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

[Song Liu <songliubraving@fb.com>]

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.

Also add warning if PageDirty() triggered for pages from readahead path.

Reported-and-tested-by: syzbot+efb9e48b9fbdc49bb34a@syzkaller.appspotmail.com
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: William Kucharski <william.kucharski@oracle.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Song Liu <songliubraving@fb.com>
---
 mm/khugepaged.c | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 3ec5333ae94d..d61368b8251e 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1601,6 +1601,33 @@ static void collapse_file(struct mm_struct *mm,
 					result = SCAN_FAIL;
 					goto xa_unlocked;
 				}
+				if (WARN_ON_ONCE(PageDirty(page))) {
+					/*
+					 * page from readahead should not
+					 * be dirty. Show warning if this
+					 * somehow happens.
+					 */
+					result = SCAN_FAIL;
+					goto out_unlock;
+				}
+			} 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);
-- 
2.17.1