[PATCH v2 2/2] btrfs: fix compressed write bio attribution

[Dennis Zhou <dennis@kernel.org>]

From a0569aebde08e31e994c92d0b70befb84f7f5563 Mon Sep 17 00:00:00 2001
From: Dennis Zhou <dennis@kernel.org>
Date: Wed, 11 Dec 2019 15:20:15 -0800

Bio attribution is handled at bio_set_dev() as once we have a device, we
have a corresponding request_queue and then can derive the current css.
In special cases, we want to attribute to bio to someone else. This can
be done by calling bio_associate_blkg_from_css() or
kthread_associate_blkcg() depending on the scenario. Btrfs does this for
compressed writeback as they are handled by kworkers, so the latter can
be done here.

Commit 1a41802701ec ("btrfs: drop bio_set_dev where not needed") removes
early bio_set_dev() calls prior to submit_stripe_bio(). This breaks the
above assumption that we'll have a request_queue when we are doing
association. To fix this, switch to using kthread_associate_blkcg().

Without this, we crash in btrfs/024:
[ 3052.093088] BUG: kernel NULL pointer dereference, address: 0000000000000510
[ 3052.107013] #PF: supervisor read access in kernel mode
[ 3052.107014] #PF: error_code(0x0000) - not-present page
[ 3052.107015] PGD 0 P4D 0
[ 3052.107021] Oops: 0000 [#1] SMP
[ 3052.138904] CPU: 42 PID: 201270 Comm: kworker/u161:0 Kdump: loaded Not tainted 5.5.0-rc1-00062-g4852d8ac90a9 #712
[ 3052.138905] Hardware name: Quanta Tioga Pass Single Side 01-0032211004/Tioga Pass Single Side, BIOS F08_3A18 12/20/2018
[ 3052.138912] Workqueue: btrfs-delalloc btrfs_work_helper
[ 3052.191375] RIP: 0010:bio_associate_blkg_from_css+0x1e/0x3c0
[ 3052.191377] Code: ff 90 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 41 54 49 89 fc 55 53 48 89 f3 48 83 ec 08 48 8b 47 08 65 ff 05 ea 6e 9f 7e <48> 8b a8 10 05 00 00 45 31 c9 45 31 c0 31 d2 31 f6 b9 02 00 00 00
[ 3052.191379] RSP: 0018:ffffc900210cfc90 EFLAGS: 00010282
[ 3052.191380] RAX: 0000000000000000 RBX: ffff88bfe5573c00 RCX: 0000000000000000
[ 3052.191382] RDX: ffff889db48ec2f0 RSI: ffff88bfe5573c00 RDI: ffff889db48ec2f0
[ 3052.191386] RBP: 0000000000000800 R08: 0000000000203bb0 R09: ffff889db16b2400
[ 3052.293364] R10: 0000000000000000 R11: ffff88a07fffde80 R12: ffff889db48ec2f0
[ 3052.293365] R13: 0000000000001000 R14: ffff889de82bc000 R15: ffff889e2b7bdcc8
[ 3052.293367] FS:  0000000000000000(0000) GS:ffff889ffba00000(0000) knlGS:0000000000000000
[ 3052.293368] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 3052.293369] CR2: 0000000000000510 CR3: 0000000002611001 CR4: 00000000007606e0
[ 3052.293370] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 3052.293371] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[ 3052.293372] PKRU: 55555554
[ 3052.293376] Call Trace:
[ 3052.402552]  btrfs_submit_compressed_write+0x137/0x390
[ 3052.402558]  submit_compressed_extents+0x40f/0x4c0
[ 3052.422401]  btrfs_work_helper+0x246/0x5a0
[ 3052.422408]  process_one_work+0x200/0x570
[ 3052.438601]  ? process_one_work+0x180/0x570
[ 3052.438605]  worker_thread+0x4c/0x3e0
[ 3052.438614]  kthread+0x103/0x140
[ 3052.460735]  ? process_one_work+0x570/0x570
[ 3052.460737]  ? kthread_mod_delayed_work+0xc0/0xc0
[ 3052.460744]  ret_from_fork+0x24/0x30

Fixes: 1a41802701ec ("btrfs: drop bio_set_dev where not needed")
Cc: David Sterba <dsterba@suse.com>
Cc: Josef Bacik <josef@toxicpanda.com>
Reported-by: Chris Murphy <chris@colorremedies.com>
Signed-off-by: Dennis Zhou <dennis@kernel.org>
---
v2: rely on kthread_associate_blkcg() instead.

 fs/btrfs/compression.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/fs/btrfs/compression.c b/fs/btrfs/compression.c
index 4ce81571f0cd..de95ad27722f 100644
--- a/fs/btrfs/compression.c
+++ b/fs/btrfs/compression.c
@@ -447,7 +447,7 @@ blk_status_t btrfs_submit_compressed_write(struct inode *inode, u64 start,
 
 	if (blkcg_css) {
 		bio->bi_opf |= REQ_CGROUP_PUNT;
-		bio_associate_blkg_from_css(bio, blkcg_css);
+		kthread_associate_blkcg(blkcg_css);
 	}
 	refcount_set(&cb->pending_bios, 1);
 
@@ -491,10 +491,8 @@ blk_status_t btrfs_submit_compressed_write(struct inode *inode, u64 start,
 			bio->bi_opf = REQ_OP_WRITE | write_flags;
 			bio->bi_private = cb;
 			bio->bi_end_io = end_compressed_bio_write;
-			if (blkcg_css) {
+			if (blkcg_css)
 				bio->bi_opf |= REQ_CGROUP_PUNT;
-				bio_associate_blkg_from_css(bio, blkcg_css);
-			}
 			bio_add_page(bio, page, PAGE_SIZE, 0);
 		}
 		if (bytes_left < PAGE_SIZE) {
@@ -521,6 +519,9 @@ blk_status_t btrfs_submit_compressed_write(struct inode *inode, u64 start,
 		bio_endio(bio);
 	}
 
+	if (blkcg_css)
+		kthread_associate_blkcg(NULL);
+
 	return 0;
 }
 
-- 
2.17.1