z3fold: stricter locking and more careful reclaim

Use temporary slots in reclaim function to avoid possible race when
freeing those.

While at it, make sure we check CLAIMED flag under page lock in the
reclaim function to make sure we are not racing with z3fold_alloc().

Link: https://lkml.kernel.org/r/20201209145151.18994-4-vitaly.wool@konsulko.com
Signed-off-by: Vitaly Wool <vitaly.wool@konsulko.com>
Cc: <stable@vger.kernel.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: vwool

mm/z3fold.c

@@ -181,6 +181,13 @@ enum z3fold_page_flags {
 	PAGE_CLAIMED, /* by either reclaim or free */
 };
 
+/*
+ * handle flags, go under HANDLE_FLAG_MASK
+ */
+enum z3fold_handle_flags {
+	HANDLES_NOFREE = 0,
+};
+
 /*
  * Forward declarations
  */
@@ -311,6 +318,12 @@ static inline void free_handle(unsigned long handle, struct z3fold_header *zhdr)
 	slots = handle_to_slots(handle);
 	write_lock(&slots->lock);
 	*(unsigned long *)handle = 0;
+
+	if (test_bit(HANDLES_NOFREE, &slots->pool)) {
+		write_unlock(&slots->lock);
+		return; /* simple case, nothing else to do */
+	}
+
 	if (zhdr->slots != slots)
 		zhdr->foreign_handles--;
 
@@ -621,6 +634,28 @@ static inline void add_to_unbuddied(struct z3fold_pool *pool,
 	}
 }
 
+static inline enum buddy get_free_buddy(struct z3fold_header *zhdr, int chunks)
+{
+	enum buddy bud = HEADLESS;
+
+	if (zhdr->middle_chunks) {
+		if (!zhdr->first_chunks &&
+		    chunks <= zhdr->start_middle - ZHDR_CHUNKS)
+			bud = FIRST;
+		else if (!zhdr->last_chunks)
+			bud = LAST;
+	} else {
+		if (!zhdr->first_chunks)
+			bud = FIRST;
+		else if (!zhdr->last_chunks)
+			bud = LAST;
+		else
+			bud = MIDDLE;
+	}
+
+	return bud;
+}
+
 static inline void *mchunk_memmove(struct z3fold_header *zhdr,
 				unsigned short dst_chunk)
 {
@@ -682,18 +717,7 @@ static struct z3fold_header *compact_single_buddy(struct z3fold_header *zhdr)
 		if (WARN_ON(new_zhdr == zhdr))
 			goto out_fail;
 
-		if (new_zhdr->first_chunks == 0) {
-			if (new_zhdr->middle_chunks != 0 &&
-					chunks >= new_zhdr->start_middle) {
-				new_bud = LAST;
-			} else {
-				new_bud = FIRST;
-			}
-		} else if (new_zhdr->last_chunks == 0) {
-			new_bud = LAST;
-		} else if (new_zhdr->middle_chunks == 0) {
-			new_bud = MIDDLE;
-		}
+		new_bud = get_free_buddy(new_zhdr, chunks);
 		q = new_zhdr;
 		switch (new_bud) {
 		case FIRST:
@@ -815,9 +839,8 @@ static void do_compact_page(struct z3fold_header *zhdr, bool locked)
 		return;
 	}
 
-	if (unlikely(PageIsolated(page) ||
-		     test_bit(PAGE_CLAIMED, &page->private) ||
-		     test_bit(PAGE_STALE, &page->private))) {
+	if (test_bit(PAGE_STALE, &page->private) ||
+	    test_and_set_bit(PAGE_CLAIMED, &page->private)) {
 		z3fold_page_unlock(zhdr);
 		return;
 	}
@@ -826,13 +849,16 @@ static void do_compact_page(struct z3fold_header *zhdr, bool locked)
 	    zhdr->mapped_count == 0 && compact_single_buddy(zhdr)) {
 		if (kref_put(&zhdr->refcount, release_z3fold_page_locked))
 			atomic64_dec(&pool->pages_nr);
-		else
+		else {
+			clear_bit(PAGE_CLAIMED, &page->private);
 			z3fold_page_unlock(zhdr);
+		}
 		return;
 	}
 
 	z3fold_compact_page(zhdr);
 	add_to_unbuddied(pool, zhdr);
+	clear_bit(PAGE_CLAIMED, &page->private);
 	z3fold_page_unlock(zhdr);
 }
 
