ext4: only dirty folios when data journaling regular files

JIRA: https://issues.redhat.com/browse/RHEL-91256

commit e26268f
Author: Brian Foster <bfoster@redhat.com>
Date:   Fri May 16 13:38:00 2025 -0400

    ext4: only dirty folios when data journaling regular files

    fstest generic/388 occasionally reproduces a crash that looks as
    follows:

    BUG: kernel NULL pointer dereference, address: 0000000000000000
    ...
    Call Trace:
     <TASK>
     ext4_block_zero_page_range+0x30c/0x380 [ext4]
     ext4_truncate+0x436/0x440 [ext4]
     ext4_process_orphan+0x5d/0x110 [ext4]
     ext4_orphan_cleanup+0x124/0x4f0 [ext4]
     ext4_fill_super+0x262d/0x3110 [ext4]
     get_tree_bdev_flags+0x132/0x1d0
     vfs_get_tree+0x26/0xd0
     vfs_cmd_create+0x59/0xe0
     __do_sys_fsconfig+0x4ed/0x6b0
     do_syscall_64+0x82/0x170
     ...

    This occurs when processing a symlink inode from the orphan list. The
    partial block zeroing code in the truncate path calls
    ext4_dirty_journalled_data() -> folio_mark_dirty(). The latter calls
    mapping->a_ops->dirty_folio(), but symlink inodes are not assigned an
    a_ops vector in ext4, hence the crash.

    To avoid this problem, update the ext4_dirty_journalled_data() helper to
    only mark the folio dirty on regular files (for which a_ops is
    assigned). This also matches the journaling logic in the ext4_symlink()
    creation path, where ext4_handle_dirty_metadata() is called directly.

    Fixes: d84c9eb ("ext4: Mark pages with journalled data dirty")
    Signed-off-by: Brian Foster <bfoster@redhat.com>
    Link: https://patch.msgid.link/20250516173800.175577-1-bfoster@redhat.com
    Signed-off-by: Theodore Ts'o <tytso@mit.edu>
    Reviewed-by: Jan Kara <jack@suse.cz>
    Cc: stable@kernel.org

Signed-off-by: CKI Backport Bot <cki-ci-bot+cki-gitlab-backport-bot@redhat.com>

Author: CKI Backport Bot

fs/ext4/inode.c

@@ -998,7 +998,12 @@ int ext4_walk_page_buffers(handle_t *handle, struct inode *inode,
  */
 static int ext4_dirty_journalled_data(handle_t *handle, struct buffer_head *bh)
 {
-	folio_mark_dirty(bh->b_folio);
+	struct folio *folio = bh->b_folio;
+	struct inode *inode = folio->mapping->host;
+
+	/* only regular files have a_ops */
+	if (S_ISREG(inode->i_mode))
+		folio_mark_dirty(folio);
 	return ext4_handle_dirty_metadata(handle, NULL, bh);
 }