linux-kernel - Re: [PATCH v2 3/3] nbd: correct the maximum value for discard sectors

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Message-ID: <20240807135625.GA242945@perftesting>
Date: Wed, 7 Aug 2024 09:56:25 -0400
From: Josef Bacik <josef@...icpanda.com>
To: Wouter Verhelst <w@...r.be>
Cc: Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
	nbd@...er.debian.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/3] nbd: correct the maximum value for discard sectors

On Tue, Aug 06, 2024 at 03:30:56PM +0200, Wouter Verhelst wrote:
> The version of the NBD protocol implemented by the kernel driver
> currently has a 32 bit field for length values. As the NBD protocol uses
> bytes as a unit of length, length values larger than 2^32 bytes cannot
> be expressed.
> 
> Update the max_hw_discard_sectors field to match that.
> 
> Signed-off-by: Wouter Verhelst <w@...r.be>
> Fixes: 268283244c0f018dec8bf4a9c69ce50684561f46
> ---
>  drivers/block/nbd.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
> index 20e9f9fdeaae..1457f0c8a4a4 100644
> --- a/drivers/block/nbd.c
> +++ b/drivers/block/nbd.c
> @@ -339,7 +339,7 @@ static int __nbd_set_size(struct nbd_device *nbd, loff_t bytesize,
>  
>  	lim = queue_limits_start_update(nbd->disk->queue);
>  	if (nbd->config->flags & NBD_FLAG_SEND_TRIM)
> -		lim.max_hw_discard_sectors = UINT_MAX;
> +		lim.max_hw_discard_sectors = UINT_MAX / blksize;

We use 512 as the "sectors" measurement throughout the block layer, so our limit
is actually

UINT32_MAX >> 9

since we can only send at most UINT32_MAX as our length.  Fix it to be that for
both patches and you should be good.  Thanks,

Josef