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From:   Damien Le Moal <Damien.LeMoal@wdc.com>
To:     Niklas Cassel <Niklas.Cassel@wdc.com>,
        Jonathan Corbet <corbet@lwn.net>, Jens Axboe <axboe@kernel.dk>,
        Keith Busch <kbusch@kernel.org>,
        Christoph Hellwig <hch@lst.de>,
        Sagi Grimberg <sagi@grimberg.me>,
        "James E.J. Bottomley" <jejb@linux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@oracle.com>
CC:     "linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
        "linux-nvme@lists.infradead.org" <linux-nvme@lists.infradead.org>,
        "linux-scsi@vger.kernel.org" <linux-scsi@vger.kernel.org>
Subject: Re: [PATCH 1/2] block: add max_open_zones to blk-sysfs
Thread-Topic: [PATCH 1/2] block: add max_open_zones to blk-sysfs
Thread-Index: AQHWQ8jtlLWiiEumE0+QMSt8tXUnYw==
Date:   Tue, 30 Jun 2020 01:49:41 +0000
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References: <20200616102546.491961-1-niklas.cassel@wdc.com>
 <20200616102546.491961-2-niklas.cassel@wdc.com>
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On 2020/06/16 19:28, Niklas Cassel wrote:=0A=
> Add a new max_open_zones definition in the sysfs documentation.=0A=
> This definition will be common for all devices utilizing the zoned block=
=0A=
> device support in the kernel.=0A=
> =0A=
> Export max open zones according to this new definition for NVMe Zoned=0A=
> Namespace devices, ZAC ATA devices (which are treated as SCSI devices by=
=0A=
> the kernel), and ZBC SCSI devices.=0A=
> =0A=
> Add the new max_open_zones struct member to the request_queue, rather=0A=
=0A=
Add the new max_open_zones member to struct request_queue...=0A=
=0A=
> than as a queue limit, since this property cannot be split across stackin=
g=0A=
> drivers.=0A=
=0A=
But device-mapper target device have a request queue too and it looks like =
your=0A=
patch is not setting any value, using the default 0 for dm-linear and dm-fl=
akey.=0A=
Attaching the new attribute directly to the request queue rather than addin=
g it=0A=
as part of the queue limits seems odd. Even if DM case is left unsupported=
=0A=
(using the default 0 =3D no limit), it may be cleaner to add the field as p=
art of=0A=
the limit struct.=0A=
=0A=
Adding the field as a device attribute rather than a queue limit, similarly=
 to=0A=
the device maximum queue depth would be another option. In such case, inclu=
ding=0A=
the field directly as part of the request queue makes more sense.=0A=
=0A=
> =0A=
> Signed-off-by: Niklas Cassel <niklas.cassel@wdc.com>=0A=
> ---=0A=
>  Documentation/block/queue-sysfs.rst |  7 +++++++=0A=
>  block/blk-sysfs.c                   | 15 +++++++++++++++=0A=
>  drivers/nvme/host/zns.c             |  1 +=0A=
>  drivers/scsi/sd_zbc.c               |  4 ++++=0A=
>  include/linux/blkdev.h              | 20 ++++++++++++++++++++=0A=
>  5 files changed, 47 insertions(+)=0A=
> =0A=
> diff --git a/Documentation/block/queue-sysfs.rst b/Documentation/block/qu=
eue-sysfs.rst=0A=
> index 6a8513af9201..f01cf8530ae4 100644=0A=
> --- a/Documentation/block/queue-sysfs.rst=0A=
