Re: OK to set PF_MEMDIE on cleanup tasks?

["Paul E. McKenney" <paulmck@us.ibm.com>]

Hello, Andrew,

On Tue, Oct 14, 2003 at 05:12:27PM -0700, Andrew Morton wrote:
> > So...  Is it considered legit to simply set PF_MEMDIE when creating
> > the cleanup task?  Or is there some reason that one should deal with
> > signal 15?
> 
> Well it's all very unconventional.  Catching SIGTERM seems like a suitable
> way to do what you want to do.

OK, since this particular case is a strictly in-kernel task, SIGTERM
should be a no-op anyway.  Unless I am missing something in the signal
delivery code, which is quite probable.  ;-)

So the magic code would then be:

	cap_raise(current->cap_effective, CAP_SYS_RAWIO);

perhaps with:

	cap_raise(current->cap_effective, CAP_SYS_ADMIN);

thrown in for good measure.

> Possibly your special process should also run as PF_MEMALLOC.  I've seen
> that done before, with success.  There is no existing API with which this
> can be set.

This would certainly head off at least some OOM deadlock situations.
On the (perhaps unlikely) chance that this was an invitation, here
is a patch to create an API.

					Thanx, Paul

diff -urN -X /home/linux/2.5/dontdiff linux-2.6.0-test7-mm1/include/linux/sched.h linux-2.6.0-test7-mm1-PF_MEMALLOC/include/linux/sched.h
--- linux-2.6.0-test7-mm1/include/linux/sched.h	2003-10-16 07:16:05.000000000 -0700
+++ linux-2.6.0-test7-mm1-PF_MEMALLOC/include/linux/sched.h	2003-10-16 09:20:34.000000000 -0700
@@ -508,6 +508,27 @@
 #define PF_LESS_THROTTLE 0x00100000	/* Throttle me less: I clean memory */
 #define PF_SYNCWRITE	0x00200000	/* I am doing a sync write */
 
+/**
+ * mark_task_memalloc - mark the specified task as deserving of preferential
+ * access to free memory.  Note that with great power comes great
+ * responsibility.
+ * @p: the task structure to be granted preferential access.
+ */
+static inline void mark_task_memalloc(task_t *p)
+{
+	p->flags |= PF_MEMALLOC;
+}
+
+/**
+ * unmark_task_memalloc - mark the specified task as no longer deserving
+ * of preferential access to free memory.
+ * @p: the task structure to have its preferential access revoked.
+ */
+static inline void unmark_task_memalloc(task_t *p)
+{
+	p->flags &= ~PF_MEMALLOC;
+}
+
 #ifdef CONFIG_SMP
 extern int set_cpus_allowed(task_t *p, cpumask_t new_mask);
 #else