@@ -1080,17 +1106,8 @@ static int z3fold_alloc(struct z3fold_pool *pool, size_t size, gfp_t gfp,
 retry:
 		zhdr = __z3fold_alloc(pool, size, can_sleep);
 		if (zhdr) {
-			if (zhdr->first_chunks == 0) {
-				if (zhdr->middle_chunks != 0 &&
-				    chunks >= zhdr->start_middle)
-					bud = LAST;
-				else
-					bud = FIRST;
-			} else if (zhdr->last_chunks == 0)
-				bud = LAST;
-			else if (zhdr->middle_chunks == 0)
-				bud = MIDDLE;
-			else {
+			bud = get_free_buddy(zhdr, chunks);
+			if (bud == HEADLESS) {
 				if (kref_put(&zhdr->refcount,
 					     release_z3fold_page_locked))
 					atomic64_dec(&pool->pages_nr);
@@ -1236,7 +1253,6 @@ static void z3fold_free(struct z3fold_pool *pool, unsigned long handle)
 		pr_err("%s: unknown bud %d\n", __func__, bud);
 		WARN_ON(1);
 		put_z3fold_header(zhdr);
-		clear_bit(PAGE_CLAIMED, &page->private);
 		return;
 	}
 
@@ -1251,8 +1267,7 @@ static void z3fold_free(struct z3fold_pool *pool, unsigned long handle)
 		z3fold_page_unlock(zhdr);
 		return;
 	}
-	if (unlikely(PageIsolated(page)) ||
-	    test_and_set_bit(NEEDS_COMPACTING, &page->private)) {
+	if (test_and_set_bit(NEEDS_COMPACTING, &page->private)) {
 		put_z3fold_header(zhdr);
 		clear_bit(PAGE_CLAIMED, &page->private);
 		return;
@@ -1316,6 +1331,10 @@ static int z3fold_reclaim_page(struct z3fold_pool *pool, unsigned int retries)
 	struct page *page = NULL;
 	struct list_head *pos;
 	unsigned long first_handle = 0, middle_handle = 0, last_handle = 0;
+	struct z3fold_buddy_slots slots __attribute__((aligned(SLOTS_ALIGN)));
+
+	rwlock_init(&slots.lock);
+	slots.pool = (unsigned long)pool | (1 << HANDLES_NOFREE);
 
 	spin_lock(&pool->lock);
 	if (!pool->ops || !pool->ops->evict || retries == 0) {
@@ -1330,35 +1349,36 @@ static int z3fold_reclaim_page(struct z3fold_pool *pool, unsigned int retries)
 		list_for_each_prev(pos, &pool->lru) {
 			page = list_entry(pos, struct page, lru);
 
-			/* this bit could have been set by free, in which case
-			 * we pass over to the next page in the pool.
-			 */
-			if (test_and_set_bit(PAGE_CLAIMED, &page->private)) {
-				page = NULL;
-				continue;
-			}
-
-			if (unlikely(PageIsolated(page))) {
-				clear_bit(PAGE_CLAIMED, &page->private);
-				page = NULL;
-				continue;
-			}
 			zhdr = page_address(page);
 			if (test_bit(PAGE_HEADLESS, &page->private))
 				break;
 
