linux-sg2042/ipc
Nadia Derbey 9eefe520c8 ipc: do not use a negative value to re-enable msgmni automatic recomputing
This patch proposes an alternative to the "magical
positive-versus-negative number trick" Andrew complained about last week
in http://lkml.org/lkml/2008/6/24/418.

This had been introduced with the patches that scale msgmni to the amount
of lowmem.  With these patches, msgmni has a registered notification
routine that recomputes msgmni value upon memory add/remove or ipc
namespace creation/ removal.

When msgmni is changed from user space (i.e.  value written to the proc
file), that notification routine is unregistered, and the way to make it
registered back is to write a negative value into the proc file.  This is
the "magical positive-versus-negative number trick".

To fix this, a new proc file is introduced: /proc/sys/kernel/auto_msgmni.
This file acts as ON/OFF for msgmni automatic recomputing.

With this patch, the process is the following:
1) kernel boots in "automatic recomputing mode"
   /proc/sys/kernel/msgmni contains the value that has been computed (depends
                           on lowmem)
   /proc/sys/kernel/automatic_msgmni contains "1"

2) echo <val> > /proc/sys/kernel/msgmni
   . sets msg_ctlmni to <val>
   . de-activates automatic recomputing (i.e. if, say, some memory is added
     msgmni won't be recomputed anymore)
   . /proc/sys/kernel/automatic_msgmni now contains "0"

3) echo "0" > /proc/sys/kernel/automatic_msgmni
   . de-activates msgmni automatic recomputing
     this has the same effect as 2) except that msg_ctlmni's value stays
     blocked at its current value)

3) echo "1" > /proc/sys/kernel/automatic_msgmni
   . recomputes msgmni's value based on the current available memory size
     and number of ipc namespaces
   . re-activates automatic recomputing for msgmni.

Signed-off-by: Nadia Derbey <Nadia.Derbey@bull.net>
Cc: Solofo Ramangalahy <Solofo.Ramangalahy@bull.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2008-07-25 10:53:42 -07:00
..
Makefile ipc: recompute msgmni on ipc namespace creation/removal 2008-04-29 08:06:12 -07:00
compat.c fix logic error in ipc compat semctl() 2007-07-06 10:23:43 -07:00
compat_mq.c Linux-2.6.12-rc2 2005-04-16 15:20:36 -07:00
ipc_sysctl.c ipc: do not use a negative value to re-enable msgmni automatic recomputing 2008-07-25 10:53:42 -07:00
ipcns_notifier.c ipc: do not use a negative value to re-enable msgmni automatic recomputing 2008-07-25 10:53:42 -07:00
mqueue.c ipc: use simple_read_from_buffer() 2008-07-25 10:53:42 -07:00
msg.c ipc: only output msgmni value at boot time 2008-06-06 11:29:12 -07:00
msgutil.c [PATCH] getting rid of all casts of k[cmz]alloc() calls 2006-12-13 09:05:58 -08:00
namespace.c ipc: recompute msgmni on ipc namespace creation/removal 2008-04-29 08:06:12 -07:00
sem.c ipc/sem.c: rewrite undo list locking 2008-07-25 10:53:42 -07:00
shm.c ipc: get rid of ipc_lock_down() 2008-07-25 10:53:42 -07:00
util.c ipc: get rid of ipc_lock_down() 2008-07-25 10:53:42 -07:00
util.h ipc: get rid of ipc_lock_down() 2008-07-25 10:53:42 -07:00