dm-verity: Convert from tasklet to BH workqueue

The only generic interface to execute asynchronously in the BH context is
tasklet; however, it's marked deprecated and has some design flaws. To
replace tasklets, BH workqueue support was recently added. A BH workqueue
behaves similarly to regular workqueues except that the queued work items
are executed in the BH context.

This commit converts dm-verity from tasklet to BH workqueue. It
backfills tasklet code that was removed with commit 0a9bab39
("dm-crypt, dm-verity: disable tasklets") and tweaks to use BH
workqueue (and does some renaming).

This is a minimal conversion which doesn't rename the related names
including the "try_verify_in_tasklet" option. If this patch is applied, a
follow-up patch would be necessary. I couldn't decide whether the option
name would need to be updated too.
Signed-off-by: Tejun Heo <tj@kernel.org>
[snitzer: rename 'use_tasklet' to 'use_bh_wq' and 'in_tasklet' to 'in_bh']
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

drivers/md/dm-verity-target.c

@@ -46,7 +46,7 @@ static unsigned int dm_verity_prefetch_cluster = DM_VERITY_DEFAULT_PREFETCH_SIZE
 
 module_param_named(prefetch_cluster, dm_verity_prefetch_cluster, uint, 0644);
 
-static DEFINE_STATIC_KEY_FALSE(use_tasklet_enabled);
+static DEFINE_STATIC_KEY_FALSE(use_bh_wq_enabled);
 
 struct dm_verity_prefetch_work {
 	struct work_struct work;
@@ -299,7 +299,7 @@ static int verity_verify_level(struct dm_verity *v, struct dm_verity_io *io,
 
 	verity_hash_at_level(v, block, level, &hash_block, &offset);
 
-	if (static_branch_unlikely(&use_tasklet_enabled) && io->in_tasklet) {
+	if (static_branch_unlikely(&use_bh_wq_enabled) && io->in_bh) {
 		data = dm_bufio_get(v->bufio, hash_block, &buf);
 		if (data == NULL) {
 			/*
@@ -327,15 +327,14 @@ static int verity_verify_level(struct dm_verity *v, struct dm_verity_io *io,
 
 		r = verity_hash(v, verity_io_hash_req(v, io),
 				data, 1 << v->hash_dev_block_bits,
-				verity_io_real_digest(v, io), !io->in_tasklet);
+				verity_io_real_digest(v, io), !io->in_bh);
 		if (unlikely(r < 0))
 			goto release_ret_r;
 
 		if (likely(memcmp(verity_io_real_digest(v, io), want_digest,
 				  v->digest_size) == 0))
 			aux->hash_verified = 1;
-		else if (static_branch_unlikely(&use_tasklet_enabled) &&
-			 io->in_tasklet) {
+		else if (static_branch_unlikely(&use_bh_wq_enabled) && io->in_bh) {
 			/*
 			 * Error handling code (FEC included) cannot be run in a
 			 * tasklet since it may sleep, so fallback to work-queue.
@@ -576,7 +575,7 @@ static int verity_verify_io(struct dm_verity_io *io)
 	struct bio *bio = dm_bio_from_per_bio_data(io, v->ti->per_io_data_size);
 	unsigned int b;
 
-	if (static_branch_unlikely(&use_tasklet_enabled) && io->in_tasklet) {
+	if (static_branch_unlikely(&use_bh_wq_enabled) && io->in_bh) {
 		/*
 		 * Copy the iterator in case we need to restart
 		 * verification in a work-queue.
@@ -616,7 +615,7 @@ static int verity_verify_io(struct dm_verity_io *io)
 			continue;
 		}
 
-		r = verity_hash_init(v, req, &wait, !io->in_tasklet);
+		r = verity_hash_init(v, req, &wait, !io->in_bh);
 		if (unlikely(r < 0))
 			return r;
 
@@ -635,8 +634,7 @@ static int verity_verify_io(struct dm_verity_io *io)
 			if (v->validated_blocks)
 				set_bit(cur_block, v->validated_blocks);
 			continue;
-		} else if (static_branch_unlikely(&use_tasklet_enabled) &&
-			   io->in_tasklet) {
+		} else if (static_branch_unlikely(&use_bh_wq_enabled) && io->in_bh) {
 			/*
 			 * Error handling code (FEC included) cannot be run in a
 			 * tasklet since it may sleep, so fallback to work-queue.
@@ -690,7 +688,7 @@ static void verity_finish_io(struct dm_verity_io *io, blk_status_t status)
 	bio->bi_end_io = io->orig_bi_end_io;
 	bio->bi_status = status;
 
-	if (!static_branch_unlikely(&use_tasklet_enabled) || !io->in_tasklet)
+	if (!static_branch_unlikely(&use_bh_wq_enabled) || !io->in_bh)
 		verity_fec_finish_io(io);
 
 	bio_endio(bio);
@@ -700,11 +698,28 @@ static void verity_work(struct work_struct *w)
 {
 	struct dm_verity_io *io = container_of(w, struct dm_verity_io, work);
 
