[patch] mm, thp: copying user pages must schedule on collapse

[David Rientjes]

We have encountered need_resched warnings in __collapse_huge_page_copy()
while doing {clear,copy}_user_highpage() over HPAGE_PMD_NR source pages.

mm->mmap_sem is held for write, but the iteration is well bounded.

Reschedule as needed.

Signed-off-by: David Rientjes <rient...@google.com>
---
 mm/khugepaged.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -612,7 +612,8 @@ static void __collapse_huge_page_copy(pte_t *pte, struct 
page *page,
                                      spinlock_t *ptl)
 {
        pte_t *_pte;
-       for (_pte = pte; _pte < pte+HPAGE_PMD_NR; _pte++) {
+       for (_pte = pte; _pte < pte + HPAGE_PMD_NR;
+                               _pte++, page++, address += PAGE_SIZE) {
                pte_t pteval = *_pte;
                struct page *src_page;
 
@@ -651,9 +652,7 @@ static void __collapse_huge_page_copy(pte_t *pte, struct 
page *page,
                        spin_unlock(ptl);
                        free_page_and_swap_cache(src_page);
                }
-
-               address += PAGE_SIZE;
-               page++;
+               cond_resched();
        }
 }