locks held by zram_verify/2095.

stack backtrace:
CPU: 5 PID: 2095 Comm: zram_verify Not tainted 4.19.0+ #390
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
Call Trace:
<IRQ>
dump_stack+0x67/0x9b
print_usage_bug+0x1bd/0x1d3
mark_lock+0x4aa/0x540
__lock_acquire+0x51d/0x1300
lock_acquire+0x90/0x180
_raw_spin_lock+0x2c/0x40
put_entry_bdev+0x1e/0x50
zram_free_page+0xf6/0x110
zram_slot_free_notify+0x42/0xa0
end_swap_bio_read+0x5b/0x170
blk_update_request+0x8f/0x340
scsi_end_request+0x2c/0x1e0
scsi_io_completion+0x98/0x650
blk_done_softirq+0x9e/0xd0
__do_softirq+0xcc/0x427
irq_exit+0xd1/0xe0
do_IRQ+0x93/0x120
common_interrupt+0xf/0xf
</IRQ>

With writeback feature, zram_slot_free_notify could be called in softirq
context by end_swap_bio_read. However, bitmap_lock is not aware of that
so lockdep yell out:

get_entry_bdev
spin_lock(bitmap->lock);
irq
softirq
end_swap_bio_read
zram_slot_free_notify
zram_slot_lock <-- deadlock prone
zram_free_page
put_entry_bdev
spin_lock(bitmap->lock); <-- deadlock prone

With akpm's suggestion (i.e. bitmap operation is already atomic), we
could remove bitmap lock. It might fail to find a empty slot if serious
contention happens. However, it's not severe problem because huge page
writeback has already possiblity to fail if there is severe memory
pressure. Worst case is just keeping the incompressible in memory, not
storage.

The other problem is zram_slot_lock in zram_slot_slot_free_notify. To
make it safe is this patch introduces zram_slot_trylock where
zram_slot_free_notify uses it. Although it's rare to be contented, this
patch adds new debug stat "miss_free" to keep monitoring how often it
happens.

Link: http://lkml.kernel.org/r/20181127055429.251614-2-minchan@kernel.org
Signed-off-by: Minchan Kim <minchan@kernel.org>
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Reviewed-by: Joey Pabalinas <joeypabalinas@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/block/zram/zram_drv.c | 38 +++++++++++++++++++----------------
drivers/block/zram/zram_drv.h | 2 +-
2 files changed, 22 insertions(+), 18 deletions(-)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 8e6a0db6555f..d1459cc1159f 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -53,6 +53,11 @@ static size_t huge_class_size;

static void zram_free_page(struct zram *zram, size_t index);

+static int zram_slot_trylock(struct zram *zram, u32 index)
+{
+ return bit_spin_trylock(ZRAM_LOCK, &zram->table[index].value);
+}
+
static void zram_slot_lock(struct zram *zram, u32 index)
{
bit_spin_lock(ZRAM_LOCK, &zram->table[index].value);
@@ -401,7 +406,6 @@ static ssize_t backing_dev_store(struct device *dev,
goto out;

reset_bdev(zram);
- spin_lock_init(&zram->bitmap_lock);

zram->old_block_size = old_block_size;
zram->bdev = bdev;
@@ -445,29 +449,24 @@ out:

static unsigned long get_entry_bdev(struct zram *zram)
{
- unsigned long entry;
-
- spin_lock(&zram->bitmap_lock);
+ unsigned long blk_idx = 1;
+retry:
/* skip 0 bit to confuse zram.handle = 0 */
- entry = find_next_zero_bit(zram->bitmap, zram->nr_pages, 1);
- if (entry == zram->nr_pages) {
- spin_unlock(&zram->bitmap_lock);
+ blk_idx = find_next_zero_bit(zram->bitmap, zram->nr_pages, blk_idx);
+ if (blk_idx == zram->nr_pages)
return 0;
- }

- set_bit(entry, zram->bitmap);
- spin_unlock(&zram->bitmap_lock);
+ if (test_and_set_bit(blk_idx, zram->bitmap))
+ goto retry;

- return entry;
+ return blk_idx;
}

static void put_entry_bdev(struct zram *zram, unsigned long entry)
{
int was_set;

- spin_lock(&zram->bitmap_lock);
was_set = test_and_clear_bit(entry, zram->bitmap);
- spin_unlock(&zram->bitmap_lock);
WARN_ON_ONCE(!was_set);
}

@@ -888,9 +887,10 @@ static ssize_t debug_stat_show(struct device *dev,

down_read(&zram->init_lock);
ret = scnprintf(buf, PAGE_SIZE,
- "version: %d\n%8llu\n",
+ "version: %d\n%8llu %8llu\n",
version,
- (u64)atomic64_read(&zram->stats.writestall));
+ (u64)atomic64_read(&zram->stats.writestall),
+ (u64)atomic64_read(&zram->stats.miss_free));
up_read(&zram->init_lock);

return ret;
@@ -1402,10 +1402,14 @@ static void zram_slot_free_notify(struct block_device *bdev,

zram = bdev->bd_disk->private_data;

- zram_slot_lock(zram, index);
+ atomic64_inc(&zram->stats.notify_free);
+ if (!zram_slot_trylock(zram, index)) {
+ atomic64_inc(&zram->stats.miss_free);
+ return;
+ }
+
zram_free_page(zram, index);
zram_slot_unlock(zram, index);
- atomic64_inc(&zram->stats.notify_free);
}

static int zram_rw_page(struct block_device *bdev, sector_t sector,
diff --git a/drivers/block/zram/zram_drv.h b/drivers/block/zram/zram_drv.h
index 72c8584b6dff..d1095dfdffa8 100644
--- a/drivers/block/zram/zram_drv.h
+++ b/drivers/block/zram/zram_drv.h
@@ -79,6 +79,7 @@ struct zram_stats {
atomic64_t pages_stored; /* no. of pages currently stored */
atomic_long_t max_used_pages; /* no. of maximum pages stored */
atomic64_t writestall; /* no. of write slow paths */
+ atomic64_t miss_free; /* no. of missed free */
};

struct zram {
@@ -110,7 +111,6 @@ struct zram {
unsigned int old_block_size;
unsigned long *bitmap;
unsigned long nr_pages;
- spinlock_t bitmap_lock;
#endif
#ifdef CONFIG_ZRAM_MEMORY_TRACKING
struct dentry *debugfs_dir;
--
2.19.1