There are two states for ubifs writing pages:
1. Dirty, Private
2. Not Dirty, Not Private

There is a third possibility which maybe related to [1] that page is
private but not dirty caused by following process:

          PA
lock(page)
ubifs_write_end
  attach_page_private		// set Private
    __set_page_dirty_nobuffers	// set Dirty
unlock(page)

write_cache_pages
  lock(page)
  clear_page_dirty_for_io(page)	// clear Dirty
  ubifs_writepage
    write_inode
    // fail, goto out, following codes are not executed
    // do_writepage
    //   set_page_writeback 	// set Writeback
    //   detach_page_private	// clear Private
    //   end_page_writeback 	// clear Writeback
    out:
    unlock(page)		// Private, Not Dirty

                                       PB
				ksys_fadvise64_64
				  generic_fadvise
				     invalidate_inode_page
				     // page is neither Dirty nor Writeback
				       invalidate_complete_page
				       // page_has_private is true
					 try_to_release_page
					   ubifs_releasepage
					     ubifs_assert(c, 0) !!!

Then we may get following assertion failed:
  UBIFS error (ubi0:0 pid 1492): ubifs_assert_failed [ubifs]:
  UBIFS assert failed: 0, in fs/ubifs/file.c:1499
  UBIFS warning (ubi0:0 pid 1492): ubifs_ro_mode [ubifs]:
  switched to read-only mode, error -22
  CPU: 2 PID: 1492 Comm: aa Not tainted 5.16.0-rc2-00012-g7bb767dee0ba-dirty
  Call Trace:
    dump_stack+0x13/0x1b
    ubifs_ro_mode+0x54/0x60 [ubifs]
    ubifs_assert_failed+0x4b/0x80 [ubifs]
    ubifs_releasepage+0x7e/0x1e0 [ubifs]
    try_to_release_page+0x57/0xe0
    invalidate_inode_page+0xfb/0x130
    invalidate_mapping_pagevec+0x12/0x20
    generic_fadvise+0x303/0x3c0
    vfs_fadvise+0x35/0x40
    ksys_fadvise64_64+0x4c/0xb0

Jump [2] to find a reproducer.

[1] https://linux-mtd.infradead.narkive.com/NQoBeT1u/patch-rfc-ubifs-fix-assert-failed-in-ubifs-set-page-dirty
[2] https://bugzilla.kernel.org/show_bug.cgi?id=215357

Fixes: 1e51764a ("UBIFS: add new flash file system")
Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>