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 <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> --- [songliubraving@xxxxxx: flush file for !is_shmem PageDirty() case in collapse_file()] https://lkml.kernel.org/linux-mm/20191030200736.3455046-1-songliubraving@xxxxxx/ --- mm/khugepaged.c | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+) 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); -- 2.17.1
