mm: hugetlb: yield when prepping struct pages

When booting with very large numbers of gigantic (i.e.  1G) pages, the
operations in the loop of gather_bootmem_prealloc, and specifically
prep_compound_gigantic_page, takes a very long time, and can cause a
softlockup if enough pages are requested at boot.

For example booting with 3844 1G pages requires prepping
(set_compound_head, init the count) over 1 billion 4K tail pages, which
takes considerable time.

Add a cond_resched() to the outer loop in gather_bootmem_prealloc() to
prevent this lockup.

Tested: Booted with softlockup_panic=1 hugepagesz=1G hugepages=3844 and
no softlockup is reported, and the hugepages are reported as
successfully setup.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Cannon Matthews <[email protected]>
Reviewed-by: Andrew Morton <[email protected]>
Reviewed-by: Mike Kravetz <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: Andres Lagar-Cavilla <[email protected]>
Cc: Peter Feiner <[email protected]>
Cc: Greg Thelen <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: loosecannon93

mm/hugetlb.c

@@ -2163,6 +2163,7 @@ static void __init gather_bootmem_prealloc(void)
 		 */
 		if (hstate_is_gigantic(h))
 			adjust_managed_page_count(page, 1 << h->order);
+		cond_resched();
 	}
 }