bcachefs: Add a mempool for btree_trans bump allocator

This allocation is required for filesystem operations to make forward
progress, thus needs a mempool.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/bcachefs.h

@@ -644,6 +644,7 @@ struct bch_fs {
 	struct mutex		btree_trans_lock;
 	struct list_head	btree_trans_list;
 	mempool_t		btree_iters_pool;
+	mempool_t		btree_trans_mem_pool;
 	struct btree_iter_buf  __percpu	*btree_iters_bufs;
 
 	struct srcu_struct	btree_trans_barrier;

fs/bcachefs/btree_iter.c

@@ -2181,7 +2181,16 @@ static int bch2_trans_preload_mem(struct btree_trans *trans, size_t size)
 	if (size > trans->mem_bytes) {
 		size_t old_bytes = trans->mem_bytes;
 		size_t new_bytes = roundup_pow_of_two(size);
-		void *new_mem = krealloc(trans->mem, new_bytes, GFP_NOFS);
+		void *new_mem;
+
+		WARN_ON_ONCE(new_bytes > BTREE_TRANS_MEM_MAX);
+
+		new_mem = krealloc(trans->mem, new_bytes, GFP_NOFS);
+		if (!new_mem && new_bytes <= BTREE_TRANS_MEM_MAX) {
+			new_mem = mempool_alloc(&trans->c->btree_trans_mem_pool, GFP_KERNEL);
+			new_bytes = BTREE_TRANS_MEM_MAX;
+			kfree(trans->mem);
+		}
 
 		if (!new_mem)
 			return -ENOMEM;
@@ -2293,8 +2302,13 @@ void bch2_trans_init(struct btree_trans *trans, struct bch_fs *c,
 	if (expected_mem_bytes) {
 		expected_mem_bytes = roundup_pow_of_two(expected_mem_bytes);
 		trans->mem = kmalloc(expected_mem_bytes, GFP_KERNEL);
-		if (trans->mem)
+
+		if (!unlikely(trans->mem)) {
+			trans->mem = mempool_alloc(&c->btree_trans_mem_pool, GFP_KERNEL);
+			trans->mem_bytes = BTREE_TRANS_MEM_MAX;
+		} else {
 			trans->mem_bytes = expected_mem_bytes;
+		}
 	}
 
 	trans->srcu_idx = srcu_read_lock(&c->btree_trans_barrier);
@@ -2324,7 +2338,11 @@ int bch2_trans_exit(struct btree_trans *trans)
 	bch2_journal_preres_put(&trans->c->journal, &trans->journal_preres);
 
 	kfree(trans->fs_usage_deltas);
-	kfree(trans->mem);
+
+	if (trans->mem_bytes == BTREE_TRANS_MEM_MAX)
+		mempool_free(trans->mem, &trans->c->btree_trans_mem_pool);
+	else
+		kfree(trans->mem);
 
 #ifdef __KERNEL__
 	/*
@@ -2332,6 +2350,7 @@ int bch2_trans_exit(struct btree_trans *trans)
 	 */
 	trans->iters = this_cpu_xchg(c->btree_iters_bufs->iter, trans->iters);
 #endif
+
 	if (trans->iters)
 		mempool_free(trans->iters, &trans->c->btree_iters_pool);
 
@@ -2407,6 +2426,7 @@ void bch2_btree_trans_to_text(struct printbuf *out, struct bch_fs *c)
 
 void bch2_fs_btree_iter_exit(struct bch_fs *c)
 {
+	mempool_exit(&c->btree_trans_mem_pool);
 	mempool_exit(&c->btree_iters_pool);
 	cleanup_srcu_struct(&c->btree_trans_barrier);
 }
@@ -2422,5 +2442,7 @@ int bch2_fs_btree_iter_init(struct bch_fs *c)
 		mempool_init_kmalloc_pool(&c->btree_iters_pool, 1,
 			sizeof(struct btree_iter) * nr +
 			sizeof(struct btree_insert_entry) * nr +
-			sizeof(struct btree_insert_entry) * nr);
+			sizeof(struct btree_insert_entry) * nr) ?:
+		mempool_init_kmalloc_pool(&c->btree_trans_mem_pool, 1,
+					  BTREE_TRANS_MEM_MAX);
 }

fs/bcachefs/btree_types.h

@@ -346,6 +346,8 @@ struct btree_insert_entry {
 #define BTREE_ITER_MAX		32
 #endif
 
+#define BTREE_TRANS_MEM_MAX	(1U << 14)
+
 struct btree_trans {
 	struct bch_fs		*c;
 #ifdef CONFIG_BCACHEFS_DEBUG