Re: [PATCH] ntfs: skip extent mft records in writeback to prevent deadlock

[Namjae Jeon]

On Thu, May 21, 2026 at 2:41 PM Hyunchul Lee <hyc.lee@xxxxxxxxx> wrote:
>
> This patch fixes the ABBA deadlock between extent_lock and extent
> mrec_lock triggered by xfstests generic/113, that occurs since the commit
> 6994acf33bae ("ntfs: use base mft_no when looking up base inode for
>                 extent record").
>
> Path A (inode writeback):
>   VFS writeback
>     -> ntfs_write_inode()
>       -> __ntfs_write_inode()
>         -> mutex_lock(&ni->extent_lock)
>         -> mutex_lock(&tni->mrec_lock)
>
> Path B (MFT folio writeback):
>   VFS writeback of $MFT dirty folios
>     -> ntfs_mft_writepages()
>       -> ntfs_write_mft_block()
>         -> ntfs_may_write_mft_record()
>           -> holds one extent mrec_lock from a previous iteration
>           -> tries to acquire another base inode extent_lock
>
> By removing all extent_lock and extent mrec_lock acquisition from the MFT
> folio writeback path, the ABBA lock ordering is eliminated:
>
> Path A: __ntfs_write_inode(): extent_lock -> mrec_lock
> Path B (removed): ntfs_write_mft_block(): mrec_lock -> extent_lock
>
> Path B is always redundant for extent records because:
>
> 1. mark_mft_record_dirty(ext_ni) does NOT dirty the MFT folio.
>    It only sets NInoDirty(ext_ni) and marks the base VFS inode dirty
>    via __mark_inode_dirty(I_DIRTY_DATASYNC), which triggers Path A.
>    Therefore, normal extent modifications never create a situation where
>    the MFT folio is dirty and Path B is not scheduled.
>
> 2. The MFT folio only gets dirtied via ntfs_mft_mark_dirty() inside
>    ntfs_mft_record_alloc(). But all identified callers in attrib.c
>    (ntfs_attr_add, ntfs_attr_record_move_away,
>    ntfs_attr_make_non_resident, ntfs_attr_record_resize) follow through
>    with mark_mft_record_dirty(), which triggers Path A to write the
>    complete record.
>
> 3. ntfs_evict_big_inode() calls ntfs_commit_inode() before freeing extent
>    inodes, ensuring all dirty extents are flushed via Path A before the
>    base inode leaves the icache.
>
> Signed-off-by: Hyunchul Lee <hyc.lee@xxxxxxxxx>
Applied it to #ntfs-next.
Thanks!