/sys/module/dm_mod/parameters/stats_current_allocated_bytes

@stats_create <range> <step> [<number_of_optional_arguments> <optional_arguments>...] [<program_id> [<aux_data>]]
Create a new region and return the region_id.

<range>
  "-"
	whole device
  "<start_sector>+<length>"
	a range of <length> 512-byte sectors
	starting with <start_sector>.

<step>
  "<area_size>"
	the range is subdivided into areas each containing
	<area_size> sectors.
  "/<number_of_areas>"
	the range is subdivided into the specified
	number of areas.

<number_of_optional_arguments>
  The number of optional arguments

<optional_arguments>
  The following optional arguments are supported:

  precise_timestamps
	use precise timer with nanosecond resolution
	instead of the "jiffies" variable.  When this argument is
	used, the resulting times are in nanoseconds instead of
	milliseconds.  Precise timestamps are a little bit slower
	to obtain than jiffies-based timestamps.
  histogram:n1,n2,n3,n4,...
	collect histogram of latencies.  The
	numbers n1, n2, etc are times that represent the boundaries
	of the histogram.  If precise_timestamps is not used, the
	times are in milliseconds, otherwise they are in
	nanoseconds.  For each range, the kernel will report the
	number of requests that completed within this range. For
	example, if we use "histogram:10,20,30", the kernel will
	report four numbers a:b:c:d. a is the number of requests
	that took 0-10 ms to complete, b is the number of requests
	that took 10-20 ms to complete, c is the number of requests
	that took 20-30 ms to complete and d is the number of
	requests that took more than 30 ms to complete.

<program_id>
  An optional parameter.  A name that uniquely identifies
  the userspace owner of the range.  This groups ranges together
  so that userspace programs can identify the ranges they
  created and ignore those created by others.
  The kernel returns this string back in the output of
  @stats_list message, but it doesn't use it for anything else.
  If we omit the number of optional arguments, program id must not
  be a number, otherwise it would be interpreted as the number of
  optional arguments.

<aux_data>
  An optional parameter.  A word that provides auxiliary data
  that is useful to the client program that created the range.
  The kernel returns this string back in the output of
  @stats_list message, but it doesn't use this value for anything.

@stats_delete <region_id>
Delete the region with the specified id.

<region_id>
  region_id returned from @stats_create

@stats_clear <region_id>
Clear all the counters except the in-flight i/o counters.

<region_id>
  region_id returned from @stats_create

@stats_list [<program_id>]
List all regions registered with @stats_create.

<program_id>
  An optional parameter.
  If this parameter is specified, only matching regions
  are returned.
  If it is not specified, all regions are returned.

Output format:
  <region_id>: <start_sector>+<length> <step> <program_id> <aux_data>
        precise_timestamps histogram:n1,n2,n3,...

The strings "precise_timestamps" and "histogram" are printed only
if they were specified when creating the region.

@stats_print <region_id> [<starting_line> <number_of_lines>]
Print counters for each step-sized area of a region.

<region_id>
  region_id returned from @stats_create

<starting_line>
  The index of the starting line in the output.
  If omitted, all lines are returned.

<number_of_lines>
  The number of lines to include in the output.
  If omitted, all lines are returned.

Output format for each step-sized area of a region:

  <start_sector>+<length>
	counters

  The first 11 counters have the same meaning as
  `/sys/block/*/stat or /proc/diskstats`.

  Please refer to Documentation/admin-guide/iostats.rst for details.

  1. the number of reads completed
  2. the number of reads merged
  3. the number of sectors read
  4. the number of milliseconds spent reading
  5. the number of writes completed
  6. the number of writes merged
  7. the number of sectors written
  8. the number of milliseconds spent writing
  9. the number of I/Os currently in progress
  10. the number of milliseconds spent doing I/Os
  11. the weighted number of milliseconds spent doing I/Os

  Additional counters:

  12. the total time spent reading in milliseconds
  13. the total time spent writing in milliseconds

@stats_print_clear <region_id> [<starting_line> <number_of_lines>]
Atomically print and then clear all the counters except the
in-flight i/o counters.	 Useful when the client consuming the
statistics does not want to lose any statistics (those updated
between printing and clearing).

<region_id>
  region_id returned from @stats_create

<starting_line>
  The index of the starting line in the output.
  If omitted, all lines are printed and then cleared.

<number_of_lines>
  The number of lines to process.
  If omitted, all lines are printed and then cleared.

@stats_set_aux <region_id> <aux_data>
Store auxiliary data aux_data for the specified region.

<region_id>
  region_id returned from @stats_create

<aux_data>
  The string that identifies data which is useful to the client
  program that created the range.  The kernel returns this
  string back in the output of @stats_list message, but it
  doesn't use this value for anything.