[PATCH] Cpuset: might sleep checking zones allowed fix

Fix an infrequently encountered 'sleeping function called
from invalid context' in the cpuset hooks in __alloc_pages.
Could sleep while interrupts disabled.

The routine cpuset_zone_allowed() is called by code in
mm/page_alloc.c __alloc_pages() to determine if a zone is
allowed in the current tasks cpuset.  This routine can sleep,
for certain GFP_KERNEL allocations, if the zone is on a memory
node not allowed in the current cpuset, but might be allowed
in a parent cpuset.

But we can't sleep in __alloc_pages() if in interrupt, nor
if called for a GFP_ATOMIC request (__GFP_WAIT not set in
gfp_flags).

The rule was intended to be:
  Don't call cpuset_zone_allowed() if you can't sleep, unless you
  pass in the __GFP_HARDWALL flag set in gfp_flag, which disables
  the code that might scan up ancestor cpusets and sleep.

This rule was being violated due to a bogus change made (by myself,
pj) to __alloc_pages() as part of the November 2005 effort to
cleanup its logic.

The bogus change can be seen at:
  http://linux.derkeiler.com/Mailing-Lists/Kernel/2005-11/4691.html
  [PATCH 01/05] mm fix __alloc_pages cpuset ALLOC_* flags

This was first noticed on a tight memory system, in code that
was disabling interrupts and doing allocation requests with
__GFP_WAIT not set, which resulted in __might_sleep() writing
complaints to the log "Debug: sleeping function called ...",
when the code in cpuset_zone_allowed() tried to take the
callback_sem cpuset semaphore.

Special thanks to Dave Chinner, for figuring this out,
and a tip of the hat to Nick Piggin who warned me of this
back in Nov 2005, before I was ready to listen.
Signed-off-by: Paul Jackson <pj@sgi.com>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

mm/page_alloc.c

@@ -949,7 +949,8 @@ __alloc_pages(gfp_t gfp_mask, unsigned int order,
 		alloc_flags |= ALLOC_HARDER;
 	if (gfp_mask & __GFP_HIGH)
 		alloc_flags |= ALLOC_HIGH;
-	alloc_flags |= ALLOC_CPUSET;
+	if (wait)
+		alloc_flags |= ALLOC_CPUSET;
 
 	/*
 	 * Go through the zonelist again. Let __GFP_HIGH and allocations