ref: a9f94671c16558d2155ae9c37cd8c4e4101fef46
dir: /README/
LIMIT(3) LIMIT(3)
NAME
limit - system limit interface
SYNOPSIS
bind #Λ /mnt/limit
/mnt/limit/ctl
/mnt/limit/clone
/mnt/limit/switch
/mnt/limit/status
/mnt/limit/groups/...
DESCRIPTION
The #Λ device provides the interface to the system's limit
mechanism. Each process has a limit group assigned. Groups
are created as children of other groups, are represented by
an id, and follow the restrictions imposed by their parents.
A group is deleted the moment there are no more processes or
segments referring to it. The group is owned by the uname
and gid of the creator process, and the attributes can be
changed. The system's group, with id 1, imposes no restric-
tions and does not increase counters.
The root directory contains the ctl, clone, switch, status
files, and a directory named groups. The current process'
limit group is always presented in the root directory, and
other groups, in the groups subdirectory, where they have
the same files, except for the groups .
The ctl file controls the limit group's attributes, namely,
the label, and the limits. Reading the file returns the
limit group's id. The following commands are supported:
label label Set the group's label; the labels need
not be unique.
mlim number Set the maximum amount of groups that
can be created from this group.
mproc number Set the maximum amount of processes that
can be forked inside this group.
mpage number Set the maximum total amount of pages
that segments inside the limit group can
have.
Reading the status file shows the group's attributes: the
label (or an empty line if not set), the parent's id, the
current limit restrictions on the left, and the counts, on
the right.
To create a new group, the clone file should be opened.
Reading from the fid returns the newly created group's id.
This creates a new reference, and when the fid is clunked,
the reference is destroyed. A reference is also created for
the parent limit group. Thus, the opener's process limit
group is updated.
To switch to an existing limit group, the switch should be
opened.
The following limits exist and are imposed: limit limits,
which represent the amount of maximum amount of limits which
can be and process limits, which represent the maximum num-
ber of processes which can exist in the group. The kernel
already imposes limits on the number of files which can be
opened, so implementing that should be redundant.
EXAMPLES
To create a new limit group and restrict the amount of sub-
limits and processes:
% <>[10] /mnt/limit/clone
% echo label mygroup >/mnt/limit/ctl
% echo mlim 1 >/mnt/limit/ctl
% echo mproc 20 >/mnt/limit/ctl
Snippet to attach to an existing limit group, looking for a
specific label:
#!/bin/rc
slabel=$1
shift
prog=$*
bind '#Λ' /mnt/limit
for (lgrpstat in `{walk -f /mnt/limit/groups | grep 'status$'}) {
lgrpdir=`{basename -d $lgrpstat}
lgrplabel=`{cat $lgrpstat | sed 1q}
if(~ $lgrplabel $slabel) {
<>[10] $lgrpdir/switch
exec $prog
}
}
echo lgrp $slabel not found
exit nolgrp
SOURCE
/sys/src/9/port/limit.c
/sys/src/9/port/devlimit.c
BUGS
Not all processes use pexit , probably the ones which are
created during boot. This has to be checked.
Spawning a lot of processes inside a limit group which is
not the root group can cause a kernel panic related to run-
lock. Needs to be investigated.