writeback, cgroup: do not reparent dax inodes

The inode switching code is not suited for dax inodes.  An attempt to
switch a dax inode to a parent writeback structure (as a part of a
writeback cleanup procedure) results in a panic like this:

  run fstests generic/270 at 2021-07-15 05:54:02
  XFS (pmem0p2): EXPERIMENTAL big timestamp feature in use.  Use at your own risk!
  XFS (pmem0p2): DAX enabled. Warning: EXPERIMENTAL, use at your own risk
  XFS (pmem0p2): EXPERIMENTAL inode btree counters feature in use. Use at your own risk!
  XFS (pmem0p2): Mounting V5 Filesystem
  XFS (pmem0p2): Ending clean mount
  XFS (pmem0p2): Quotacheck needed: Please wait.
  XFS (pmem0p2): Quotacheck: Done.
  XFS (pmem0p2): xlog_verify_grant_tail: space > BBTOB(tail_blocks)
  XFS (pmem0p2): xlog_verify_grant_tail: space > BBTOB(tail_blocks)
  XFS (pmem0p2): xlog_verify_grant_tail: space > BBTOB(tail_blocks)
  BUG: unable to handle page fault for address: 0000000005b0f669
  #PF: supervisor read access in kernel mode
  #PF: error_code(0x0000) - not-present page
  PGD 0 P4D 0
  Oops: 0000 [#1] SMP PTI
  CPU: 13 PID: 10479 Comm: kworker/13:16 Not tainted 5.14.0-rc1-master-8096acd7442e+ #8
  Hardware name: HP ProLiant DL360 Gen9/ProLiant DL360 Gen9, BIOS P89 09/13/2016
  Workqueue: inode_switch_wbs inode_switch_wbs_work_fn
  RIP: 0010:inode_do_switch_wbs+0xaf/0x470
  Code: 00 30 0f 85 c1 03 00 00 0f 1f 44 00 00 31 d2 48 c7 c6 ff ff ff ff 48 8d 7c 24 08 e8 eb 49 1a 00 48 85 c0 74 4a bb ff ff ff ff <48> 8b 50 08 48 8d 4a ff 83 e2 01 48 0f 45 c1 48 8b 00 a8 08 0f 85
  RSP: 0018:ffff9c66691abdc8 EFLAGS: 00010002
  RAX: 0000000005b0f661 RBX: 00000000ffffffff RCX: ffff89e6a21382b0
  RDX: 0000000000000001 RSI: ffff89e350230248 RDI: ffffffffffffffff
  RBP: ffff89e681d19400 R08: 0000000000000000 R09: 0000000000000228
  R10: ffffffffffffffff R11: ffffffffffffffc0 R12: ffff89e6a2138130
  R13: ffff89e316af7400 R14: ffff89e316af6e78 R15: ffff89e6a21382b0
  FS:  0000000000000000(0000) GS:ffff89ee5fb40000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 0000000005b0f669 CR3: 0000000cb2410004 CR4: 00000000001706e0
  Call Trace:
   inode_switch_wbs_work_fn+0xb6/0x2a0
   process_one_work+0x1e6/0x380
   worker_thread+0x53/0x3d0
   kthread+0x10f/0x130
   ret_from_fork+0x22/0x30
  Modules linked in: xt_CHECKSUM xt_MASQUERADE xt_conntrack ipt_REJECT nf_reject_ipv4 nft_compat nft_chain_nat nf_nat nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 nft_counter nf_tables nfnetlink bridge stp llc rfkill sunrpc intel_rapl_msr intel_rapl_common sb_edac x86_pkg_temp_thermal intel_powerclamp coretemp kvm_intel ipmi_ssif kvm mgag200 i2c_algo_bit iTCO_wdt irqbypass drm_kms_helper iTCO_vendor_support acpi_ipmi rapl syscopyarea sysfillrect intel_cstate ipmi_si sysimgblt ioatdma dax_pmem_compat fb_sys_fops ipmi_devintf device_dax i2c_i801 pcspkr intel_uncore hpilo nd_pmem cec dax_pmem_core dca i2c_smbus acpi_tad lpc_ich ipmi_msghandler acpi_power_meter drm fuse xfs libcrc32c sd_mod t10_pi crct10dif_pclmul crc32_pclmul crc32c_intel tg3 ghash_clmulni_intel serio_raw hpsa hpwdt scsi_transport_sas wmi dm_mirror dm_region_hash dm_log dm_mod
  CR2: 0000000005b0f669
  ---[ end trace ed2105faff8384f3 ]---
  RIP: 0010:inode_do_switch_wbs+0xaf/0x470
  Code: 00 30 0f 85 c1 03 00 00 0f 1f 44 00 00 31 d2 48 c7 c6 ff ff ff ff 48 8d 7c 24 08 e8 eb 49 1a 00 48 85 c0 74 4a bb ff ff ff ff <48> 8b 50 08 48 8d 4a ff 83 e2 01 48 0f 45 c1 48 8b 00 a8 08 0f 85
  RSP: 0018:ffff9c66691abdc8 EFLAGS: 00010002
  RAX: 0000000005b0f661 RBX: 00000000ffffffff RCX: ffff89e6a21382b0
  RDX: 0000000000000001 RSI: ffff89e350230248 RDI: ffffffffffffffff
  RBP: ffff89e681d19400 R08: 0000000000000000 R09: 0000000000000228
  R10: ffffffffffffffff R11: ffffffffffffffc0 R12: ffff89e6a2138130
  R13: ffff89e316af7400 R14: ffff89e316af6e78 R15: ffff89e6a21382b0
  FS:  0000000000000000(0000) GS:ffff89ee5fb40000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 0000000005b0f669 CR3: 0000000cb2410004 CR4: 00000000001706e0
  Kernel panic - not syncing: Fatal exception
  Kernel Offset: 0x15200000 from 0xffffffff81000000 (relocation range: 0xffffffff80000000-0xffffffffbfffffff)
  ---[ end Kernel panic - not syncing: Fatal exception ]---

The crash happens on an attempt to iterate over attached pagecache pages
and check the dirty flag: a dax inode's xarray contains pfn's instead of
generic struct page pointers.

This happens for DAX and not for other kinds of non-page entries in the
inodes because it's a tagged iteration, and shadow/swap entries are
never tagged; only DAX entries get tagged.

Fix the problem by bailing out (with the false return value) of
inode_prepare_sbs_switch() if a dax inode is passed.

[[email protected]: changelog addition]

Link: https://lkml.kernel.org/r/[email protected]
Fixes: c22d70a ("writeback, cgroup: release dying cgwbs by switching attached inodes")
Signed-off-by: Roman Gushchin <[email protected]>
Reported-by: Murphy Zhou <[email protected]>
Reported-by: Darrick J. Wong <[email protected]>
Tested-by: Darrick J. Wong <[email protected]>
Tested-by: Murphy Zhou <[email protected]>
Acked-by: Matthew Wilcox (Oracle) <[email protected]>
Cc: Jan Kara <[email protected]>
Cc: Dave Chinner <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

fs/fs-writeback.c

@@ -521,6 +521,9 @@ static bool inode_prepare_wbs_switch(struct inode *inode,
 	 */
 	smp_mb();
 
+	if (IS_DAX(inode))
+		return false;
+
 	/* while holding I_WB_SWITCH, no one else can update the association */
 	spin_lock(&inode->i_lock);
 	if (!(inode->i_sb->s_flags & SB_ACTIVE) ||