mdev primer

The following is the doc related to mdev which i get from busybox 1.6.1
distribution :

————-
MDEV Primer
————-

For those of us who know how to use mdev, a primer might seem lame. For
everyone else, mdev is a weird black box that they hear is awesome, but
can't
seem to get their head around how it works. Thus, a primer.

———–
Basic Use
———–

Mdev has two primary uses: initial population and dynamic updates. Both
require sysfs support in the kernel and have it mounted at /sys. For
dynamic
updates, you also need to have hotplugging enabled in your kernel.

Here's a typical code snippet from the init script:
[1] mount -t sysfs sysfs /sys
[2] echo /bin/mdev > /proc/sys/kernel/hotplug
[3] mdev -s

Of course, a more "full" setup would entail executing this before the
previous
code snippet:
[4] mount -t tmpfs mdev /dev
[5] mkdir /dev/pts
[6] mount -t devpts devpts /dev/pts

The simple explanation here is that [1] you need to have /sys mounted
before
executing mdev. Then you [2] instruct the kernel to execute /bin/mdev
whenever
a device is added or removed so that the device node can be created or
destroyed. Then you [3] seed /dev with all the device nodes that were
created
while the system was booting.

For the "full" setup, you want to [4] make sure /dev is a tmpfs
filesystem
(assuming you're running out of flash). Then you want to [5] create the
/dev/pts mount point and finally [6] mount the devpts filesystem on it.

————-
MDEV Config (/etc/mdev.conf)
————-

Mdev has an optional config file for controlling ownership/permissions
of
device nodes if your system needs something more than the default
root/root
660 permissions.

The file has the format:
<device regex> <uid>:<gid> <octal permissions>
For example:
hd[a-z][0-9]* 0:3 660

The config file parsing stops at the first matching line. If no line is
matched, then the default of 0:0 660 is used. To set your own default,
simply
create your own total match like so:
.* 1:1 777

If you also enable support for executing your own commands, then the
file has
the format:
<device regex> <uid>:<gid> <octal permissions> [<@|$|*> <command>]
The special characters have the meaning:
@ Run after creating the device.
$ Run before removing the device.
* Run both after creating and before removing the device.

The command is executed via the system() function (which means you're
giving a
command to the shell), so make sure you have a shell installed
at /bin/sh.

For your convenience, the shell env var $MDEV is set to the device name.
So if
the device 'hdc' was matched, MDEV would be set to "hdc

/proc/meminfo

/proc/meminfo annotated:

fs/proc/proc_misc.c: meminfo_read_proc()

MemTotal: Total usable ram (i.e. physical ram minus a few reserved
bits and the kernel binary code)

MemFree: The sum of LowFree+HighFree

Buffers: Relatively temporary storage for raw disk blocks
shouldn't get tremendously large (20MB or so)

Cached: in-memory cache for files read from the disk (the
pagecache). Doesn't include SwapCached

SwapCached: Memory that once was swapped out, is swapped back in but
still also is in the swapfile (if memory is needed it
doesn't need to be swapped out AGAIN because it is already
in the swapfile. This saves I/O)

(All page frames are linked in inactive or active list)
Active: Memory that has been used more recently and usually not
reclaimed unless absolutely necessary.
Inactive: Memory which has been less recently used. It is more
eligible to be reclaimed for other purposes

HighTotal:
HighFree: Highmem is all memory above ~860MB of physical memory
Highmem areas are for use by userspace programs, or
for the pagecache. The kernel must use tricks to access
this memory, making it slower to access than lowmem.
LowTotal:
LowFree: Lowmem is memory which can be used for everything that
highmem can be used for, but it is also available for the
kernel's use for its own data structures. Among many
other things, it is where everything from the Slab is
allocated. Bad things happen when you're out of lowmem.

SwapTotal: total amount of swap space available
SwapFree: Memory which has been evicted from RAM, and is temporarily
on the disk
Dirty: Memory which is waiting to get written back to the disk
Writeback: Memory which is actively being written back to the disk
Mapped: files which have been mmaped, such as libraries
Slab: in-kernel data structures cache
CommitLimit: Based on the overcommit ratio ('vm.overcommit_ratio'),
this is the total amount of memory currently available to
be allocated on the system. This limit is only adhered to
if strict overcommit accounting is enabled (mode 2 in
'vm.overcommit_memory').
The CommitLimit is calculated with the following formula:
CommitLimit = ('vm.overcommit_ratio' * Physical RAM) +
Swap
For example, on a system with 1G of physical RAM and 7G
of swap with a `vm.overcommit_ratio` of 30 it would
yield a CommitLimit of 7.3G.
For more details, see the memory overcommit documentation
in vm/overcommit-accounting.
Committed_AS: The amount of memory presently allocated on the system.
The committed memory is a sum of all of the memory which
has been allocated by processes, even if it has not been
"used" by them as of yet. A process which malloc()'s 1G
of memory, but only touches 300M of it will only show up
as using 300M of memory even if it has the address space
allocated for the entire 1G. This 1G is memory which has
been "committed" to by the VM and can be used at any time
by the allocating application. With strict overcommit
enabled on the system (mode 2 in 'vm.overcommit_memory'),
allocations which would exceed the CommitLimit (detailed
above) will not be permitted. This is useful if one needs
to guarantee that processes will not fail due to lack of
memory once that memory has been successfully allocated.
PageTables: amount of memory dedicated to the lowest level of page
tables.
VmallocTotal: total size of vmalloc memory area
VmallocUsed: amount of vmalloc area which is used
VmallocChunk: largest contigious block of vmalloc area which is free