> +++ b/Documentation/block/queue-sysfs.rst=0A=
> @@ -117,6 +117,13 @@ Maximum number of elements in a DMA scatter/gather l=
ist with integrity=0A=
>  data that will be submitted by the block layer core to the associated=0A=
>  block driver.=0A=
>  =0A=
> +max_open_zones (RO)=0A=
> +-------------------=0A=
> +For zoned block devices (zoned attribute indicating "host-managed" or=0A=
> +"host-aware"), the sum of zones belonging to any of the zone states:=0A=
> +EXPLICIT OPEN or IMPLICIT OPEN, is limited by this value.=0A=
> +If this value is 0, there is no limit.=0A=
> +=0A=
>  max_sectors_kb (RW)=0A=
>  -------------------=0A=
>  This is the maximum number of kilobytes that the block layer will allow=
=0A=
> diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c=0A=
> index 02643e149d5e..fa42961e9678 100644=0A=
> --- a/block/blk-sysfs.c=0A=
> +++ b/block/blk-sysfs.c=0A=
> @@ -305,6 +305,11 @@ static ssize_t queue_nr_zones_show(struct request_qu=
eue *q, char *page)=0A=
>  	return queue_var_show(blk_queue_nr_zones(q), page);=0A=
>  }=0A=
>  =0A=
> +static ssize_t queue_max_open_zones_show(struct request_queue *q, char *=
page)=0A=
> +{=0A=
> +	return queue_var_show(queue_max_open_zones(q), page);=0A=
> +}=0A=
> +=0A=
>  static ssize_t queue_nomerges_show(struct request_queue *q, char *page)=
=0A=
>  {=0A=
>  	return queue_var_show((blk_queue_nomerges(q) << 1) |=0A=
> @@ -667,6 +672,11 @@ static struct queue_sysfs_entry queue_nr_zones_entry=
 =3D {=0A=
>  	.show =3D queue_nr_zones_show,=0A=
>  };=0A=
>  =0A=
> +static struct queue_sysfs_entry queue_max_open_zones_entry =3D {=0A=
> +	.attr =3D {.name =3D "max_open_zones", .mode =3D 0444 },=0A=
> +	.show =3D queue_max_open_zones_show,=0A=
> +};=0A=
> +=0A=
>  static struct queue_sysfs_entry queue_nomerges_entry =3D {=0A=
>  	.attr =3D {.name =3D "nomerges", .mode =3D 0644 },=0A=
>  	.show =3D queue_nomerges_show,=0A=
> @@ -765,6 +775,7 @@ static struct attribute *queue_attrs[] =3D {=0A=
>  	&queue_nonrot_entry.attr,=0A=
>  	&queue_zoned_entry.attr,=0A=
>  	&queue_nr_zones_entry.attr,=0A=
> +	&queue_max_open_zones_entry.attr,=0A=
>  	&queue_nomerges_entry.attr,=0A=
>  	&queue_rq_affinity_entry.attr,=0A=
>  	&queue_iostats_entry.attr,=0A=
> @@ -792,6 +803,10 @@ static umode_t queue_attr_visible(struct kobject *ko=
bj, struct attribute *attr,=0A=
>  		(!q->mq_ops || !q->mq_ops->timeout))=0A=
>  			return 0;=0A=
>  =0A=
> +	if (attr =3D=3D &queue_max_open_zones_entry.attr &&=0A=
> +	    !blk_queue_is_zoned(q))=0A=
> +		return 0;=0A=
> +=0A=
>  	return attr->mode;=0A=
>  }=0A=
>  =0A=
> diff --git a/drivers/nvme/host/zns.c b/drivers/nvme/host/zns.c=0A=
> index c08f6281b614..af156529f3b6 100644=0A=
> --- a/drivers/nvme/host/zns.c=0A=
> +++ b/drivers/nvme/host/zns.c=0A=
> @@ -82,6 +82,7 @@ int nvme_update_zone_info(struct gendisk *disk, struct =
nvme_ns *ns,=0A=
>  =0A=
>  	q->limits.zoned =3D BLK_ZONED_HM;=0A=
>  	blk_queue_flag_set(QUEUE_FLAG_ZONE_RESETALL, q);=0A=
> +	blk_queue_max_open_zones(q, le32_to_cpu(id->mor) + 1);=0A=
>  free_data:=0A=
>  	kfree(id);=0A=
>  	return status;=0A=
