linux-kernel - Re: [PATCH v6 2/8] sbitmap: fix off-by-one when wrapping hint

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Message-ID: <aB173vMuGb_Sq2vx@fedora>
Date: Fri, 9 May 2025 11:51:58 +0800
From: Ming Lei <ming.lei@...hat.com>
To: Uday Shankar <ushankar@...estorage.com>
Cc: Jens Axboe <axboe@...nel.dk>,
	Caleb Sander Mateos <csander@...estorage.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Shuah Khan <shuah@...nel.org>, Jonathan Corbet <corbet@....net>,
	linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-kselftest@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH v6 2/8] sbitmap: fix off-by-one when wrapping hint

On Wed, May 07, 2025 at 03:49:36PM -0600, Uday Shankar wrote:
> In update_alloc_hint_after_get, we wrap the new hint back to 0 one bit
> too early. This breaks round robin tag allocation (BLK_MQ_F_TAG_RR) -
> some tags get skipped, so we don't get round robin tags even in the
> simple case of single-threaded load on a single hctx. Fix the off-by-one
> in the wrapping condition so that round robin tag allocation works
> properly.
> 
> The same pattern occurs in __sbitmap_get_word, so fix it there too.
> 
> Signed-off-by: Uday Shankar <ushankar@...estorage.com>
> ---
>  lib/sbitmap.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
> index d3412984170c03dc6600bbe53f130404b765ac5a..aa1cec78b9649f1f3e8ef2d617dd7ee724391a8c 100644
> --- a/lib/sbitmap.c
> +++ b/lib/sbitmap.c
> @@ -51,7 +51,7 @@ static inline void update_alloc_hint_after_get(struct sbitmap *sb,
>  	} else if (nr == hint || unlikely(sb->round_robin)) {
>  		/* Only update the hint if we used it. */
>  		hint = nr + 1;
> -		if (hint >= depth - 1)
> +		if (hint >= depth)
>  			hint = 0;
>  		this_cpu_write(*sb->alloc_hint, hint);

This may help for round robin.

>  	}
> @@ -182,7 +182,7 @@ static int __sbitmap_get_word(unsigned long *word, unsigned long depth,
>  			break;
>  
>  		hint = nr + 1;
> -		if (hint >= depth - 1)
> +		if (hint >= depth)
>  			hint = 0;

I guess round robin may need to return -1 if 'hint >= depth'.

Also the existing behavior starts from adding sbitmap, maybe Jens
has idea why hint is checked against 'depth -1' instead of 'depth'.



Thanks,
Ming