-	io->in_tasklet = false;
+	io->in_bh = false;
 
 	verity_finish_io(io, errno_to_blk_status(verity_verify_io(io)));
 }
 
+static void verity_bh_work(struct work_struct *w)
+{
+	struct dm_verity_io *io = container_of(w, struct dm_verity_io, bh_work);
+	int err;
+
+	io->in_bh = true;
+	err = verity_verify_io(io);
+	if (err == -EAGAIN || err == -ENOMEM) {
+		/* fallback to retrying with work-queue */
+		INIT_WORK(&io->work, verity_work);
+		queue_work(io->v->verify_wq, &io->work);
+		return;
+	}
+
+	verity_finish_io(io, errno_to_blk_status(err));
+}
+
 static void verity_end_io(struct bio *bio)
 {
 	struct dm_verity_io *io = bio->bi_private;
@@ -717,8 +732,13 @@ static void verity_end_io(struct bio *bio)
 		return;
 	}
 
+	if (static_branch_unlikely(&use_bh_wq_enabled) && io->v->use_bh_wq) {
+		INIT_WORK(&io->bh_work, verity_bh_work);
+		queue_work(system_bh_wq, &io->bh_work);
+	} else {
 		INIT_WORK(&io->work, verity_work);
 		queue_work(io->v->verify_wq, &io->work);
+	}
 }
 
 /*
@@ -885,7 +905,7 @@ static void verity_status(struct dm_target *ti, status_type_t type,
 			args++;
 		if (v->validated_blocks)
 			args++;
-		if (v->use_tasklet)
+		if (v->use_bh_wq)
 			args++;
 		if (v->signature_key_desc)
 			args += DM_VERITY_ROOT_HASH_VERIFICATION_OPTS;
@@ -912,7 +932,7 @@ static void verity_status(struct dm_target *ti, status_type_t type,
 			DMEMIT(" " DM_VERITY_OPT_IGN_ZEROES);
 		if (v->validated_blocks)
 			DMEMIT(" " DM_VERITY_OPT_AT_MOST_ONCE);
-		if (v->use_tasklet)
+		if (v->use_bh_wq)
 			DMEMIT(" " DM_VERITY_OPT_TASKLET_VERIFY);
 		sz = verity_fec_status_table(v, sz, result, maxlen);
 		if (v->signature_key_desc)
@@ -1031,8 +1051,8 @@ static void verity_dtr(struct dm_target *ti)
 
 	kfree(v->signature_key_desc);
 
-	if (v->use_tasklet)
-		static_branch_dec(&use_tasklet_enabled);
+	if (v->use_bh_wq)
+		static_branch_dec(&use_bh_wq_enabled);
 
 	kfree(v);
 
@@ -1166,8 +1186,8 @@ static int verity_parse_opt_args(struct dm_arg_set *as, struct dm_verity *v,
 			continue;
 
 		} else if (!strcasecmp(arg_name, DM_VERITY_OPT_TASKLET_VERIFY)) {
-			v->use_tasklet = true;
-			static_branch_inc(&use_tasklet_enabled);
+			v->use_bh_wq = true;
+			static_branch_inc(&use_bh_wq_enabled);
 			continue;
 
 		} else if (verity_is_fec_opt_arg(arg_name)) {
@@ -1338,7 +1358,7 @@ static int verity_ctr(struct dm_target *ti, unsigned int argc, char **argv)
 	}
 
 	v->tfm = crypto_alloc_ahash(v->alg_name, 0,
-				    v->use_tasklet ? CRYPTO_ALG_ASYNC : 0);
+				    v->use_bh_wq ? CRYPTO_ALG_ASYNC : 0);
 	if (IS_ERR(v->tfm)) {
 		ti->error = "Cannot initialize hash function";
 		r = PTR_ERR(v->tfm);
@@ -1463,7 +1483,7 @@ static int verity_ctr(struct dm_target *ti, unsigned int argc, char **argv)
 	v->bufio = dm_bufio_client_create(v->hash_dev->bdev,
 		1 << v->hash_dev_block_bits, 1, sizeof(struct buffer_aux),
 		dm_bufio_alloc_callback, NULL,
-		v->use_tasklet ? DM_BUFIO_CLIENT_NO_SLEEP : 0);
+		v->use_bh_wq ? DM_BUFIO_CLIENT_NO_SLEEP : 0);
 	if (IS_ERR(v->bufio)) {
 		ti->error = "Cannot initialize dm-bufio";
 		r = PTR_ERR(v->bufio);
@@ -1482,7 +1502,7 @@ static int verity_ctr(struct dm_target *ti, unsigned int argc, char **argv)
 	 * reducing wait times when reading from a dm-verity device.
 	 *
 	 * Also as required for the "try_verify_in_tasklet" feature: WQ_HIGHPRI
-	 * allows verify_wq to preempt softirq since verification in tasklet
+	 * allows verify_wq to preempt softirq since verification in BH workqueue
 	 * will fall-back to using it for error handling (or if the bufio cache
 	 * doesn't have required hashes).
 	 */

drivers/md/dm-verity.h

@@ -54,7 +54,7 @@ struct dm_verity {
 	unsigned char levels;	/* the number of tree levels */
 	unsigned char version;
 	bool hash_failed:1;	/* set if hash of any block failed */
-	bool use_tasklet:1;	/* try to verify in tasklet before work-queue */
+	bool use_bh_wq:1;	/* try to verify in BH wq before normal work-queue */
 	unsigned int digest_size;	/* digest size for the current hash algorithm */
 	unsigned int ahash_reqsize;/* the size of temporary space for crypto */
 	enum verity_mode mode;	/* mode for handling verification errors */
@@ -84,9 +84,10 @@ struct dm_verity_io {
 
 	sector_t block;
 	unsigned int n_blocks;
-	bool in_tasklet;
+	bool in_bh;
 
 	struct work_struct work;
+	struct work_struct bh_work;
 
 	char *recheck_buffer;