[Devel] [PATCH 08/17] Pid-NS(V3) Define/use pid->upid_list list.

[sukadev at us.ibm.com]

Pavel Emelianov [xemul at openvz.org] wrote:
| sukadev at us.ibm.com wrote:
| > Subject: [PATCH 08/17] Pid-NS(V3) Define/use pid->upid_list list.
| > 
| > From: Sukadev Bhattiprolu <sukadev at us.ibm.com>
| > 
| > 
| > With multiple pid namespaces, a process would be known by several pid_t
| > values, one in each pid namespace. To represent this, we introduce a
| > 'struct upid' which associates a single pid_t value with a single pid
| > namespace.
| > 
| > We then replace the pid->nr field in 'struct pid' with a list of struct upid'
| > entries (referred to as 'pid->upid_list'). This list represents the multiple
| > pid_t values of the process, one in each namespace. The current patch adds
| > just one element to this list, corresponding to 'init_pid_ns'. Subsequent
| > patches implement multiple pid namespaces and add more elements to the list.
| > 
| > The 'struct upid' also replaces 'struct pid' in the pid_hash table to enable us
| > to find processes given a pid_t from any namespace (i.e we find 'struct upid'
| > for a given pid_t and from the 'struct upid', we find the 'struct pid' of the
| > process)
| > 
| > We finally reimplement find_pid() and pid_to_nr() to use pid->upid_list
| > and remove unused fields from 'struct pid'.
| > 
| > Changelog:
| > 	2.6.21-mm2-pidns3:
| > 
| > 	- 'struct upid' used to be called 'struct pid_nr' and a list of these
| > 	   were hanging off of 'struct pid'. So, we renamed 'struct pid_nr'
| > 	   and now hold them in a statically sized array in 'struct pid' since
| > 	   the number of 'struct upid's for a process is known at process-
| > 	   creation time.
| > 	  
| > 	2.6.21-rc3-mm2:
| > 
| > 	- [Eric Biederman] Combine all logical changes into one patch
| > 	- [Eric Biederman] Implement __pid_nr(pid_ns, pid) for use in procfs.
| > 	  (now called pid_to_nr_in_ns()).
| > 	- [Serge Hallyn]: Remove (!pid_nr) check in free_pid_nr()
| > 
| > Signed-off-by: Cedric Le Goater <clg at fr.ibm.com>
| > Signed-off-by: Sukadev Bhattiprolu <sukadev at us.ibm.com>
| > ---
| >  fs/proc/array.c               |   30 ++++++--
| >  fs/proc/base.c                |    9 ++
| >  include/linux/init_task.h     |   14 +++-
| >  include/linux/pid.h           |   62 ++++++++++++++---
| >  include/linux/pid_namespace.h |   15 ++++
| >  kernel/fork.c                 |    2 
| >  kernel/pid.c                  |  145 +++++++++++++++++++++++++++++++++---------
| >  7 files changed, 220 insertions(+), 57 deletions(-)
| > 
| > Index: lx26-22-rc4-mm2/include/linux/pid.h
| > ===================================================================
| > --- lx26-22-rc4-mm2.orig/include/linux/pid.h	2007-06-15 18:44:50.000000000 -0700
| > +++ lx26-22-rc4-mm2/include/linux/pid.h	2007-06-15 19:47:58.000000000 -0700
| > @@ -16,6 +16,25 @@ enum pid_type
| >  	PIDTYPE_MAX
| >  };
| >  
| > +struct pid_namespace;
| > +
| > +/*
| > + * A struct upid holds a process identifier (or pid->nr) for a given
| > + * pid namespace.
| > + *
| > + * A list of 'struct upid' entries is stored in the struct pid. This list
| > + * is used to get the process identifier associated with the pid
| > + * namespace it is being seen from.
| > + */
| > +struct upid
| > +{
| > +	/* Try to keep pid_chain in the same cacheline as nr for find_pid */
| > +	struct hlist_node pid_chain; 	/* link hash collisions on pid_hash */
| > +	int nr;	 			/* user space pid number */
| > +	struct pid_namespace *pid_ns; 	/* pid namespace in which nr is valid */
| > +	struct pid *pid; 		/* back to task's unique kernel pid */
| > +};
| > +
| >  /*
| >   * What is struct pid?
| >   *
| > @@ -48,12 +67,11 @@ enum pid_type
| >  struct pid
| >  {
| >  	atomic_t count;
| > -	/* Try to keep pid_chain in the same cacheline as nr for find_pid */
| > -	int nr;
| > -	struct hlist_node pid_chain;
| >  	/* lists of tasks that use this pid */
| >  	struct hlist_head tasks[PIDTYPE_MAX];
| >  	struct rcu_head rcu;
| > +	int num_upids;
| > +	struct upid upid_list[1];
| 
| Further in your patches you define MAX_NESTED_PID_NS. What for, you
| use the linked list here!?

Hmm. I don't understand. upid_list[] is an array (and not a linked
list). Are you saying the '_list' in 'upid_list' is misleading ?

Placing a limit like MAX_NESTED_PID_NS simplifies allocation of
'struct pid'.


| 
| >  };
| >  
| >  extern struct pid init_struct_pid;
| 
| [snip]
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