Re: [PATCH] btrfs: nofs inode allocations

[Filipe Manana <fdmanana@gmail.com>]

On Tue, Sep 10, 2019 at 7:22 PM Josef Bacik <josef@toxicpanda.com> wrote:
>
> 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>

To aid the stable team figure out to where to backport things, adding
a Fixes tag (Fixes: 712e36c5f2a7fa ("btrfs: use GFP_KERNEL in
btrfs_alloc_inode")) or a " CC: stable@vger.kernel.org #  4.16+" would
help.

> ---
>  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;

Should be unsigned int (it's what memalloc_nofs_save() returns exactly).

Anyway, the change itself looks good to me.

Reviewed-by: Filipe Manana <fdmanana@suse.com>

>         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
>


-- 
Filipe David Manana,

“Whether you think you can, or you think you can't — you're right.”