fs, proc: unconditional cond_resched when reading smaps

If there are large numbers of hugepages to iterate while reading
/proc/pid/smaps, the page walk never does cond_resched().  On archs
without split pmd locks, there can be significant and observable
contention on mm->page_table_lock which cause lengthy delays without
rescheduling.

Always reschedule in smaps_pte_range() if necessary since the pagewalk
iteration can be expensive.

Link: http://lkml.kernel.org/r/alpine.DEB.2.10.1708211405520.131071@chino.kir.corp.google.com
Signed-off-by: David Rientjes <rientjes@google.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/task_mmu.c

@@ -611,11 +611,11 @@ static int smaps_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
 		if (pmd_present(*pmd))
 			smaps_pmd_entry(pmd, addr, walk);
 		spin_unlock(ptl);
-		return 0;
+		goto out;
 	}
 
 	if (pmd_trans_unstable(pmd))
-		return 0;
+		goto out;
 	/*
 	 * The mmap_sem held all the way back in m_start() is what
 	 * keeps khugepaged out of here and from collapsing things
@@ -625,6 +625,7 @@ static int smaps_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
 	for (; addr != end; pte++, addr += PAGE_SIZE)
 		smaps_pte_entry(pte, addr, walk);
 	pte_unmap_unlock(pte - 1, ptl);
+out:
 	cond_resched();
 	return 0;
 }