Update Linux to v5.4.2
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+=============
+DM statistics
+=============
+
+Device Mapper supports the collection of I/O statistics on user-defined
+regions of a DM device. If no regions are defined no statistics are
+collected so there isn't any performance impact. Only bio-based DM
+devices are currently supported.
+
+Each user-defined region specifies a starting sector, length and step.
+Individual statistics will be collected for each step-sized area within
+the range specified.
+
+The I/O statistics counters for each step-sized area of a region are
+in the same format as `/sys/block/*/stat` or `/proc/diskstats` (see:
+Documentation/admin-guide/iostats.rst). But two extra counters (12 and 13) are
+provided: total time spent reading and writing. When the histogram
+argument is used, the 14th parameter is reported that represents the
+histogram of latencies. All these counters may be accessed by sending
+the @stats_print message to the appropriate DM device via dmsetup.
+
+The reported times are in milliseconds and the granularity depends on
+the kernel ticks. When the option precise_timestamps is used, the
+reported times are in nanoseconds.
+
+Each region has a corresponding unique identifier, which we call a
+region_id, that is assigned when the region is created. The region_id
+must be supplied when querying statistics about the region, deleting the
+region, etc. Unique region_ids enable multiple userspace programs to
+request and process statistics for the same DM device without stepping
+on each other's data.
+
+The creation of DM statistics will allocate memory via kmalloc or
+fallback to using vmalloc space. At most, 1/4 of the overall system
+memory may be allocated by DM statistics. The admin can see how much
+memory is used by reading:
+
+ /sys/module/dm_mod/parameters/stats_current_allocated_bytes
+
+Messages
+========
+
+ @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.
+
+Examples
+========
+
+Subdivide the DM device 'vol' into 100 pieces and start collecting
+statistics on them::
+
+ dmsetup message vol 0 @stats_create - /100
+
+Set the auxiliary data string to "foo bar baz" (the escape for each
+space must also be escaped, otherwise the shell will consume them)::
+
+ dmsetup message vol 0 @stats_set_aux 0 foo\\ bar\\ baz
+
+List the statistics::
+
+ dmsetup message vol 0 @stats_list
+
+Print the statistics::
+
+ dmsetup message vol 0 @stats_print 0
+
+Delete the statistics::
+
+ dmsetup message vol 0 @stats_delete 0