Re: [PATCH v3 1/4] block: Add concurrent positioning ranges support

Damien Le Moal
2021-08-10T11:03:49Z

On 2021/08/10 17:24, Christoph Hellwig wrote:
> On Mon, Jul 26, 2021 at 10:38:03AM +0900, Damien Le Moal wrote:
>> The Concurrent Positioning Ranges VPD page (for SCSI) and Log (for ATA)
>> contain parameters describing the number of sets of contiguous LBAs that
>> can be served independently by a single LUN multi-actuator disk. This
>> patch provides the blk_queue_set_cranges() function allowing a device
>> driver to signal to the block layer that a disk has multiple actuators,
>> each one serving a contiguous range of sectors. To describe the set
>> of sector ranges representing the different actuators of a device, the
>> data type struct blk_cranges is introduced.
>>
>> For a device with multiple actuators, a struct blk_cranges is attached
>> to the device request queue by the blk_queue_set_cranges() function. The
>> function blk_alloc_cranges() is provided for drivers to allocate this
>> structure.
>>
>> The blk_cranges structure contains kobjects (struct kobject) to register
>> with sysfs the set of sector ranges defined by a device. On initial
>> device scan, this registration is done from blk_register_queue() using
>> the block layer internal function blk_register_cranges(). If a driver
>> calls blk_queue_set_cranges() for a registered queue, e.g. when a device
>> is revalidated, blk_queue_set_cranges() will execute
>> blk_register_cranges() to update the queue sysfs attribute files.
>>
>> The sysfs file structure created starts from the cranges sub-directory
>> and contains the start sector and number of sectors served by an
>> actuator, with the information for each actuator grouped in one
>> directory per actuator. E.g. for a dual actuator drive, we have:
>>
>> $ tree /sys/block/sdk/queue/cranges/
>> /sys/block/sdk/queue/cranges/
>> |-- 0
>> | |-- nr_sectors
>> | `-- sector
>> `-- 1
>> |-- nr_sectors
>> `-- sector
>>
>> For a regular single actuator device, the cranges directory does not
>> exist.
>>
>> Device revalidation may lead to changes to this structure and to the
>> attribute values. When manipulated, the queue sysfs_lock and
>> sysfs_dir_lock are held for atomicity, similarly to how the blk-mq and
>> elevator sysfs queue sub-directories are protected.
>>
>> The code related to the management of cranges is added in the new
>> file block/blk-cranges.c.
>>
>> Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
>> ---
>> block/Makefile | 2 +-
>> block/blk-cranges.c | 295 +++++++++++++++++++++++++++++++++++++++++
>> block/blk-sysfs.c | 13 ++
>> block/blk.h | 3 +
>> include/linux/blkdev.h | 29 ++++
>> 5 files changed, 341 insertions(+), 1 deletion(-)
>> create mode 100644 block/blk-cranges.c
>>
>> diff --git a/block/Makefile b/block/Makefile
>> index bfbe4e13ca1e..e477e6ca9ea6 100644
>> --- a/block/Makefile
>> +++ b/block/Makefile
>> @@ -9,7 +9,7 @@ obj-$(CONFIG_BLOCK) := bio.o elevator.o blk-core.o blk-sysfs.o \
>> blk-lib.o blk-mq.o blk-mq-tag.o blk-stat.o \
>> blk-mq-sysfs.o blk-mq-cpumap.o blk-mq-sched.o ioctl.o \
>> genhd.o ioprio.o badblocks.o partitions/ blk-rq-qos.o \
>> - disk-events.o
>> + disk-events.o blk-cranges.o
>>
>> obj-$(CONFIG_BOUNCE) += bounce.o
>> obj-$(CONFIG_BLK_SCSI_REQUEST) += scsi_ioctl.o
>> diff --git a/block/blk-cranges.c b/block/blk-cranges.c
>> new file mode 100644
>> index 000000000000..deaa09e564f7
>> --- /dev/null
>> +++ b/block/blk-cranges.c
>> @@ -0,0 +1,295 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +/*
>> + * Block device concurrent positioning ranges.
>> + *
>> + * Copyright (C) 2021 Western Digital Corporation or its Affiliates.
>> + */
>> +#include <linux/kernel.h>
>> +#include <linux/blkdev.h>
>> +#include <linux/slab.h>
>> +#include <linux/init.h>
>> +
>> +#include "blk.h"
>> +
>> +static ssize_t blk_crange_sector_show(struct blk_crange *cr, char *page)
>> +{
>> + return sprintf(page, "%llu\n", cr->sector);
>> +}
>> +
>> +static ssize_t blk_crange_nr_sectors_show(struct blk_crange *cr, char *page)
>> +{
>> + return sprintf(page, "%llu\n", cr->nr_sectors);
>> +}
>> +
>> +struct blk_crange_sysfs_entry {
>> + struct attribute attr;
>> + ssize_t (*show)(struct blk_crange *cr, char *page);
>> +};
>> +
>> +static struct blk_crange_sysfs_entry blk_crange_sector_entry = {
>> + .attr = { .name = "sector", .mode = 0444 },
>> + .show = blk_crange_sector_show,
>> +};
>> +
>> +static struct blk_crange_sysfs_entry blk_crange_nr_sectors_entry = {
>> + .attr = { .name = "nr_sectors", .mode = 0444 },
>> + .show = blk_crange_nr_sectors_show,
>> +};
>> +
>> +static struct attribute *blk_crange_attrs[] = {
>> + &blk_crange_sector_entry.attr,
>> + &blk_crange_nr_sectors_entry.attr,
>> + NULL,
>> +};
>> +ATTRIBUTE_GROUPS(blk_crange);
>> +
>> +static ssize_t blk_crange_sysfs_show(struct kobject *kobj,
>> + struct attribute *attr, char *page)
>> +{
>> + struct blk_crange_sysfs_entry *entry;
>> + struct blk_crange *cr;
>> + struct request_queue *q;
>> + ssize_t ret;
>> +
>> + entry = container_of(attr, struct blk_crange_sysfs_entry, attr);
>> + cr = container_of(kobj, struct blk_crange, kobj);
>> + q = cr->queue;
>
> Nit: I'd find this a little eaier to read if the variables were
> initialized at declaration time:
>
> struct blk_crange_sysfs_entry *entry =
> container_of(attr, struct blk_crange_sysfs_entry, attr);
> struct blk_crange *cr = container_of(kobj, struct blk_crange, kobj);
> struct request_queue *q = cr->queue;
>
> (or maybe even don't bother with the q local variable).

