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

[Nilay Shroff]

On 3/16/26 12:33 PM, Yuto Ohnuki wrote:
> 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@xxxxxxxxxxxxxxxxxxxxxxxxx
> 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@xxxxxxxxxx>

Looks good to me:
Reviewed-by: Nilay Shroff <nilay@xxxxxxxxxxxxx>