linux-kernel - Re: [PATCH] block: remove test of io priority level

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Message-ID: <adca39d3-04fc-45f5-8e34-0d06714f0ff9@kernel.org>
Date: Tue, 29 Apr 2025 19:50:11 +0900
From: Damien Le Moal <dlemoal@...nel.org>
To: Aaron Lu <ziqianlu@...edance.com>, Jens Axboe <axboe@...nel.dk>
Cc: Kexin Wei <ys.weikexin@....com>, linux-block@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] block: remove test of io priority level

On 4/29/25 17:29, Aaron Lu wrote:
> Ever since commit eca2040972b4("scsi: block: ioprio: Clean up interface
> definition"), the io priority level is masked and can no longer be larger
> than IOPRIO_NR_LEVELS so remove this now useless test.
> 
> The actual test of io prio level is done in ioprio_value() where any
> invalid input of class/level/hint will result in an invalid class being
> passed to the syscall, this is introduced in commit 01584c1e2337("scsi: 
> block: Improve ioprio value validity checks").
> 
> Reported-by: Kexin Wei <ys.weikexin@....com>
> Cc: Damien Le Moal <dlemoal@...nel.org>
> Signed-off-by: Aaron Lu <ziqianlu@...edance.com>
> ---
> Kexin reported a LTP/ioprio_set03 case failure, where the test would
> pass IOPRIO_CLASS_BE with priority level 8 and see if kernel would
> return error. Turned out she is using an old kernel header where the
> change introduced in commit 01584c1e2337("scsi: block: Improve ioprio
> value validity checks") isn't available. During troubleshooting, I find
> this priority level test confusing and misleading so I think it should
> be removed.

What is confusing and misleading about the fact that we support only 8 priority
levels (0 to 7) and should check for it ?

With that said, the test is indeed redundant for the BE and RT class because we
have:

int ioprio_check_cap(int ioprio)
{
	int class = IOPRIO_PRIO_CLASS(ioprio);
	int level = IOPRIO_PRIO_LEVEL(ioprio);

And the macro IOPRIO_PRIO_LEVEL() will mask the level value to something between
0 and 7, always. So necessarily, level will always be lower than
IOPRIO_NR_LEVELS. So please reword your commit message to explain that rather
than describe what a user may or may not use when setting an ioprio field.
And also simplify the patch:

diff --git a/block/ioprio.c b/block/ioprio.c
index 73301a261429..f0ee2798539c 100644
--- a/block/ioprio.c
+++ b/block/ioprio.c
@@ -46,12 +46,8 @@ int ioprio_check_cap(int ioprio)
                         */
                        if (!capable(CAP_SYS_ADMIN) && !capable(CAP_SYS_NICE))
                                return -EPERM;
-                       fallthrough;
-                       /* rt has prio field too */
-               case IOPRIO_CLASS_BE:
-                       if (level >= IOPRIO_NR_LEVELS)
-                               return -EINVAL;
                        break;
+               case IOPRIO_CLASS_BE:
                case IOPRIO_CLASS_IDLE:
                        break;
                case IOPRIO_CLASS_NONE:


>  block/ioprio.c | 5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
> 
> diff --git a/block/ioprio.c b/block/ioprio.c
> index 73301a261429f..60364d3faf800 100644
> --- a/block/ioprio.c
> +++ b/block/ioprio.c
> @@ -46,11 +46,8 @@ int ioprio_check_cap(int ioprio)
>  			 */
>  			if (!capable(CAP_SYS_ADMIN) && !capable(CAP_SYS_NICE))
>  				return -EPERM;
> -			fallthrough;
> -			/* rt has prio field too */
> +			break;
>  		case IOPRIO_CLASS_BE:
> -			if (level >= IOPRIO_NR_LEVELS)
> -				return -EINVAL;
>  			break;
>  		case IOPRIO_CLASS_IDLE:
>  			break;


-- 
Damien Le Moal
Western Digital Research