The patch below does not apply to the 5.4-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 769030e11847c5412270c0726ff21d3a1f0a3131 Mon Sep 17 00:00:00 2001 From: Filipe Manana fdmanana@suse.com Date: Mon, 1 Aug 2022 14:57:52 +0100 Subject: [PATCH] btrfs: fix warning during log replay when bumping inode link count

During log replay, at add_link(), we may increment the link count of another inode that has a reference that conflicts with a new reference for the inode currently being processed.

During log replay, at add_link(), we may drop (unlink) a reference from some inode in the subvolume tree if that reference conflicts with a new reference found in the log for the inode we are currently processing.

After the unlink, If the link count has decreased from 1 to 0, then we increment the link count to prevent the inode from being deleted if it's evicted by an iput() call, because we may have references to add to that inode later on (and we will fixup its link count later during log replay).

However incrementing the link count from 0 to 1 triggers a warning:

$ cat fs/inode.c (...) void inc_nlink(struct inode *inode) { if (unlikely(inode->i_nlink == 0)) { WARN_ON(!(inode->i_state & I_LINKABLE)); atomic_long_dec(&inode->i_sb->s_remove_count); } (...)

The I_LINKABLE flag is only set when creating an O_TMPFILE file, so it's never set during log replay.

Most of the time, the warning isn't triggered even if we dropped the last reference of the conflicting inode, and this is because:

1) The conflicting inode was previously marked for fixup, through a call to link_to_fixup_dir(), which increments the inode's link count;

2) And the last iput() on the inode has not triggered eviction of the inode, nor was eviction triggered after the iput(). So at add_link(), even if we unlink the last reference of the inode, its link count ends up being 1 and not 0.

So this means that if eviction is triggered after link_to_fixup_dir() is called, at add_link() we will read the inode back from the subvolume tree and have it with a correct link count, matching the number of references it has on the subvolume tree. So if when we are at add_link() the inode has exactly one reference only, its link count is 1, and after the unlink its link count becomes 0.

So fix this by using set_nlink() instead of inc_nlink(), as the former accepts a transition from 0 to 1 and it's what we use in other similar contexts (like at link_to_fixup_dir().

Also make add_inode_ref() use set_nlink() instead of inc_nlink() to bump the link count from 0 to 1.

The warning is actually harmless, but it may scare users. Josef also ran into it recently.

CC: stable@vger.kernel.org # 5.1+ Signed-off-by: Filipe Manana fdmanana@suse.com Reviewed-by: David Sterba dsterba@suse.com Signed-off-by: David Sterba dsterba@suse.com

diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c index c1fdd6ef3f4a..9205c4a5ca81 100644 --- a/fs/btrfs/tree-log.c +++ b/fs/btrfs/tree-log.c @@ -1459,7 +1459,7 @@ static int add_link(struct btrfs_trans_handle *trans, * on the inode will not free it. We will fixup the link count later. */ if (other_inode->i_nlink == 0) - inc_nlink(other_inode); + set_nlink(other_inode, 1); add_link: ret = btrfs_add_link(trans, BTRFS_I(dir), BTRFS_I(inode), name, namelen, 0, ref_index); @@ -1602,7 +1602,7 @@ static noinline int add_inode_ref(struct btrfs_trans_handle *trans, * free it. We will fixup the link count later. */ if (!ret && inode->i_nlink == 0) - inc_nlink(inode); + set_nlink(inode, 1); } if (ret < 0) goto out;