btrfs: fix and document the zoned device choice in alloc_new_bio

Zone Append bios only need a valid block device in struct bio, but
not the device in the btrfs_bio.  Use the information from
btrfs_zoned_get_device to set up bi_bdev and fix zoned writes on
multi-device file system with non-homogeneous capabilities and remove
the pointless btrfs_bio.device assignment.

Add big fat comments explaining what is going on here.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Naohiro Aota <naohiro.aota@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index 78486bbd1ac90254ea421effeb8dd3df3fcec611..49f789627d0051f2755224b51f770e978a530aa4 100644 (file)
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -3334,24 +3334,37 @@ static int alloc_new_bio(struct btrfs_inode *inode,
        ret = calc_bio_boundaries(bio_ctrl, inode, file_offset);
        if (ret < 0)
                goto error;
-       if (wbc) {
-               struct block_device *bdev;
 
-               bdev = fs_info->fs_devices->latest_dev->bdev;
-               bio_set_dev(bio, bdev);
-               wbc_init_bio(wbc, bio);
-       }
-       if (bio_op(bio) == REQ_OP_ZONE_APPEND) {
-               struct btrfs_device *device;
+       if (wbc) {
+               /*
+                * For Zone append we need the correct block_device that we are
+                * going to write to set in the bio to be able to respect the
+                * hardware limitation.  Look it up here:
+                */
+               if (bio_op(bio) == REQ_OP_ZONE_APPEND) {
+                       struct btrfs_device *dev;
+
+                       dev = btrfs_zoned_get_device(fs_info, disk_bytenr,
+                                                    fs_info->sectorsize);
+                       if (IS_ERR(dev)) {
+                               ret = PTR_ERR(dev);
+                               goto error;
+                       }
 
-               device = btrfs_zoned_get_device(fs_info, disk_bytenr,
-                                               fs_info->sectorsize);
-               if (IS_ERR(device)) {
-                       ret = PTR_ERR(device);
-                       goto error;
+                       bio_set_dev(bio, dev->bdev);
+               } else {
+                       /*
+                        * Otherwise pick the last added device to support
+                        * cgroup writeback.  For multi-device file systems this
+                        * means blk-cgroup policies have to always be set on the
+                        * last added/replaced device.  This is a bit odd but has
+                        * been like that for a long time.
+                        */
+                       bio_set_dev(bio, fs_info->fs_devices->latest_dev->bdev);
                }
-
-               btrfs_bio(bio)->device = device;
+               wbc_init_bio(wbc, bio);
+       } else {
+               ASSERT(bio_op(bio) != REQ_OP_ZONE_APPEND);
        }
        return 0;
 error: