[v5,2/2] mm/thp: flush file for !is_shmem PageDirty() case in collapse_file()

Commit Message

Song Liu Nov. 6, 2019, 6:09 a.m. UTC

For non-shmem file THPs, khugepaged only collapses read only .text mapping
(VM_DENYWRITE). These pages should not be dirty except the case where the
file hasn't been flushed since first write.

Call filemap_flush() in collapse_file() to accelerate the write back in
such cases.

Cc: Kirill A. Shutemov <[e-mail]>
Cc: Hugh Dickins <[e-mail]>
Cc: William Kucharski <[e-mail]>
Cc: Johannes Weiner <[e-mail]>
Cc: Andrew Morton <[e-mail]>
Cc: Matthew Wilcox <[e-mail]>
Signed-off-by: Song Liu <[e-mail]>

---

[[e-mail]: flush file for !is_shmem PageDirty() case in collapse_file()]
https://lkml.kernel.org/linux-mm/[e-mail]/

---
 mm/khugepaged.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

--
2.17.1

Patch

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 40215795d641..513cce897794 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1601,6 +1601,24 @@  static void collapse_file(struct mm_struct *mm,
 					result = SCAN_FAIL;
 					goto xa_unlocked;
 				}
+			} else if (PageDirty(page)) {
+				/*
+				 * khugepaged only works on read-only fd,
+				 * so this page is dirty because it hasn't
+				 * been flushed since first write. There
+				 * won't be new dirty pages.
+				 *
+				 * Trigger async flush here and hope the
+				 * writeback is done when khugepaged
+				 * revisits this page.
+				 *
+				 * This is a one-off situation. We are not
+				 * forcing writeback in loop.
+				 */
+				xas_unlock_irq(&xas);
+				filemap_flush(mapping);
+				result = SCAN_FAIL;
+				goto xa_unlocked;
 			} else if (trylock_page(page)) {
 				get_page(page);
 				xas_unlock_irq(&xas);