btrfs: handle allocation failure in btrfs_wq_submit_bio gracefully

btrfs_wq_submit_bio is used for writeback under memory pressure.
Instead of failing the I/O when we can't allocate the async_submit_bio,
just punt back to the synchronous submission path.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Tested-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/disk-io.c

@@ -759,16 +759,23 @@ static void run_one_async_free(struct btrfs_work *work)
 	kfree(async);
 }
 
-blk_status_t btrfs_wq_submit_bio(struct inode *inode, struct bio *bio,
+/*
-				 int mirror_num, u64 dio_file_offset,
+ * Submit bio to an async queue.
-				 extent_submit_bio_start_t *submit_bio_start)
+ *
+ * Retrun:
+ * - true if the work has been succesfuly submitted
+ * - false in case of error
+ */
+bool btrfs_wq_submit_bio(struct inode *inode, struct bio *bio, int mirror_num,
+			 u64 dio_file_offset,
+			 extent_submit_bio_start_t *submit_bio_start)
 {
 	struct btrfs_fs_info *fs_info = BTRFS_I(inode)->root->fs_info;
 	struct async_submit_bio *async;
 
 	async = kmalloc(sizeof(*async), GFP_NOFS);
 	if (!async)
-		return BLK_STS_RESOURCE;
+		return false;
 
 	async->inode = inode;
 	async->bio = bio;
@@ -786,7 +793,7 @@ blk_status_t btrfs_wq_submit_bio(struct inode *inode, struct bio *bio,
 		btrfs_queue_work(fs_info->hipri_workers, &async->work);
 	else
 		btrfs_queue_work(fs_info->workers, &async->work);
-	return 0;
+	return true;
 }
 
 static blk_status_t btree_csum_one_bio(struct bio *bio)
@@ -840,25 +847,23 @@ void btrfs_submit_metadata_bio(struct inode *inode, struct bio *bio, int mirror_
 		btrfs_submit_bio(fs_info, bio, mirror_num);
 		return;
 	}
-	if (!should_async_write(fs_info, BTRFS_I(inode))) {
-		ret = btree_csum_one_bio(bio);
-		if (!ret) {
-			btrfs_submit_bio(fs_info, bio, mirror_num);
-			return;
-		}
-	} else {
-		/*
-		 * kthread helpers are used to submit writes so that
-		 * checksumming can happen in parallel across all CPUs
-		 */
-		ret = btrfs_wq_submit_bio(inode, bio, mirror_num, 0,
-					  btree_submit_bio_start);
-	}
 
+	/*
+	 * Kthread helpers are used to submit writes so that checksumming can
+	 * happen in parallel across all CPUs.
+	 */
+	if (should_async_write(fs_info, BTRFS_I(inode)) &&
+	    btrfs_wq_submit_bio(inode, bio, mirror_num, 0, btree_submit_bio_start))
+		return;
+
+	ret = btree_csum_one_bio(bio);
 	if (ret) {
 		bio->bi_status = ret;
 		bio_endio(bio);
+		return;
 	}
+
+	btrfs_submit_bio(fs_info, bio, mirror_num);
 }
 
 #ifdef CONFIG_MIGRATION

fs/btrfs/disk-io.h

@@ -114,9 +114,9 @@ int btrfs_buffer_uptodate(struct extent_buffer *buf, u64 parent_transid,
 			  int atomic);
 int btrfs_read_extent_buffer(struct extent_buffer *buf, u64 parent_transid,
 			     int level, struct btrfs_key *first_key);
-blk_status_t btrfs_wq_submit_bio(struct inode *inode, struct bio *bio,
+bool btrfs_wq_submit_bio(struct inode *inode, struct bio *bio, int mirror_num,
-				 int mirror_num, u64 dio_file_offset,
+			 u64 dio_file_offset,
-				 extent_submit_bio_start_t *submit_bio_start);
+			 extent_submit_bio_start_t *submit_bio_start);
 blk_status_t btrfs_submit_bio_done(void *private_data, struct bio *bio,
 			  int mirror_num);
 int btrfs_alloc_log_tree_node(struct btrfs_trans_handle *trans,

fs/btrfs/inode.c

@@ -2674,11 +2674,10 @@ void btrfs_submit_data_write_bio(struct inode *inode, struct bio *bio, int mirro
 	if (!(bi->flags & BTRFS_INODE_NODATASUM) &&
 	    !test_bit(BTRFS_FS_STATE_NO_CSUMS, &fs_info->fs_state) &&
 	    !btrfs_is_data_reloc_root(bi->root)) {
-		if (!atomic_read(&bi->sync_writers)) {
+		if (!atomic_read(&bi->sync_writers) &&
-			ret = btrfs_wq_submit_bio(inode, bio, mirror_num, 0,
+		    btrfs_wq_submit_bio(inode, bio, mirror_num, 0,
-						  btrfs_submit_bio_start);
+					btrfs_submit_bio_start))
-			goto out;
+			return;
-		}
 
 		ret = btrfs_csum_one_bio(bi, bio, (u64)-1, false);
 		if (ret)
@@ -8027,9 +8026,11 @@ static inline blk_status_t btrfs_submit_dio_bio(struct bio *bio,
 
 	if (btrfs_op(bio) == BTRFS_MAP_WRITE) {
 		/* Check btrfs_submit_data_write_bio() for async submit rules */
-		if (async_submit && !atomic_read(&BTRFS_I(inode)->sync_writers))
+		if (async_submit && !atomic_read(&BTRFS_I(inode)->sync_writers) &&
-			return btrfs_wq_submit_bio(inode, bio, 0, file_offset,
+		    btrfs_wq_submit_bio(inode, bio, 0, file_offset,
-					btrfs_submit_bio_start_direct_io);
+					btrfs_submit_bio_start_direct_io))
+			return BLK_STS_OK;
+
 		/*
 		 * If we aren't doing async submit, calculate the csum of the
 		 * bio now.