[PATCH] btrfs: nofs inode allocations

Josef Bacik
2019-09-09T14:12:08Z

A user reported a lockdep splat

======================================================
WARNING: possible circular locking dependency detected
5.2.11-gentoo #2 Not tainted
------------------------------------------------------
kswapd0/711 is trying to acquire lock:
000000007777a663 (sb_internal){.+.+}, at: start_transaction+0x3a8/0x500

but task is already holding lock:
000000000ba86300 (fs_reclaim){+.+.}, at: __fs_reclaim_acquire+0x0/0x30

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #1 (fs_reclaim){+.+.}:
kmem_cache_alloc+0x1f/0x1c0
btrfs_alloc_inode+0x1f/0x260
alloc_inode+0x16/0xa0
new_inode+0xe/0xb0
btrfs_new_inode+0x70/0x610
btrfs_symlink+0xd0/0x420
vfs_symlink+0x9c/0x100
do_symlinkat+0x66/0xe0
do_syscall_64+0x55/0x1c0
entry_SYSCALL_64_after_hwframe+0x49/0xbe

-> #0 (sb_internal){.+.+}:
__sb_start_write+0xf6/0x150
start_transaction+0x3a8/0x500
btrfs_commit_inode_delayed_inode+0x59/0x110
btrfs_evict_inode+0x19e/0x4c0
evict+0xbc/0x1f0
inode_lru_isolate+0x113/0x190
__list_lru_walk_one.isra.4+0x5c/0x100
list_lru_walk_one+0x32/0x50
prune_icache_sb+0x36/0x80
super_cache_scan+0x14a/0x1d0
do_shrink_slab+0x131/0x320
shrink_node+0xf7/0x380
balance_pgdat+0x2d5/0x640
kswapd+0x2ba/0x5e0
kthread+0x147/0x160
ret_from_fork+0x24/0x30

other info that might help us debug this:

Possible unsafe locking scenario:

CPU0 CPU1
---- ----
lock(fs_reclaim);
lock(sb_internal);
lock(fs_reclaim);
lock(sb_internal);
*** DEADLOCK ***

3 locks held by kswapd0/711:
#0: 000000000ba86300 (fs_reclaim){+.+.}, at: __fs_reclaim_acquire+0x0/0x30
#1: 000000004a5100f8 (shrinker_rwsem){++++}, at: shrink_node+0x9a/0x380
#2: 00000000f956fa46 (&type->s_umount_key#30){++++}, at: super_cache_scan+0x35/0x1d0

stack backtrace:
CPU: 7 PID: 711 Comm: kswapd0 Not tainted 5.2.11-gentoo #2
Hardware name: Dell Inc. Precision Tower 3620/0MWYPT, BIOS 2.4.2 09/29/2017
Call Trace:
dump_stack+0x85/0xc7
print_circular_bug.cold.40+0x1d9/0x235
__lock_acquire+0x18b1/0x1f00
lock_acquire+0xa6/0x170
? start_transaction+0x3a8/0x500
__sb_start_write+0xf6/0x150
? start_transaction+0x3a8/0x500
start_transaction+0x3a8/0x500
btrfs_commit_inode_delayed_inode+0x59/0x110
btrfs_evict_inode+0x19e/0x4c0
? var_wake_function+0x20/0x20
evict+0xbc/0x1f0
inode_lru_isolate+0x113/0x190
? discard_new_inode+0xc0/0xc0
__list_lru_walk_one.isra.4+0x5c/0x100
? discard_new_inode+0xc0/0xc0
list_lru_walk_one+0x32/0x50
prune_icache_sb+0x36/0x80
super_cache_scan+0x14a/0x1d0
do_shrink_slab+0x131/0x320
shrink_node+0xf7/0x380
balance_pgdat+0x2d5/0x640
kswapd+0x2ba/0x5e0
? __wake_up_common_lock+0x90/0x90
kthread+0x147/0x160
? balance_pgdat+0x640/0x640
? __kthread_create_on_node+0x160/0x160
ret_from_fork+0x24/0x30

This is because btrfs_new_inode() calls new_inode() under the
transaction. We could probably move the new_inode() outside of this but
for now just wrap it in memalloc_nofs_save().

Reported-by: Zdenek Sojka <zsojka@seznam.cz>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
fs/btrfs/inode.c | 3 +++
1 file changed, 3 insertions(+)

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index aece5dd0e7a8..bf40d1085e4e 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -6291,13 +6291,16 @@ static struct inode *btrfs_new_inode(struct btrfs_trans_handle *trans,
u32 sizes[2];
int nitems = name ? 2 : 1;
unsigned long ptr;
+ unsigned long nofs_flag;
int ret;

path = btrfs_alloc_path();
if (!path)
return ERR_PTR(-ENOMEM);

+ nofs_flag = memalloc_nofs_save();
inode = new_inode(fs_info->sb);
+ memalloc_nofs_restore(nofs_flag);
if (!inode) {
btrfs_free_path(path);
return ERR_PTR(-ENOMEM);
--
2.21.0