7.0-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Yuto Ohnuki <redacted>

[ Upstream commit e9b004ff83067cdf96774b45aea4b239ace99a2f ]

wbt_init_enable_default() uses WARN_ON_ONCE to check for failures from
wbt_alloc() and wbt_init(). However, both are expected failure paths:

- wbt_alloc() can return NULL under memory pressure (-ENOMEM)
- wbt_init() can fail with -EBUSY if wbt is already registered

syzbot triggers this by injecting memory allocation failures during MTD
partition creation via ioctl(BLKPG), causing a spurious warning.

wbt_init_enable_default() is a best-effort initialization called from
blk_register_queue() with a void return type. Failure simply means the
disk operates without writeback throttling, which is harmless.

Replace WARN_ON_ONCE with plain if-checks, consistent with how
wbt_set_lat() in the same file already handles these failures. Add a
pr_warn() for the wbt_init() failure to retain diagnostic information
without triggering a full stack trace.

Reported-by: syzbot+71fcf20f7c1e5043d78c@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=71fcf20f7c1e5043d78c
Fixes: 41afaeeda509 ("blk-wbt: fix possible deadlock to nest pcpu_alloc_mutex under q_usage_counter")
Signed-off-by: Yuto Ohnuki <redacted>
Reviewed-by: Yu Kuai <yukuai@fnnas.com>
Reviewed-by: Nilay Shroff <redacted>
Link: https://patch.msgid.link/20260316070358.65225-2-ytohnuki@amazon.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 block/blk-wbt.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/block/blk-wbt.c b/block/blk-wbt.c
index 33006edfccd44..dcc2438ca16dc 100644
--- a/block/blk-wbt.c
+++ b/block/blk-wbt.c

@@ -782,10 +782,11 @@ void wbt_init_enable_default(struct gendisk *disk)
 		return;
 
 	rwb = wbt_alloc();
-	if (WARN_ON_ONCE(!rwb))
+	if (!rwb)
 		return;
 
-	if (WARN_ON_ONCE(wbt_init(disk, rwb))) {
+	if (wbt_init(disk, rwb)) {
+		pr_warn("%s: failed to enable wbt\n", disk->disk_name);
 		wbt_free(rwb);
 		return;
 	}

-- 
2.53.0