Btrfs: btrfs_submit_direct_hook: Handle map_length < bio vector length

In subpagesize-blocksize scenario, map_length can be less than the length of a
bio vector. Such a condition may cause btrfs_submit_direct_hook() to submit a
zero length bio. Fix this by comparing map_length against block size rather
than with bv_len.

Signed-off-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -8244,9 +8244,11 @@ static int btrfs_submit_direct_hook(int rw, struct btrfs_dio_private *dip,
 	u64 file_offset = dip->logical_offset;
 	u64 submit_len = 0;
 	u64 map_length;
-	int nr_pages = 0;
-	int ret;
+	u32 blocksize = root->sectorsize;
 	int async_submit = 0;
+	int nr_sectors;
+	int ret;
+	int i;
 
 	map_length = orig_bio->bi_iter.bi_size;
 	ret = btrfs_map_block(root->fs_info, rw, start_sector << 9,
@@ -8276,9 +8278,12 @@ static int btrfs_submit_direct_hook(int rw, struct btrfs_dio_private *dip,
 	atomic_inc(&dip->pending_bios);
 
 	while (bvec <= (orig_bio->bi_io_vec + orig_bio->bi_vcnt - 1)) {
-		if (map_length < submit_len + bvec->bv_len ||
-		    bio_add_page(bio, bvec->bv_page, bvec->bv_len,
-				 bvec->bv_offset) < bvec->bv_len) {
+		nr_sectors = BTRFS_BYTES_TO_BLKS(root->fs_info, bvec->bv_len);
+		i = 0;
+next_block:
+		if (unlikely(map_length < submit_len + blocksize ||
+		    bio_add_page(bio, bvec->bv_page, blocksize,
+			    bvec->bv_offset + (i * blocksize)) < blocksize)) {
 			/*
 			 * inc the count before we submit the bio so
 			 * we know the end IO handler won't happen before
@@ -8299,7 +8304,6 @@ static int btrfs_submit_direct_hook(int rw, struct btrfs_dio_private *dip,
 			file_offset += submit_len;
 
 			submit_len = 0;
-			nr_pages = 0;
 
 			bio = btrfs_dio_bio_alloc(orig_bio->bi_bdev,
 						  start_sector, GFP_NOFS);
@@ -8317,9 +8321,14 @@ static int btrfs_submit_direct_hook(int rw, struct btrfs_dio_private *dip,
 				bio_put(bio);
 				goto out_err;
 			}
+
+			goto next_block;
 		} else {
-			submit_len += bvec->bv_len;
-			nr_pages++;
+			submit_len += blocksize;
+			if (--nr_sectors) {
+				i++;
+				goto next_block;
+			}
 			bvec++;
 		}
 	}