mm/huge_memory.c: __split_huge_page() use atomic ClearPageDirty()

From: Hugh Dickins <[email protected]>
Subject: mm/huge_memory.c: __split_huge_page() use atomic ClearPageDirty()

Swapping load on huge=always tmpfs (with khugepaged tuned up to be very eager, but I'm not sure that is relevant) soon hung uninterruptibly, waiting for page lock in shmem_getpage_gfp()'s find_lock_entry(), most often when "cp -a" was trying to write to a smallish file. Debug showed that the page in question was not locked, and page->mapping NULL by now, but page->index consistent with having been in a huge page before.

Reproduced in minutes on a 4.15 kernel, even with 4.17's 605ca5ede764 ("mm/huge_memory.c: reorder operations in __split_huge_page_tail()") added in; but took hours to reproduce on a 4.17 kernel (no idea why).

The culprit proved to be the __ClearPageDirty() on tails beyond i_size in __split_huge_page(): the non-atomic __bitoperation may have been safe when 4.8's baa355fd3314 ("thp: file pages support for split_huge_page()") introduced it, but liable to erase PageWaiters after 4.10's 62906027091f ("mm: add PageWaiters indicating tasks are waiting for a page bit").

---

 mm/huge_memory.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff -puN mm/huge_memory.c~mm-huge_memoryc-__split_huge_page-use-atomic-clearpagedirty mm/huge_memory.c
@@ -2431,7 +2431,7 @@ static void __split_huge_page(struct pag
 		__split_huge_page_tail(head, i, lruvec, list);
 		/* Some pages can be beyond i_size: drop them from page cache */
 		if (head[i].index >= end) {
-			__ClearPageDirty(head + i);
+			ClearPageDirty(head + i);
 			__delete_from_page_cache(head + i, NULL);
 			if (IS_ENABLED(CONFIG_SHMEM) && PageSwapBacked(head))
 				shmem_uncharge(head->mapping->host, 1);

_