[PATCH v2] blk-wbt: remove WARN_ON_ONCE from wbt_init_enable_default()

[Yuto Ohnuki <ytohnuki@amazon.com>]

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 <ytohnuki@amazon.com>
---
Changes in v2:
- Add pr_warn() for wbt_init() failure as suggested by Yu Kuai
- Link to v1: https://lore.kernel.org/all/20260315143137.13547-2-ytohnuki@amazon.com/
---
 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 33006edfccd4..dcc2438ca16d 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;
 	}
-- 
2.50.1




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