[PATCH v12 0/4] ext4: deferred iput framework for EA inodes

[Yun Zhou]

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().
Annotated with __must_check to ensure callers handle the failure case.

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.

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

v12:
 - Drop patch 5 (dedup array for corrupted fs duplicate entries).
 - Simplify ext4_put_ea_inode() to take only an inode argument (sb is
   derived from inode->i_sb).

v11:
 - Patch 1: add __must_check annotation to iput_if_not_last().
 - Patch 2: remove ext4_drain_ea_inode_work() wrapper, use direct
   flush_delayed_work() at drain points.  Re-arm is not possible
   because check_igot_inode() in __ext4_iget() already rejects EA
   inodes with extended attributes, so evicting an EA inode never
   enters ext4_xattr_delete_inode().  Drop the ext4_evict_inode()
   guard (was patch 5 in v10) -- it is unnecessary given the above.
   Remove ext4_xattr_inode_array_free_deferred() intermediate function
   -- mechanism is introduced without converting any call site.
 - Patch 2: add comment on ext4_put_ea_inode() documenting why the
   inode cannot be double-queued to s_ea_inode_to_free (reviewer
   request).
 - Patch 2: simplify ext4_ea_inode_work() by removing 'next' variable.
 - Patch 5: replace per-call llist (i_ea_iput_node reuse) with a simple
   on-stack ino array + __GFP_NOFAIL dynamic growth.  This eliminates
   all concurrent access concerns on i_ea_iput_node and avoids the
   need for EXT4_STATE_EA_DEC_REF or ihold tricks.  Only EA inodes
   whose nlink drops to 0 are tracked, so legitimate dedup with
   ref_count > 1 is correctly processed multiple times.

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).
 - 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 (4):
  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()

 fs/ext4/ext4.h     |  13 +++-
 fs/ext4/inode.c    |   6 +-
 fs/ext4/super.c    |  18 +++++-
 fs/ext4/xattr.c    | 154 +++++++++++++++++++++------------------------
 fs/ext4/xattr.h    |   9 +--
 include/linux/fs.h |  13 ++++
 6 files changed, 117 insertions(+), 96 deletions(-)

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2.43.0