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

[Yu Kuai]

Hi,

在 2026/3/15 22:31, Yuto Ohnuki 写道:
> 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.
>
> 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>
> ---
>   block/blk-wbt.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/block/blk-wbt.c b/block/blk-wbt.c
> index 33006edfccd4..4337cbb7380c 100644
> --- a/block/blk-wbt.c
> +++ b/block/blk-wbt.c
> @@ -782,10 +782,10 @@ 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)) {

Can you also add a warning message that disk failed to enable wbt?

Otherwise this patch looks good to me.

>   		wbt_free(rwb);
>   		return;
>   	}

-- 
Thansk,
Kuai