btrfs: allocate new inode in NOFS context

BugLink: https://bugs.launchpad.net/bugs/1848750
commit 11a19a90870ea5496a8ded69b86f5b476b6d3355 upstream.

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);

Signed-off-by: Connor Kuehl <connor.kuehl@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index d51d9466feb0025748316eb529e768b4005f6f9a..b3453adb214da0292978dea6256bf886153d46ab 100644 (file)
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -6276,13 +6276,16 @@ static struct inode *btrfs_new_inode(struct btrfs_trans_handle *trans,
        u32 sizes[2];
        int nitems = name ? 2 : 1;
        unsigned long ptr;
+       unsigned int 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);