[PATCH] nilfs2: fix null pointer dereference at nilfs_segctor_do_construct()

[Ryusuke Konishi]

After commit c3aab9a0bd91 ("mm/filemap.c: don't initiate writeback if
mapping has no dirty pages"), the following null pointer dereference
has been reported on nilfs2:

 BUG: kernel NULL pointer dereference, address: 00000000000000a8
 #PF: supervisor read access in kernel mode
 #PF: error_code(0x0000) - not-present page
 PGD 0 P4D 0
 Oops: 0000 [#1] SMP PTI
 ...
 RIP: 0010:percpu_counter_add_batch+0xa/0x60
 ...
 Call Trace:
  __test_set_page_writeback+0x2d3/0x330
  nilfs_segctor_do_construct+0x10d3/0x2110 [nilfs2]
  nilfs_segctor_construct+0x168/0x260 [nilfs2]
  nilfs_segctor_thread+0x127/0x3b0 [nilfs2]
  kthread+0xf8/0x130
  ...

This crash turned out to be caused by set_page_writeback() call for
segment summary buffers at nilfs_segctor_prepare_write().

set_page_writeback() can call inc_wb_stat(inode_to_wb(inode),
WB_WRITEBACK) where inode_to_wb(inode) is NULL if the inode of
underlying block device does not have an associated wb.

This fixes the issue by calling inode_attach_wb() in advance to ensure
to associate the bdev inode with its wb.

Reported-by: Walton Hoops <me@xxxxxxxxxxxxxxx>
Reported-by: Tomas Hlavaty <tom@xxxxxxxxxx>
Reported-by: ARAI Shun-ichi <hermes@xxxxxxxxxxxxxxx>
Reported-by: Hideki EIRAKU <hdk1983@xxxxxxxxx>
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@xxxxxxxxx>
Tested-by: Ryusuke Konishi <konishi.ryusuke@xxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx # v5.4+
---
 fs/nilfs2/segment.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/fs/nilfs2/segment.c b/fs/nilfs2/segment.c
index 445eef4..91b58c8 100644
--- a/fs/nilfs2/segment.c
+++ b/fs/nilfs2/segment.c
@@ -2780,6 +2780,8 @@ int nilfs_attach_log_writer(struct super_block *sb, struct nilfs_root *root)
 	if (!nilfs->ns_writer)
 		return -ENOMEM;
 
+	inode_attach_wb(nilfs->ns_bdev->bd_inode, NULL);
+
 	err = nilfs_segctor_start_thread(nilfs->ns_writer);
 	if (err) {
 		kfree(nilfs->ns_writer);
-- 
1.8.3.1