ext4: deferred iput framework for EA inodes

This series introduces a deferred-iput framework for EA inodes to
eliminate a class of lock ordering issues in ext4 xattr code.

The problem: iput() on EA inodes while holding xattr_sem or a jbd2
handle can trigger eviction, which may acquire those same locks or
s_writepages_rwsem, creating circular dependencies.  The immediate
deadlock (during mount-time orphan cleanup) is fixed by two separate
patches already reviewed and posted:

  ext4: skip extra isize expansion during mount to prevent deadlock
  ext4: set EXT4_STATE_NO_EXPAND in ext4_evict_inode

This series provides the structural fix that makes the code safe
regardless of calling context:

Patch 1 adds a VFS helper iput_if_not_last() which drops an inode
reference only if it is not the last one, using atomic_add_unless().
This provides a proper VFS abstraction for filesystems that need to
conditionally defer final iput.

Patch 2 introduces ext4_put_ea_inode() using iput_if_not_last() as
a fast path (single atomic, zero overhead for the common case).  If
this is the last reference, the inode is linked onto a per-sb llist
(via i_ea_iput_node embedded in ext4_inode_info, union with xattr_sem
which is unused for EA inodes) and a delayed worker (1 jiffie) performs
the final iput() in a clean context.  No per-iput allocation needed.
Also moves init_rwsem(xattr_sem) from init_once to ext4_alloc_inode
to handle slab reuse after the union field has been overwritten.

Patch 3 converts all EA inode iput() calls in xattr code to use
ext4_put_ea_inode() uniformly -- no exceptions to reason about.

Patch 4 removes the now-redundant ea_inode_array mechanism (parameter
threading, struct, expand/free functions), replaced entirely by direct
ext4_put_ea_inode() calls.  This is a net code reduction.

Patch 5 prevents a potential ABBA deadlock on corrupted filesystems
where multiple xattr entries reference the same EA inode.  It tracks
processed EA inodes on a per-call llist (reusing i_ea_iput_node) and
skips duplicates before iget, deferring the actual ext4_put_ea_inode()
until after the loop completes.  This covers both intra-block and
cross ibody/block duplicates in ext4_xattr_delete_inode().

Link: https://syzkaller.appspot.com/bug?extid=5d19358d7eb30ffb0cc5

v10:
 - New patch 5: prevent deadlock from duplicate EA inode references
   on corrupted filesystems.  Track processed EA inodes on a per-call
   llist to skip duplicates before iget, and defer ext4_put_ea_inode()
   until after the loop to avoid queuing an inode for eviction while
   the same loop may still iget it.
 - Patch 2: move ext4_init_ea_inode_work() before ext4_multi_mount_protect()
   so that failed_mount3a drain does not hit an uninitialized delayed_work
   when MMP check fails.

v9:
 - Add iput_if_not_last() as proper VFS helper (per reviewer: don't
   let filesystems manipulate inode refcount without VFS abstraction).
 - Use iput_if_not_last() + llist_node embedded in ext4_inode_info
   (union with xattr_sem) to avoid per-iput allocation entirely.
 - Convert ALL EA inode iput() calls uniformly -- no exceptions.
 - Remove entire ea_inode_array mechanism.
 - Add WARN_ON_ONCE in ext4_put_ea_inode() to catch misuse on non-EA
   inodes (protects the xattr_sem union safety).
 - Fix worker re-arm: ext4_drain_ea_inode_work() loops to handle
   nested EA inode evictions re-scheduling work.
 - Move INIT_DELAYED_WORK before journal loading (fast commit replay
   may trigger evictions).
 - Drain before ext4_quotas_off() for correct quota accounting.
 - Add flush in failed_mount_wq and failed_mount3a error paths for
   journal replay case.
 - Move init_rwsem(xattr_sem) from init_once to ext4_alloc_inode to
   handle slab object reuse after union overwrite.
 - Encapsulate worker init into ext4_init_ea_inode_work(), making
   ext4_ea_inode_work() static to xattr.c.

Yun Zhou (5):
  fs: add iput_if_not_last() helper
  ext4: introduce ext4_put_ea_inode() for safe deferred iput
  ext4: convert all EA inode iput() calls to ext4_put_ea_inode()
  ext4: remove ea_inode_array mechanism in favor of ext4_put_ea_inode()
  ext4: prevent deadlock from duplicate EA inode references on corrupted
    fs

 fs/ext4/ext4.h     |  13 ++-
 fs/ext4/inode.c    |   6 +-
 fs/ext4/super.c    |  19 +++-
 fs/ext4/xattr.c    | 214 +++++++++++++++++++++++++++------------------
 fs/ext4/xattr.h    |  21 +++--
 include/linux/fs.h |  13 +++
 6 files changed, 185 insertions(+), 101 deletions(-)

-- 
2.43.0