[PATCH] z3fold: fix possible reclaim races

[Vitaly Wool <vitalywool@gmail.com>]

Reclaim and free can race on an object which is basically fine but
in order for reclaim to be able to  map "freed" object we need to
encode object length in the handle. handle_to_chunks() is then
introduced to extract object length from a handle and use it during
mapping.

Moreover, to avoid racing on a z3fold "headless" page release, we
should not try to free that page in z3fold_free() if the reclaim
bit is set. Also, in the unlikely case of trying to reclaim a page
being freed, we should not proceed with that page.

While at it, fix the page accounting in reclaim function.

This patch supersedes "[PATCH] z3fold: fix reclaim lock-ups".

Signed-off-by: Vitaly Wool <vitaly.vul@sony.com>
Reviewed-by: Snild Dolkow <snild@sony.com>

---
 mm/z3fold.c | 101 ++++++++++++++++++++++++++++++++--------------------
 1 file changed, 62 insertions(+), 39 deletions(-)

diff --git a/mm/z3fold.c b/mm/z3fold.c
index ff73ef8124bf..5999727ee17c 100644
--- a/mm/z3fold.c
+++ b/mm/z3fold.c
@@ -99,6 +99,7 @@ struct z3fold_header {
 #define NCHUNKS		((PAGE_SIZE - ZHDR_SIZE_ALIGNED) >> CHUNK_SHIFT)
 
 #define BUDDY_MASK	(0x3)
+#define BUDDY_SHIFT	2
 
 /**
  * struct z3fold_pool - stores metadata for each z3fold pool
@@ -145,7 +146,7 @@ enum z3fold_page_flags {
 	MIDDLE_CHUNK_MAPPED,
 	NEEDS_COMPACTING,
 	PAGE_STALE,
-	UNDER_RECLAIM
+	PAGE_CLAIMED, /* by either reclaim or free */
 };
 
 /*****************
@@ -174,7 +175,7 @@ static struct z3fold_header *init_z3fold_page(struct page *page,
 	clear_bit(MIDDLE_CHUNK_MAPPED, &page->private);
 	clear_bit(NEEDS_COMPACTING, &page->private);
 	clear_bit(PAGE_STALE, &page->private);
-	clear_bit(UNDER_RECLAIM, &page->private);
+	clear_bit(PAGE_CLAIMED, &page->private);
 
 	spin_lock_init(&zhdr->page_lock);
 	kref_init(&zhdr->refcount);
@@ -223,8 +224,11 @@ static unsigned long encode_handle(struct z3fold_header *zhdr, enum buddy bud)
 	unsigned long handle;
 
 	handle = (unsigned long)zhdr;
-	if (bud != HEADLESS)
-		handle += (bud + zhdr->first_num) & BUDDY_MASK;
+	if (bud != HEADLESS) {
+		handle |= (bud + zhdr->first_num) & BUDDY_MASK;
+		if (bud == LAST)
+			handle |= (zhdr->last_chunks << BUDDY_SHIFT);
+	}
 	return handle;
 }
 
@@ -234,6 +238,12 @@ static struct z3fold_header *handle_to_z3fold_header(unsigned long handle)
 	return (struct z3fold_header *)(handle & PAGE_MASK);
 }
 
+/* only for LAST bud, returns zero otherwise */
+static unsigned short handle_to_chunks(unsigned long handle)
+{
+	return (handle & ~PAGE_MASK) >> BUDDY_SHIFT;
+}
+
 /*
  * (handle & BUDDY_MASK) < zhdr->first_num is possible in encode_handle
  *  but that doesn't matter. because the masking will result in the
@@ -720,37 +730,39 @@ static void z3fold_free(struct z3fold_pool *pool, unsigned long handle)
 	page = virt_to_page(zhdr);
 
