Re: [PATCH 3/3] block: Export max_dev_sectors_kb in sysfs

[Ewan Milne]

On Wed, 2015-12-16 at 17:53 -0500, Martin K. Petersen wrote:
> Some storage devices report a maximum transfer length which indicates
> the maximum size of an I/O request that the device can process. This
> limit is enforced in combination with the controller's max_hw_sectors
> and DMA constraints to ensure that we do not issue a command too big for
> the device.
> 
> Export the max_dev_sectors_kb queue limit in sysfs and update the
> documentation accordingly.
> 
> Signed-off-by: Martin K. Petersen 
> ---
>  Documentation/ABI/testing/sysfs-block |  9 +
>  Documentation/block/queue-sysfs.txt   | 11 +--
>  block/blk-settings.c  |  4 +++-
>  block/blk-sysfs.c | 13 +
>  4 files changed, 34 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/ABI/testing/sysfs-block 
> b/Documentation/ABI/testing/sysfs-block
> index 71d184dbb70d..4f284d38c085 100644
> --- a/Documentation/ABI/testing/sysfs-block
> +++ b/Documentation/ABI/testing/sysfs-block
> @@ -235,3 +235,12 @@ Description:
>   write_same_max_bytes is 0, write same is not supported
>   by the device.
>  
> +What:/sys/block//queue/max_dev_sectors_kb
> +Date:December 2015
> +Contact: Martin K. Petersen 
> +Description:
> + Some storage devices report the maximum size that the
> + device can process in a single READ or WRITE
> + request. This limit is used in combination with
> + constraints set by the controller driver to limit the
> + size of filesystem requests.
> diff --git a/Documentation/block/queue-sysfs.txt 
> b/Documentation/block/queue-sysfs.txt
> index e5d914845be6..a078995f3eae 100644
> --- a/Documentation/block/queue-sysfs.txt
> +++ b/Documentation/block/queue-sysfs.txt
> @@ -55,9 +55,15 @@ logical_block_size (RO)
>  ---
>  This is the logcal block size of the device, in bytes.
>  
> +max_dev_sectors_kb (R)
> +--
> +This is the maximum number of kilobytes supported by the storage device
> +for a READ or WRITE request.
> +
>  max_hw_sectors_kb (RO)
>  --
> -This is the maximum number of kilobytes supported in a single data transfer.
> +This is the maximum number of kilobytes supported by the storage
> +controller in a single data transfer.
>  
>  max_integrity_segments (RO)
>  ---
> @@ -68,7 +74,8 @@ max_sectors_kb (RW)
>  ---
>  This is the maximum number of kilobytes that the block layer will allow
>  for a filesystem request. Must be smaller than or equal to the maximum
> -size allowed by the hardware.
> +size allowed by the storage controller (max_hw_sectors_kb) and the
> +maximum size allowed by the storage device (max_dev_sectors_kb).
>  
>  max_segments (RO)
>  -
> diff --git a/block/blk-settings.c b/block/blk-settings.c
> index dd4973583978..362b0179c86a 100644
> --- a/block/blk-settings.c
> +++ b/block/blk-settings.c
> @@ -232,7 +232,9 @@ EXPORT_SYMBOL(blk_queue_bounce_limit);
>   *max_sectors is a soft limit imposed by the block layer for
>   *filesystem type requests.  This value can be overridden on a
>   *per-device basis in /sys/block//queue/max_sectors_kb.
> - *The soft limit can not exceed max_hw_sectors.
> + *
> + *The soft limit's lower bound is the page cache size and it can not
> + *exceed neither max_hw_sectors, nor max_dev_sectors.
>   **/
>  void blk_queue_max_hw_sectors(struct request_queue *q, unsigned int 
> max_hw_sectors)
>  {
> diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
> index e140cc487ce1..c289f9f6dcd9 100644
> --- a/block/blk-sysfs.c
> +++ b/block/blk-sysfs.c
> @@ -225,6 +225,13 @@ static ssize_t queue_max_hw_sectors_show(struct 
> request_queue *q, char *page)
>   return queue_var_show(max_hw_sectors_kb, (page));
>  }
>  
> +static ssize_t queue_max_dev_sectors_show(struct request_queue *q, char 
> *page)
> +{
> + int max_dev_sectors_kb = q->limits.max_dev_sectors >> 1;

Consider adding inline queue_max_dev_sectors() to blkdev.h and using it
in the line above?

> +
> + return queue_var_show(max_dev_sectors_kb, (page));
> +}
> +
>  #define QUEUE_SYSFS_BIT_FNS(name, flag, neg) \
>  static ssize_t   
> \
>  queue_show_##name(struct request_queue *q, char *page)   
> \
> @@ -371,6 +378,11 @@ static struct queue_sysfs_entry 
> queue_max_hw_sectors_entry = {
>   .show = queue_max_hw_sectors_show,
>  };
>  
> +static struct queue_sysfs_entry queue_max_dev_sectors_entry = {
> + .attr = {.name = "max_dev_sectors_kb", .mode = S_IRUGO },
> + .show = queue_max_dev_sectors_show,
> +};
> +
>  static struct queue_sysfs_entry queue_max_segments_entry = {
>   .attr = {.name = "max_segments", .mode = S_IRUGO },
> 

RE: [PATCH 3/3] block: Export max_dev_sectors_kb in sysfs

[Elliott, Robert (Persistent Memory)]

> -Original Message-
> From: linux-block-ow...@vger.kernel.org [mailto:linux-block-
> ow...@vger.kernel.org] On Behalf Of Martin K. Petersen
> Sent: Wednesday, December 16, 2015 4:54 PM
> To: linux-scsi@vger.kernel.org; linux-bl...@vger.kernel.org
> Cc: Martin K. Petersen 
> Subject: [PATCH 3/3] block: Export max_dev_sectors_kb in sysfs
> 
...
> diff --git a/Documentation/block/queue-sysfs.txt
...
> @@ -55,9 +55,15 @@ logical_block_size (RO)
>  ---
>  This is the logcal block size of the device, in bytes.

Since you're in the vicinity, consider adding an i to logcal.
> 
> +max_dev_sectors_kb (R)
> +--

The others use (RO) rather than just (R).

> +This is the maximum number of kilobytes supported by the storage
> +device for a READ or WRITE request.
> +
>  max_hw_sectors_kb (RO)
>  --
> -This is the maximum number of kilobytes supported in a single data transfer.
> +This is the maximum number of kilobytes supported by the storage
> +controller in a single data transfer.
...
> @@ -68,7 +74,8 @@ max_sectors_kb (RW)
>  ---
>  This is the maximum number of kilobytes that the block layer will allow
>  for a filesystem request. Must be smaller than or equal to the maximum
> -size allowed by the hardware.
> +size allowed by the storage controller (max_hw_sectors_kb) and the
> +maximum size allowed by the storage device (max_dev_sectors_kb).

Each one uses a different phrase:
* for a READ or WRITE request
* in a single data transfer
* for a filesystem request

Is some subtle difference being implied?


Since there are existing sysfs file with similar names, you're
stuck using _kb for the new one.  However, I suggest that the
Documentation at least mention these are really KiB, not kilobyte,
units.

---
Robert Elliott, HPE Persistent Memory


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