linux-kernel - Re: [PATCH v13 13/13] dm: add power-of-2 target for zoned devices with non power-of-2 zone sizes

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Message-ID: <e42a0579-61b2-7b77-08cb-6723278490cc@samsung.com>
Date:   Wed, 14 Sep 2022 21:16:30 +0200
From:   Pankaj Raghav <p.raghav@...sung.com>
To:     Mike Snitzer <snitzer@...hat.com>
CC:     <hch@....de>, <agk@...hat.com>, <damien.lemoal@...nsource.wdc.com>,
        <axboe@...nel.dk>, <snitzer@...nel.org>,
        <linux-kernel@...r.kernel.org>, <Johannes.Thumshirn@....com>,
        <linux-nvme@...ts.infradead.org>, <pankydev8@...il.com>,
        <matias.bjorling@....com>, <linux-block@...r.kernel.org>,
        <bvanassche@....org>, <gost.dev@...sung.com>,
        <dm-devel@...hat.com>, <hare@...e.de>, <jaegeuk@...nel.org>,
        Damien Le Moal <damien.lemoal@....com>
Subject: Re: [PATCH v13 13/13] dm: add power-of-2 target for zoned devices
 with non power-of-2 zone sizes

>> +
>> +	if (is_power_of_2(zone_size))
>> +		DMWARN("%pg: underlying device has a power-of-2 number of sectors per zone",
>> +		       dmh->dev->bdev);
>> +
>> +	dmh->zone_size = zone_size;
>> +	dmh->zone_size_po2 = 1 << get_count_order_long(zone_size);
>> +	dmh->zone_size_po2_shift = ilog2(dmh->zone_size_po2);
>> +	dmh->zone_size_diff = dmh->zone_size_po2 - dmh->zone_size;
>> +	ti->private = dmh;
>> +	ti->max_io_len = dmh->zone_size_po2;
>> +	dmh->nr_zones = npo2_zone_no(dmh, ti->len);
>> +	ti->len = dmh->zone_size_po2 * dmh->nr_zones;
>> +
>> +	return 0;
>> +}
> 
> The above error paths need to unwind the references or any other
> resources acquired before failing.  Please see other targets for how
> they handle sequencing of the needed operations (e.g. dm_put_device)
> in the error path by using gotos, etc.
> 

Ok. That makes sense, and it should be pretty straight forward to do that.

>> +
>> +static void dm_po2z_io_hints(struct dm_target *ti, struct queue_limits *limits)
>> +{
>> +	struct dm_po2z_target *dmh = ti->private;
>> +
>> +	limits->chunk_sectors = dmh->zone_size_po2;
>> +}
> 
> Are you certain you shouldn't at least be exposing a different
> logical_block_size to upper layers?
> 
To be honest, I tested my patches in QEMU with 4k Logical block size and on
a device with 4k LBA size.

I did a quick test with 512B LBA size in QEMU, and I didn't see any
failures when I ran my normal test suite.

Do you see any problem with exposing the same LBA as the underlying device?

>> +
>> +static void dm_po2z_status(struct dm_target *ti, status_type_t type,
>> +			   unsigned int status_flags, char *result,
>> +			   unsigned int maxlen)
>> +{
>> +	struct dm_po2z_target *dmh = ti->private;
>> +	size_t sz = 0;
>> +
>> +	switch (type) {
>> +	case STATUSTYPE_INFO:
>> +		DMEMIT("%s %lld", dmh->dev->name,
>> +		       (unsigned long long)dmh->zone_size_po2);
>> +		break;
> 
> Wouldn't it be worthwhile to expose the zone sectors (native npo2 vs
> simulated po2?) You merely roundup but never expose what you're using
> (unless I'm missing something about generic "zoned" device
> capabilities).
>

BLKREPORTZONE ioctl is typically used to get the zone information from a
zoned block device, which should expose the npo2 zone sectors(zone
capacity) in this case.

But I do see the value of exposing the dmh->zone_size instead of
dmh->zone_size_po2 as the latter can be easily calculated from the former
or it can be retrieved by reading the chunk_sectors. I will fix that up.


> Mike
>