[PATCH v3 1/7] zram: fix lockdep warning of free block handling

[Minchan Kim <minchan@kernel.org>]

[  254.519728] ================================
[  254.520311] WARNING: inconsistent lock state
[  254.520898] 4.19.0+ #390 Not tainted
[  254.521387] --------------------------------
[  254.521732] inconsistent {SOFTIRQ-ON-W} -> {IN-SOFTIRQ-W} usage.
[  254.521732] zram_verify/2095 [HC0[0]:SC1[1]:HE1:SE0] takes:
[  254.521732] 00000000b1828693 (&(&zram->bitmap_lock)->rlock){+.?.}, at: put_entry_bdev+0x1e/0x50
[  254.521732] {SOFTIRQ-ON-W} state was registered at:
[  254.521732]   _raw_spin_lock+0x2c/0x40
[  254.521732]   zram_make_request+0x755/0xdc9
[  254.521732]   generic_make_request+0x373/0x6a0
[  254.521732]   submit_bio+0x6c/0x140
[  254.521732]   __swap_writepage+0x3a8/0x480
[  254.521732]   shrink_page_list+0x1102/0x1a60
[  254.521732]   shrink_inactive_list+0x21b/0x3f0
[  254.521732]   shrink_node_memcg.constprop.99+0x4f8/0x7e0
[  254.521732]   shrink_node+0x7d/0x2f0
[  254.521732]   do_try_to_free_pages+0xe0/0x300
[  254.521732]   try_to_free_pages+0x116/0x2b0
[  254.521732]   __alloc_pages_slowpath+0x3f4/0xf80
[  254.521732]   __alloc_pages_nodemask+0x2a2/0x2f0
[  254.521732]   __handle_mm_fault+0x42e/0xb50
[  254.521732]   handle_mm_fault+0x55/0xb0
[  254.521732]   __do_page_fault+0x235/0x4b0
[  254.521732]   page_fault+0x1e/0x30
[  254.521732] irq event stamp: 228412
[  254.521732] hardirqs last  enabled at (228412): [<ffffffff98245846>] __slab_free+0x3e6/0x600
[  254.521732] hardirqs last disabled at (228411): [<ffffffff98245625>] __slab_free+0x1c5/0x600
[  254.521732] softirqs last  enabled at (228396): [<ffffffff98e0031e>] __do_softirq+0x31e/0x427
[  254.521732] softirqs last disabled at (228403): [<ffffffff98072051>] irq_exit+0xd1/0xe0
[  254.521732]
[  254.521732] other info that might help us debug this:
[  254.521732]  Possible unsafe locking scenario:
[  254.521732]
[  254.521732]        CPU0
[  254.521732]        ----
[  254.521732]   lock(&(&zram->bitmap_lock)->rlock);
[  254.521732]   <Interrupt>
[  254.521732]     lock(&(&zram->bitmap_lock)->rlock);
[  254.521732]
[  254.521732]  *** DEADLOCK ***
[  254.521732]
[  254.521732] no locks held by zram_verify/2095.
[  254.521732]
[  254.521732] stack backtrace:
[  254.521732] CPU: 5 PID: 2095 Comm: zram_verify Not tainted 4.19.0+ #390
[  254.521732] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
[  254.521732] Call Trace:
[  254.521732]  <IRQ>
[  254.521732]  dump_stack+0x67/0x9b
[  254.521732]  print_usage_bug+0x1bd/0x1d3
[  254.521732]  mark_lock+0x4aa/0x540
[  254.521732]  ? check_usage_backwards+0x160/0x160
[  254.521732]  __lock_acquire+0x51d/0x1300
[  254.521732]  ? free_debug_processing+0x24e/0x400
[  254.521732]  ? bio_endio+0x6d/0x1a0
[  254.521732]  ? lockdep_hardirqs_on+0x9b/0x180
[  254.521732]  ? lock_acquire+0x90/0x180
[  254.521732]  lock_acquire+0x90/0x180
[  254.521732]  ? put_entry_bdev+0x1e/0x50
[  254.521732]  _raw_spin_lock+0x2c/0x40
[  254.521732]  ? put_entry_bdev+0x1e/0x50
[  254.521732]  put_entry_bdev+0x1e/0x50
[  254.521732]  zram_free_page+0xf6/0x110
[  254.521732]  zram_slot_free_notify+0x42/0xa0
[  254.521732]  end_swap_bio_read+0x5b/0x170
[  254.521732]  blk_update_request+0x8f/0x340
[  254.521732]  scsi_end_request+0x2c/0x1e0
[  254.521732]  scsi_io_completion+0x98/0x650
[  254.521732]  blk_done_softirq+0x9e/0xd0
[  254.521732]  __do_softirq+0xcc/0x427
[  254.521732]  irq_exit+0xd1/0xe0
[  254.521732]  do_IRQ+0x93/0x120
[  254.521732]  common_interrupt+0xf/0xf
[  254.521732]  </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. Thanks.

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.

Signed-off-by: Minchan Kim <minchan@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 4879595200e1..21a7046958a3 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);
@@ -399,7 +404,6 @@ 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;
@@ -443,29 +447,24 @@ static ssize_t backing_dev_store(struct device *dev,
 
 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);
 }
 
@@ -886,9 +885,10 @@ 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;
@@ -1400,10 +1400,14 @@ 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;
-- 
2.20.0.rc0.387.gc7a69e6b6c-goog