[PATCH] cpuset: update cpuset_zones_allowed comment

 * So only GFP_KERNEL allocations, if all nodes in the cpuset are

 * short of memory, might require taking the callback_mutex mutex.

 *

 * The first loop over the zonelist in mm/page_alloc.c:__alloc_pages()

 * calls here with __GFP_HARDWALL always set in gfp_mask, enforcing

 * hardwall cpusets - no allocation on a node outside the cpuset is

 * allowed (unless in interrupt, of course).

 *

 * The second loop doesn't even call here for GFP_ATOMIC requests

 * (if the __alloc_pages() local variable 'wait' is set).  That check

 * and the checks below have the combined affect in the second loop of

 * the __alloc_pages() routine that:

 * The first call here from mm/page_alloc:get_page_from_freelist()

 * has __GFP_HARDWALL set in gfp_mask, enforcing hardwall cpusets, so

 * no allocation on a node outside the cpuset is allowed (unless in

 * interrupt, of course).

 *

 * The second pass through get_page_from_freelist() doesn't even call

 * here for GFP_ATOMIC calls.  For those calls, the __alloc_pages()

 * variable 'wait' is not set, and the bit ALLOC_CPUSET is not set

 * in alloc_flags.  That logic and the checks below have the combined

 * affect that:

 *	in_interrupt - any node ok (current task context irrelevant)

 *	GFP_ATOMIC   - any node ok

 *	GFP_KERNEL   - any node in enclosing mem_exclusive cpuset ok

 *	GFP_USER     - only nodes in current tasks mems allowed ok.

 *

 * Rule:

 *    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.

 **/

int __cpuset_zone_allowed(struct zone *z, gfp_t gfp_mask)