linux-kernel - Re: [PATCH] mmc: core: Fix off-by-one error in mmc_do_calc_max

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Date:	Mon, 01 Jun 2015 13:36:45 +0300
From:	Adrian Hunter <adrian.hunter@...el.com>
To:	David Jander <david@...tonic.nl>,
	Ulf Hansson <ulf.hansson@...aro.org>,
	Sascha Hauer <s.hauer@...gutronix.de>
CC:	Johan Rudholm <johan.rudholm@...s.com>,
	Javier Martinez Canillas <javier.martinez@...labora.co.uk>,
	linux-mmc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mmc: core: Fix off-by-one error in mmc_do_calc_max_discard()

On 01/06/15 12:20, David Jander wrote:
> qty is the maximum number of discard that _do_ fit in the timeout, not
> the first amount that does _not_ fit anymore.
> This seemingly harmless error has a very severe performance impact when
> the timeout value is enough for only 1 erase group.
> 
> Signed-off-by: David Jander <david@...tonic.nl>
> ---
>  drivers/mmc/core/core.c | 7 ++-----
>  1 file changed, 2 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/mmc/core/core.c b/drivers/mmc/core/core.c
> index 92e7671..1f9573b 100644
> --- a/drivers/mmc/core/core.c
> +++ b/drivers/mmc/core/core.c
> @@ -2234,16 +2234,13 @@ static unsigned int mmc_do_calc_max_discard(struct mmc_card *card,
>  	if (!qty)
>  		return 0;
>  
> -	if (qty == 1)
> -		return 1;
> -
>  	/* Convert qty to sectors */
>  	if (card->erase_shift)
> -		max_discard = --qty << card->erase_shift;
> +		max_discard = qty << card->erase_shift;
>  	else if (mmc_card_sd(card))
>  		max_discard = qty;
>  	else
> -		max_discard = --qty * card->erase_size;
> +		max_discard = qty * card->erase_size;
>  
>  	return max_discard;
>  }
> 

This keeps coming up but there is more to it than that.  See here:

	http://marc.info/?l=linux-mmc&m=142504164427546

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