 	if (test_bit(PAGE_HEADLESS, &page->private)) {
-		/* HEADLESS page stored */
-		bud = HEADLESS;
-	} else {
-		z3fold_page_lock(zhdr);
-		bud = handle_to_buddy(handle);
-
-		switch (bud) {
-		case FIRST:
-			zhdr->first_chunks = 0;
-			break;
-		case MIDDLE:
-			zhdr->middle_chunks = 0;
-			zhdr->start_middle = 0;
-			break;
-		case LAST:
-			zhdr->last_chunks = 0;
-			break;
-		default:
-			pr_err("%s: unknown bud %d\n", __func__, bud);
-			WARN_ON(1);
-			z3fold_page_unlock(zhdr);
-			return;
+		/* 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)) {
+			spin_lock(&pool->lock);
+			list_del(&page->lru);
+			spin_unlock(&pool->lock);
+			free_z3fold_page(page);
+			atomic64_dec(&pool->pages_nr);
 		}
+		return;
 	}
 
-	if (bud == HEADLESS) {
-		spin_lock(&pool->lock);
-		list_del(&page->lru);
-		spin_unlock(&pool->lock);
-		free_z3fold_page(page);
-		atomic64_dec(&pool->pages_nr);
+	/* Non-headless case */
+	z3fold_page_lock(zhdr);
+	bud = handle_to_buddy(handle);
+
+	switch (bud) {
+	case FIRST:
+		zhdr->first_chunks = 0;
+		break;
+	case MIDDLE:
+		zhdr->middle_chunks = 0;
+		break;
+	case LAST:
+		zhdr->last_chunks = 0;
+		break;
+	default:
+		pr_err("%s: unknown bud %d\n", __func__, bud);
+		WARN_ON(1);
+		z3fold_page_unlock(zhdr);
 		return;
 	}
 
@@ -758,7 +770,7 @@ static void z3fold_free(struct z3fold_pool *pool, unsigned long handle)
 		atomic64_dec(&pool->pages_nr);
 		return;
 	}
-	if (test_bit(UNDER_RECLAIM, &page->private)) {
+	if (test_bit(PAGE_CLAIMED, &page->private)) {
 		z3fold_page_unlock(zhdr);
 		return;
 	}
@@ -836,20 +848,30 @@ 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))
+				continue;
+
+			zhdr = page_address(page);
 			if (test_bit(PAGE_HEADLESS, &page->private))
-				/* candidate found */
 				break;
 
-			zhdr = page_address(page);
-			if (!z3fold_page_trylock(zhdr))
+			if (!z3fold_page_trylock(zhdr)) {
+				zhdr = NULL;
 				continue; /* can't evict at this point */
+			}
 			kref_get(&zhdr->refcount);
 			list_del_init(&zhdr->buddy);
 			zhdr->cpu = -1;
-			set_bit(UNDER_RECLAIM, &page->private);
 			break;
 		}
 
+		if (!zhdr)
+			break;
+
 		list_del_init(&page->lru);
 		spin_unlock(&pool->lock);
 
@@ -898,6 +920,7 @@ static int z3fold_reclaim_page(struct z3fold_pool *pool, unsigned int retries)
 		if (test_bit(PAGE_HEADLESS, &page->private)) {
 			if (ret == 0) {
 				free_z3fold_page(page);
+				atomic64_dec(&pool->pages_nr);
 				return 0;
 			}
 			spin_lock(&pool->lock);
@@ -905,7 +928,7 @@ static int z3fold_reclaim_page(struct z3fold_pool *pool, unsigned int retries)
 			spin_unlock(&pool->lock);
 		} else {
 			z3fold_page_lock(zhdr);
-			clear_bit(UNDER_RECLAIM, &page->private);
+			clear_bit(PAGE_CLAIMED, &page->private);
 			if (kref_put(&zhdr->refcount,
 					release_z3fold_page_locked)) {
 				atomic64_dec(&pool->pages_nr);
@@ -964,7 +987,7 @@ static void *z3fold_map(struct z3fold_pool *pool, unsigned long handle)
 		set_bit(MIDDLE_CHUNK_MAPPED, &page->private);
 		break;
 	case LAST:
-		addr += PAGE_SIZE - (zhdr->last_chunks << CHUNK_SHIFT);
+		addr += PAGE_SIZE - (handle_to_chunks(handle) << CHUNK_SHIFT);
 		break;
 	default:
 		pr_err("unknown buddy id %d\n", buddy);
-- 
2.17.1