ext4: reject delalloc to nodelalloc before applying remount options

Commit 97f5ec3b166d ("ext4: prevent delalloc to nodelalloc on
remount") rejects switching a mounted filesystem from delalloc to
nodelalloc.  However, it performs the check after ext4_apply_options()
has already cleared EXT4_MOUNT_DELALLOC in the live superblock.  The
failure path eventually restores the bit, but leaves a window where
other CPUs can observe nodelalloc.

test_opt() reads s_mount_opt directly without a lock shared with
remount.  If a concurrent truncate's __es_remove_extent() hits this
window and observes nodelalloc, it sets count_reserved to false.
Delayed extent status entries are then removed without calculating
their cluster reservations, leaving reserved at zero.
ext4_es_remove_extent() therefore calls ext4_da_release_space() with
zero, leaving i_reserved_data_blocks and s_dirtyclusters_counter
elevated.  When the inode is later evicted after unlink,
ext4_destroy_inode() reports:

  i_reserved_data_blocks (...) not cleared!

CPU 0                             CPU 1
                                  ksys_truncate()
                                    ...
                                    ext4_es_remove_extent()
ext4_reconfigure()
  ext4_check_opt_consistency()
  __ext4_remount()
    ext4_apply_options()
      clear EXT4_MOUNT_DELALLOC
                                      __es_remove_extent()
                                        test_opt() sees !DELALLOC
                                        count_reserved = false
    reject delalloc -> nodelalloc
    restore EXT4_MOUNT_DELALLOC
                                        remove delayed ES
                                    ext4_da_release_space(0)

Follow the pre-apply validation approach used by
ext4_check_quota_consistency() and reject the transition in
ext4_check_opt_consistency(), before ext4_apply_options() changes live
state.  Use mask_s_mount_opt to determine whether delalloc/nodelalloc
was specified and ctx_test_mount_opt() to check the final parsed value.

Fixes: 97f5ec3b166d ("ext4: prevent delalloc to nodelalloc on remount")
Cc: stable@vger.kernel.org
Signed-off-by: guzebing <guzebing1612@gmail.com>
Reviewed-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/20260814034855.1573759-1-guzebing1612@gmail.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

0 files changed, 0 insertions, 0 deletions