linux-kernel - Re: [PATCH v3 11/11] dm-zoned: ensure only power of 2 zone sizes are allowed

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Message-ID: <d8e86c32-f122-01df-168e-648179766c55@samsung.com>
Date:   Wed, 11 May 2022 16:39:17 +0200
From:   Pankaj Raghav <p.raghav@...sung.com>
To:     <dsterba@...e.cz>, <jaegeuk@...nel.org>, <hare@...e.de>,
        <dsterba@...e.com>, <axboe@...nel.dk>, <hch@....de>,
        <damien.lemoal@...nsource.wdc.com>, <snitzer@...nel.org>,
        Chris Mason <clm@...com>, Josef Bacik <josef@...icpanda.com>,
        <bvanassche@....org>, <linux-fsdevel@...r.kernel.org>,
        <matias.bjorling@....com>, Jens Axboe <axboe@...com>,
        <gost.dev@...sung.com>, <jonathan.derrick@...ux.dev>,
        <jiangbo.365@...edance.com>, <linux-nvme@...ts.infradead.org>,
        <dm-devel@...hat.com>, Naohiro Aota <naohiro.aota@....com>,
        <linux-kernel@...r.kernel.org>,
        Johannes Thumshirn <jth@...nel.org>,
        "Sagi Grimberg" <sagi@...mberg.me>,
        Alasdair Kergon <agk@...hat.com>,
        <linux-block@...r.kernel.org>, Chaitanya Kulkarni <kch@...dia.com>,
        "Keith Busch" <kbusch@...nel.org>, <linux-btrfs@...r.kernel.org>,
        Luis Chamberlain <mcgrof@...nel.org>
Subject: Re: [PATCH v3 11/11] dm-zoned: ensure only power of 2 zone sizes
 are allowed

Hi David,

On 2022-05-09 20:54, David Sterba wrote:>> diff --git
a/drivers/md/dm-zone.c b/drivers/md/dm-zone.c
>> index 3e7b1fe15..27dc4ddf2 100644
>> --- a/drivers/md/dm-zone.c
>> +++ b/drivers/md/dm-zone.c
>> @@ -231,6 +231,18 @@ static int dm_revalidate_zones(struct mapped_device *md, struct dm_table *t)
>>  	struct request_queue *q = md->queue;
>>  	unsigned int noio_flag;
>>  	int ret;
>> +	struct block_device *bdev = md->disk->part0;
>> +	sector_t zone_sectors;
>> +	char bname[BDEVNAME_SIZE];
>> +
>> +	zone_sectors = bdev_zone_sectors(bdev);
>> +
>> +	if (!is_power_of_2(zone_sectors)) {
> 
> is_power_of_2 takes 'unsigned long' and sector_t is u64, so this is not
> 32bit clean and we had an actual bug where value 1<<48 was not
> recognized as power of 2.
> 
Good catch. Now I understand why btrfs has a helper for is_power_of_two_u64.

But the zone size can never be more than 32bit value so the zone size
sect will never greater than unsigned long.

With that said, we have two options:

1.) We can put a comment explaining that even though it is 32 bit
unsafe, zone size sect can never be a 32bit value

or

2) We should move the btrfs only helper `is_power_of_two_u64` to some
common header and use it everywhere.

Let me know your thoughts.