We've been seeing the following panic in production

BUG: kernel NULL pointer dereference, address: 0000000000000065
PGD 2f485f067 P4D 2f485f067 PUD 2cc5d8067 PMD 0
RIP: 0010:ff_layout_cancel_io+0x3a/0x90 [nfs_layout_flexfiles]
Call Trace:
 <TASK>
 ? __die+0x78/0xc0
 ? page_fault_oops+0x286/0x380
 ? __rpc_execute+0x2c3/0x470 [sunrpc]
 ? rpc_new_task+0x42/0x1c0 [sunrpc]
 ? exc_page_fault+0x5d/0x110
 ? asm_exc_page_fault+0x22/0x30
 ? ff_layout_free_layoutreturn+0x110/0x110 [nfs_layout_flexfiles]
 ? ff_layout_cancel_io+0x3a/0x90 [nfs_layout_flexfiles]
 ? ff_layout_cancel_io+0x6f/0x90 [nfs_layout_flexfiles]
 pnfs_mark_matching_lsegs_return+0x1b0/0x360 [nfsv4]
 pnfs_error_mark_layout_for_return+0x9e/0x110 [nfsv4]
 ? ff_layout_send_layouterror+0x50/0x160 [nfs_layout_flexfiles]
 nfs4_ff_layout_prepare_ds+0x11f/0x290 [nfs_layout_flexfiles]
 ff_layout_pg_init_write+0xf0/0x1f0 [nfs_layout_flexfiles]
 __nfs_pageio_add_request+0x154/0x6c0 [nfs]
 nfs_pageio_add_request+0x26b/0x380 [nfs]
 nfs_do_writepage+0x111/0x1e0 [nfs]
 nfs_writepages_callback+0xf/0x30 [nfs]
 write_cache_pages+0x17f/0x380
 ? nfs_pageio_init_write+0x50/0x50 [nfs]
 ? nfs_writepages+0x6d/0x210 [nfs]
 ? nfs_writepages+0x6d/0x210 [nfs]
 nfs_writepages+0x125/0x210 [nfs]
 do_writepages+0x67/0x220
 ? generic_perform_write+0x14b/0x210
 filemap_fdatawrite_wbc+0x5b/0x80
 file_write_and_wait_range+0x6d/0xc0
 nfs_file_fsync+0x81/0x170 [nfs]
 ? nfs_file_mmap+0x60/0x60 [nfs]
 __x64_sys_fsync+0x53/0x90
 do_syscall_64+0x3d/0x90
 entry_SYSCALL_64_after_hwframe+0x46/0xb0

Inspecting the core with drgn I was able to pull this

  >>> prog.crashed_thread().stack_trace()[0]
  #0 at 0xffffffffa079657a (ff_layout_cancel_io+0x3a/0x84) in ff_layout_cancel_io at fs/nfs/flexfilelayout/flexfilelayout.c:2021:27
  >>> prog.crashed_thread().stack_trace()[0]['idx']
  (u32)1
  >>> prog.crashed_thread().stack_trace()[0]['flseg'].mirror_array[1].mirror_ds
  (struct nfs4_ff_layout_ds *)0xffffffffffffffed

This is clear from the stack trace, we call nfs4_ff_layout_prepare_ds()
which could error out initializing the mirror_ds, and then we go to
clean it all up and our check is only for if (!mirror->mirror_ds).  This
is inconsistent with the rest of the users of mirror_ds, which have

  if (IS_ERR_OR_NULL(mirror_ds))

to keep from tripping over this exact scenario.  Fix this up in
ff_layout_cancel_io() to make sure we don't panic when we get an error.
I also spot checked all the other instances of checking mirror_ds and we
appear to be doing the correct checks everywhere, only unconditionally
dereferencing mirror_ds when we know it would be valid.
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Fixes: b739a5bd ("NFSv4/flexfiles: Cancel I/O if the layout is recalled or revoked")
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>