+			if (kref_get_unless_zero(&zhdr->refcount) == 0) {
+				zhdr = NULL;
+				break;
+			}
 			if (!z3fold_page_trylock(zhdr)) {
-				clear_bit(PAGE_CLAIMED, &page->private);
+				if (kref_put(&zhdr->refcount,
+						release_z3fold_page))
+					atomic64_dec(&pool->pages_nr);
 				zhdr = NULL;
 				continue; /* can't evict at this point */
 			}
-			if (zhdr->foreign_handles) {
-				clear_bit(PAGE_CLAIMED, &page->private);
-				z3fold_page_unlock(zhdr);
+
+			/* test_and_set_bit is of course atomic, but we still
+			 * need to do it under page lock, otherwise checking
+			 * that bit in __z3fold_alloc wouldn't make sense
+			 */
+			if (zhdr->foreign_handles ||
+			    test_and_set_bit(PAGE_CLAIMED, &page->private)) {
+				if (kref_put(&zhdr->refcount,
+						release_z3fold_page))
+					atomic64_dec(&pool->pages_nr);
+				else
+					z3fold_page_unlock(zhdr);
 				zhdr = NULL;
 				continue; /* can't evict such page */
 			}
-			kref_get(&zhdr->refcount);
 			list_del_init(&zhdr->buddy);
 			zhdr->cpu = -1;
 			break;
@@ -1380,12 +1400,16 @@ static int z3fold_reclaim_page(struct z3fold_pool *pool, unsigned int retries)
 			first_handle = 0;
 			last_handle = 0;
 			middle_handle = 0;
+			memset(slots.slot, 0, sizeof(slots.slot));
 			if (zhdr->first_chunks)
-				first_handle = encode_handle(zhdr, FIRST);
+				first_handle = __encode_handle(zhdr, &slots,
+								FIRST);
 			if (zhdr->middle_chunks)
-				middle_handle = encode_handle(zhdr, MIDDLE);
+				middle_handle = __encode_handle(zhdr, &slots,
+								MIDDLE);
 			if (zhdr->last_chunks)
-				last_handle = encode_handle(zhdr, LAST);
+				last_handle = __encode_handle(zhdr, &slots,
+								LAST);
 			/*
 			 * it's safe to unlock here because we hold a
 			 * reference to this page
@@ -1400,19 +1424,16 @@ static int z3fold_reclaim_page(struct z3fold_pool *pool, unsigned int retries)
 			ret = pool->ops->evict(pool, middle_handle);
 			if (ret)
 				goto next;
-			free_handle(middle_handle, zhdr);
 		}
 		if (first_handle) {
 			ret = pool->ops->evict(pool, first_handle);
 			if (ret)
 				goto next;
-			free_handle(first_handle, zhdr);
 		}
 		if (last_handle) {
 			ret = pool->ops->evict(pool, last_handle);
 			if (ret)
 				goto next;
-			free_handle(last_handle, zhdr);
 		}
 next:
 		if (test_bit(PAGE_HEADLESS, &page->private)) {
@@ -1426,9 +1447,11 @@ static int z3fold_reclaim_page(struct z3fold_pool *pool, unsigned int retries)
 			spin_unlock(&pool->lock);
 			clear_bit(PAGE_CLAIMED, &page->private);
 		} else {
+			struct z3fold_buddy_slots *slots = zhdr->slots;
 			z3fold_page_lock(zhdr);
 			if (kref_put(&zhdr->refcount,
 					release_z3fold_page_locked)) {
+				kmem_cache_free(pool->c_handle, slots);
 				atomic64_dec(&pool->pages_nr);
 				return 0;
 			}
@@ -1544,8 +1567,7 @@ static bool z3fold_page_isolate(struct page *page, isolate_mode_t mode)
 	VM_BUG_ON_PAGE(!PageMovable(page), page);
 	VM_BUG_ON_PAGE(PageIsolated(page), page);
 
-	if (test_bit(PAGE_HEADLESS, &page->private) ||
-	    test_bit(PAGE_CLAIMED, &page->private))
+	if (test_bit(PAGE_HEADLESS, &page->private))
 		return false;
 
 	zhdr = page_address(page);
@@ -1557,6 +1579,8 @@ static bool z3fold_page_isolate(struct page *page, isolate_mode_t mode)
 	if (zhdr->mapped_count != 0 || zhdr->foreign_handles != 0)
 		goto out;
 
+	if (test_and_set_bit(PAGE_CLAIMED, &page->private))
+		goto out;
 	pool = zhdr_to_pool(zhdr);
 	spin_lock(&pool->lock);
 	if (!list_empty(&zhdr->buddy))
@@ -1583,16 +1607,17 @@ static int z3fold_page_migrate(struct address_space *mapping, struct page *newpa
 
 	VM_BUG_ON_PAGE(!PageMovable(page), page);
 	VM_BUG_ON_PAGE(!PageIsolated(page), page);
+	VM_BUG_ON_PAGE(!test_bit(PAGE_CLAIMED, &page->private), page);
 	VM_BUG_ON_PAGE(!PageLocked(newpage), newpage);
 
 	zhdr = page_address(page);
 	pool = zhdr_to_pool(zhdr);
 
-	if (!z3fold_page_trylock(zhdr)) {
+	if (!z3fold_page_trylock(zhdr))
 		return -EAGAIN;
-	}
 	if (zhdr->mapped_count != 0 || zhdr->foreign_handles != 0) {
 		z3fold_page_unlock(zhdr);
+		clear_bit(PAGE_CLAIMED, &page->private);
 		return -EBUSY;
 	}
 	if (work_pending(&zhdr->work)) {
@@ -1634,6 +1659,7 @@ static int z3fold_page_migrate(struct address_space *mapping, struct page *newpa
 	queue_work_on(new_zhdr->cpu, pool->compact_wq, &new_zhdr->work);
 
 	page_mapcount_reset(page);
+	clear_bit(PAGE_CLAIMED, &page->private);
 	put_page(page);
 	return 0;
 }
@@ -1657,6 +1683,7 @@ static void z3fold_page_putback(struct page *page)
 	spin_lock(&pool->lock);
 	list_add(&page->lru, &pool->lru);
 	spin_unlock(&pool->lock);
+	clear_bit(PAGE_CLAIMED, &page->private);
 	z3fold_page_unlock(zhdr);
 }