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

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Message-ID: <effc195f-9f0e-4e0d-95ab-5bae22702f95@kernel.org>
Date: Wed, 7 Aug 2024 10:20:20 -0700
From: Damien Le Moal <dlemoal@...nel.org>
To: Josef Bacik <josef@...icpanda.com>, 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 2024/08/07 6:56, Josef Bacik wrote:
> 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

UINT_MAX >> SECTOR_SHIFT

would be better.

> 
> 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
> 

-- 
Damien Le Moal
Western Digital Research