[PATCH] ext4: skip extra isize expansion while unmounting

syzbot reports a WARN from ext4_xattr_inode_create() reached through the
unmount path:

  EXT4-fs warning (device loop0): ext4_xattr_inode_create:1485: refuse to
  create EA inode when umounting
  WARNING: fs/ext4/xattr.c:1486 at ext4_xattr_inode_lookup_create
   ext4_xattr_block_set
   ext4_expand_extra_isize_ea
   __ext4_expand_extra_isize
   __ext4_mark_inode_dirty
   ext4_dirty_inode
   __mark_inode_dirty
   sync_lazytime
   iput
   dentry_kill
   shrink_dentry_list
   shrink_dcache_for_umount
   generic_shutdown_super
   kill_block_super
   ext4_kill_sb

shrink_dcache_for_umount() clears s_root before generic_shutdown_super()
clears SB_ACTIVE, so during the dcache shrink the last iput() of a
lazytime inode still redirties it and reaches the isize expansion.  The
expansion can move xattrs out to a block, and creating the EA inode for
them needs s_root, which ext4_xattr_inode_create() refuses without.

ext4_try_to_expand_extra_isize() already declines to expand when the
superblock is not active, but that test does not cover this window.
Decline while s_root is gone as well.  The expansion is best effort and
__ext4_mark_inode_dirty() ignores its return value, so nothing else
changes; the inode can be expanded on a later mount.

Running the syzbot reproducer for 60 seconds produced 3583 splats before
this change and none after it, with the same number of mount cycles.

Reported-by: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=4b03894b6ec5753ddf24
Fixes: f31173c19901 ("ext4: refuse to create ea block when umounted")
Signed-off-by: Hemanth Selam <[email protected]>
---
 fs/ext4/inode.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index bd4b778df9eb..6e523a5c8230 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -6598,8 +6598,14 @@ static int ext4_try_to_expand_extra_isize(struct inode *inode,
 	 * When !SB_ACTIVE, iput triggers write_inode_now() which acquires
 	 * s_writepages_rwsem, causing a deadlock with the caller's active
 	 * jbd2 handle (lock order: s_writepages_rwsem -> jbd2_handle).
+	 *
+	 * Skip it while unmounting as well.  shrink_dcache_for_umount()
+	 * clears s_root before generic_shutdown_super() clears SB_ACTIVE, and
+	 * the last iput() of a lazytime inode in that window redirties it and
+	 * lands here.  Moving xattrs out to a block then needs a new EA inode,
+	 * which ext4_xattr_inode_create() refuses without s_root.
 	 */
-	if (unlikely(!(inode->i_sb->s_flags & SB_ACTIVE)))
+	if (unlikely(!(inode->i_sb->s_flags & SB_ACTIVE) || !inode->i_sb->s_root))
 		return -EBUSY;
 
 	/*
-- 
2.43.7