mm/z3fold.c: claim page in the beginning of free

There's a really hard to reproduce race in z3fold between z3fold_free()
and z3fold_reclaim_page().  z3fold_reclaim_page() can claim the page
after z3fold_free() has checked if the page was claimed and
z3fold_free() will then schedule this page for compaction which may in
turn lead to random page faults (since that page would have been
reclaimed by then).

Fix that by claiming page in the beginning of z3fold_free() and not
forgetting to clear the claim in the end.

[[email protected]: v2]
  Link: http://lkml.kernel.org/r/20190928113456.152742cf@bigdell
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Vitaly Wool <[email protected]>
Reported-by: Markus Linnala <[email protected]>
Cc: Dan Streetman <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Henry Burns <[email protected]>
Cc: Shakeel Butt <[email protected]>
Cc: Markus Linnala <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/z3fold.c

@@ -998,17 +998,19 @@ static void z3fold_free(struct z3fold_pool *pool, unsigned long handle)
 	struct z3fold_header *zhdr;
 	struct page *page;
 	enum buddy bud;
+	bool page_claimed;
 
 	zhdr = handle_to_z3fold_header(handle);
 	page = virt_to_page(zhdr);
+	page_claimed = test_and_set_bit(PAGE_CLAIMED, &page->private);
 
 	if (test_bit(PAGE_HEADLESS, &page->private)) {
 		/* if a headless page is under reclaim, just leave.
 		 * NB: we use test_and_set_bit for a reason: if the bit
 		 * has not been set before, we release this page
 		 * immediately so we don't care about its value any more.
 		 */
-		if (!test_and_set_bit(PAGE_CLAIMED, &page->private)) {
+		if (!page_claimed) {
 			spin_lock(&pool->lock);
 			list_del(&page->lru);
 			spin_unlock(&pool->lock);
@@ -1044,13 +1046,15 @@ static void z3fold_free(struct z3fold_pool *pool, unsigned long handle)
 		atomic64_dec(&pool->pages_nr);
 		return;
 	}
-	if (test_bit(PAGE_CLAIMED, &page->private)) {
+	if (page_claimed) {
+		/* the page has not been claimed by us */
 		z3fold_page_unlock(zhdr);
 		return;
 	}
 	if (unlikely(PageIsolated(page)) ||
 	    test_and_set_bit(NEEDS_COMPACTING, &page->private)) {
 		z3fold_page_unlock(zhdr);
+		clear_bit(PAGE_CLAIMED, &page->private);
 		return;
 	}
 	if (zhdr->cpu < 0 || !cpu_online(zhdr->cpu)) {
@@ -1060,10 +1064,12 @@ static void z3fold_free(struct z3fold_pool *pool, unsigned long handle)
 		zhdr->cpu = -1;
 		kref_get(&zhdr->refcount);
 		do_compact_page(zhdr, true);
+		clear_bit(PAGE_CLAIMED, &page->private);
 		return;
 	}
 	kref_get(&zhdr->refcount);
 	queue_work_on(zhdr->cpu, pool->compact_wq, &zhdr->work);
+	clear_bit(PAGE_CLAIMED, &page->private);
 	z3fold_page_unlock(zhdr);
 }