OK.

>
>> +/*
>> + * Dummy release function to make kobj happy.
>> + */
>> +static void blk_cranges_sysfs_nop_release(struct kobject *kobj)
>> +{
>> +}
>
> How do we ensure the kobj is still alive while it is accessed?

q->sysfs_lock ensures that. This mutex is taken whenever revalidate registers
new ranges (see blk_queue_set_cranges below), and is taken also when the ranges
are unregistered (on revalidate if the ranges changed and when the request queue
is unregistered). And blk_crange_sysfs_show() takes that lock too. So the kobj
cannot be freed while it is being accessed (the sysfs inode lock also prevents
it since kobj_del() will take the inode lock).

>
>> +void blk_queue_set_cranges(struct gendisk *disk, struct blk_cranges *cr)
>
> s/blk_queue/disk/

Hmmm... The argument is a gendisk, but it is the request queue that is modified.
So following other functions like this, isn't blk_queue_ prefix better ?

>
>> mutex_lock(&q->sysfs_lock);
>> +
>> + ret = blk_register_cranges(disk, NULL);
>> + if (ret) {
>> + mutex_unlock(&q->sysfs_lock);
>> + mutex_unlock(&q->sysfs_dir_lock);
>> + kobject_del(&q->kobj);
>> + blk_trace_remove_sysfs(dev);
>> + kobject_put(&dev->kobj);
>> + return ret;
>> + }
>> +
>> if (q->elevator) {
>> ret = elv_register_queue(q, false);
>> if (ret) {
>> + blk_unregister_cranges(disk);
>> mutex_unlock(&q->sysfs_lock);
>> mutex_unlock(&q->sysfs_dir_lock);
>> kobject_del(&q->kobj);
>
> Please use a goto instead of duplicating the code.

I had tried that but it made a mess. Will have another look at it.

--
Damien Le Moal
Western Digital Research