mm: add cond_resched() in gather_pte_stats()

The other pagetable walks in task_mmu.c have a cond_resched() after
walking their ptes: add a cond_resched() in gather_pte_stats() too, for
reading /proc/<id>/numa_maps.  Only pagemap_pmd_range() has a
cond_resched() in its (unusually expensive) pmd_trans_huge case: more
should probably be added, but leave them unchanged for now.

Link: http://lkml.kernel.org/r/alpine.LSU.2.11.1612052157400.13021@eggly.anvils
Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/task_mmu.c

@@ -1588,6 +1588,7 @@ static int gather_pte_stats(pmd_t *pmd, unsigned long addr,
 
 	} while (pte++, addr += PAGE_SIZE, addr != end);
 	pte_unmap_unlock(orig_pte, ptl);
+	cond_resched();
 	return 0;
 }
 #ifdef CONFIG_HUGETLB_PAGE