> diff --git a/drivers/scsi/sd_zbc.c b/drivers/scsi/sd_zbc.c=0A=
> index 183a20720da9..aa3564139b40 100644=0A=
> --- a/drivers/scsi/sd_zbc.c=0A=
> +++ b/drivers/scsi/sd_zbc.c=0A=
> @@ -717,6 +717,10 @@ int sd_zbc_read_zones(struct scsi_disk *sdkp, unsign=
ed char *buf)=0A=
>  	/* The drive satisfies the kernel restrictions: set it up */=0A=
>  	blk_queue_flag_set(QUEUE_FLAG_ZONE_RESETALL, q);=0A=
>  	blk_queue_required_elevator_features(q, ELEVATOR_F_ZBD_SEQ_WRITE);=0A=
> +	if (sdkp->zones_max_open =3D=3D U32_MAX)=0A=
> +		blk_queue_max_open_zones(q, 0);=0A=
> +	else=0A=
> +		blk_queue_max_open_zones(q, sdkp->zones_max_open);=0A=
=0A=
This is correct only for host-managed drives. Host-aware models define the=
=0A=
"OPTIMAL NUMBER OF OPEN SEQUENTIAL WRITE PREFERRED ZONES" instead of a maxi=
mum=0A=
number of open sequential write required zones.=0A=
=0A=
Since the standard does not actually explicitly define what the value of th=
e=0A=
maximum number of open sequential write required zones should be for a=0A=
host-aware drive, I would suggest to always have the max_open_zones value s=
et to=0A=
0 for host-aware disks.=0A=
=0A=
>  	nr_zones =3D round_up(sdkp->capacity, zone_blocks) >> ilog2(zone_blocks=
);=0A=
>  =0A=
>  	/* READ16/WRITE16 is mandatory for ZBC disks */=0A=
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h=0A=
> index 8fd900998b4e..2f332f00501d 100644=0A=
> --- a/include/linux/blkdev.h=0A=
> +++ b/include/linux/blkdev.h=0A=
> @@ -520,6 +520,7 @@ struct request_queue {=0A=
>  	unsigned int		nr_zones;=0A=
>  	unsigned long		*conv_zones_bitmap;=0A=
>  	unsigned long		*seq_zones_wlock;=0A=
> +	unsigned int		max_open_zones;=0A=
>  #endif /* CONFIG_BLK_DEV_ZONED */=0A=
>  =0A=
>  	/*=0A=
> @@ -729,6 +730,17 @@ static inline bool blk_queue_zone_is_seq(struct requ=
est_queue *q,=0A=
>  		return true;=0A=
>  	return !test_bit(blk_queue_zone_no(q, sector), q->conv_zones_bitmap);=
=0A=
>  }=0A=
> +=0A=
> +static inline void blk_queue_max_open_zones(struct request_queue *q,=0A=
> +		unsigned int max_open_zones)=0A=
> +{=0A=
> +	q->max_open_zones =3D max_open_zones;=0A=
> +}=0A=
> +=0A=
> +static inline unsigned int queue_max_open_zones(const struct request_que=
ue *q)=0A=
> +{=0A=
> +	return q->max_open_zones;=0A=
> +}=0A=
>  #else /* CONFIG_BLK_DEV_ZONED */=0A=
>  static inline unsigned int blk_queue_nr_zones(struct request_queue *q)=
=0A=
>  {=0A=
> @@ -744,6 +756,14 @@ static inline unsigned int blk_queue_zone_no(struct =
request_queue *q,=0A=
>  {=0A=
>  	return 0;=0A=
>  }=0A=
> +static inline void blk_queue_max_open_zones(struct request_queue *q,=0A=
> +		unsigned int max_open_zones)=0A=
> +{=0A=
> +}=0A=
=0A=
Why is this one necessary ? For the !CONFIG_BLK_DEV_ZONED case, no driver s=
hould=0A=
ever call this function.=0A=
=0A=
> +static inline unsigned int queue_max_open_zones(const struct request_que=
ue *q)=0A=
> +{=0A=
> +	return 0;=0A=
> +}=0A=
>  #endif /* CONFIG_BLK_DEV_ZONED */=0A=
>  =0A=
>  static inline bool rq_is_sync(struct request *rq)=0A=
> =0A=
=0A=
=0A=
-- =0A=
Damien Le Moal=0A=
Western Digital Research=0A=