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@xxxxxxxxxxxxxxxxxxxxxxxxx
Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: William Kucharski <william.kucharski@xxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Signed-off-by: Song Liu <songliubraving@xxxxxx>
---
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
