[PATCH v2 3/3] bcache: support overlay bcache

[mingzhe.zou@easystack.cn]

From: Dongsheng Yang <dongsheng.yang@easystack.cn>

If we want to build a bcache device with backing device of a bcache flash device,
we will fail with creating a duplicated sysfs filename.

E.g:
(1) we create bcache0 with vdc, then there is "/sys/block/bcache0/bcache" as a link to "/sys/block/vdc/bcache"
 $ readlink /sys/block/bcache0/bcache
../../../pci0000:00/0000:00:0b.0/virtio4/block/vdc/bcache

(2) if we continue to create bcache1 with bcache0:
 $ make-bcache -B /dev/bcache0

We will fail to register bdev with "sysfs: cannot create duplicate filename '/devices/virtual/block/bcache0/bcache'"

How this commit solving this problem?
E.g:
   we have vdf as cache disk, vdc as backing disk.

 $ make-bcache -C /dev/vdf -B /dev/vdc --wipe-bcache
 $ echo 100G > /sys/block/vdf/bcache_cache/set/flash_vol_create
 $ lsblk
NAME                       MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
vdf                        252:80   0   50G  0 disk
├─bcache0                  251:0    0  100G  0 disk
└─bcache1                  251:128  0  100G  0 disk
vdc                        252:32   0  100G  0 disk
└─bcache0                  251:0    0  100G  0 disk

(a) rename sysfs file to more meaningful name:
(a.2) bcahce_cache -> sysfs filename under cache disk (/sys/block/vdf/bcache_cache)
(a.1) bcache_fdev -> flash bcache device (/sys/block/bcache1/bcache_fdev)
(a.4) bcache_bdev -> sysfs filename for backing disk (/sys/block/vdc/bcache_bdev)
(a.3) bcache_cdev -> link to /sys/block/vdc/bcache_bdev (/sys/block/bcache0/bcache_cdev)

(b) create ->bcache lagacy link file for backward compatability
$ ll /sys/block/vdc/bcache
lrwxrwxrwx 1 root root 0 Oct 26 11:21 /sys/block/vdc/bcache -> bcache_bdev
$ ll /sys/block/bcache0/bcache
lrwxrwxrwx 1 root root 0 Oct 26 11:21 /sys/block/bcache0/bcache -> ../../../pci0000:00/0000:00:0b.0/virtio4/block/vdc/bcache_bdev
$ ll /sys/block/bcache1/bcache
lrwxrwxrwx 1 root root 0 Oct 26 11:19 /sys/block/bcache1/bcache -> bcache_fdev
$ ll /sys/block/vdf/bcache
lrwxrwxrwx 1 root root 0 Oct 26 11:17 /sys/block/vdf/bcache -> bcache_cache

These link are created with sysfs_create_link_nowarn(), that means, we dont
care about the failure when creating if these links are already created.
Because these lagacy sysfile are only for backwards compatability in no-overlay usecase
of bcache, in the no-overlay use, bcache will never create duplicated link.

In overlay usecase after this commit, please dont use bcache link any more, instead
use bcache_cdev, bcache_fdev, bcache_bdev or bcache_cache.

Then we can create a cached_dev with bcache1 (flash dev) as backing dev.
$ make-bcache -B /dev/bcache1
$ lsblk
NAME                       MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
vdf                        252:80   0   50G  0 disk
├─bcache0                  251:0    0  100G  0 disk
└─bcache1                  251:128  0  100G  0 disk
  └─bcache2                251:256  0  100G  0 disk

As a result there is a cached device bcache2 with backing device of a flash device bcache1.
        ----------------------------
        | bcache2 (cached_dev)     |
        | ------------------------ |
        | |   sdb (cache_dev)    | |
        | ------------------------ |
        | ------------------------ |
        | |   bcache1 (flash_dev)| |
        | ------------------------ |
        ----------------------------

Signed-off-by: Dongsheng Yang <dongsheng.yang@easystack.cn>
Signed-off-by: mingzhe <mingzhe.zou@easystack.cn>
---
 drivers/md/bcache/super.c | 38 ++++++++++++++++++++++++++++++++++----
 1 file changed, 34 insertions(+), 4 deletions(-)

diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
index ba3909bb6bea..8a922ce91d1e 100644
--- a/drivers/md/bcache/super.c
+++ b/drivers/md/bcache/super.c
@@ -1087,12 +1087,19 @@ int bch_cached_dev_run(struct cached_dev *dc)
 
 	if (sysfs_create_link(&d->kobj, &disk_to_dev(d->disk)->kobj, "dev") ||
 	    sysfs_create_link(&disk_to_dev(d->disk)->kobj,
-			      &d->kobj, "bcache")) {
+			      &d->kobj, "bcache_cdev")) {
 		pr_err("Couldn't create bcache dev <-> disk sysfs symlinks\n");
 		ret = -ENOMEM;
 		goto out;
 	}
 
+	ret = sysfs_create_link_nowarn(&disk_to_dev(d->disk)->kobj,
+				       &d->kobj, "bcache");
+	if (ret && ret != -EEXIST) {
+		pr_err("Couldn't create lagacy disk sysfs ->bcache symlinks\n");
+		goto out;
+	}
+
 	dc->status_update_thread = kthread_run(cached_dev_status_update,
 					       dc, "bcache_status_update");
 	if (IS_ERR(dc->status_update_thread)) {
@@ -1461,8 +1468,16 @@ static int register_bdev(struct cache_sb *sb, struct cache_sb_disk *sb_disk,
 		goto err;
 
 	err = "error creating kobject";
-	if (kobject_add(&dc->disk.kobj, bdev_kobj(bdev), "bcache"))
+	if (kobject_add(&dc->disk.kobj, bdev_kobj(bdev), "bcache_bdev"))
 		goto err;
+
+	err = "error creating lagacy sysfs link";
+	ret = sysfs_create_link_nowarn(bdev_kobj(bdev), &dc->disk.kobj, "bcache");
+	if (ret && ret != -EEXIST) {
+		pr_err("Couldn't create lagacy disk sysfs ->bcache");
+		goto err;
+	}
+
 	if (bch_cache_accounting_add_kobjs(&dc->accounting, &dc->disk.kobj))
 		goto err;
 
@@ -1524,6 +1539,7 @@ static void flash_dev_flush(struct closure *cl)
 
 static int flash_dev_run(struct cache_set *c, struct uuid_entry *u)
 {
+	int ret;
 	int err = -ENOMEM;
 	struct bcache_device *d = kzalloc(sizeof(struct bcache_device),
 					  GFP_KERNEL);
@@ -1546,10 +1562,17 @@ static int flash_dev_run(struct cache_set *c, struct uuid_entry *u)
 	if (err)
 		goto err;
 
-	err = kobject_add(&d->kobj, &disk_to_dev(d->disk)->kobj, "bcache");
+	err = kobject_add(&d->kobj, &disk_to_dev(d->disk)->kobj, "bcache_fdev");
 	if (err)
 		goto err;
 
+	ret = sysfs_create_link_nowarn(&disk_to_dev(d->disk)->kobj,
+				       &d->kobj, "bcache");
+	if (ret && ret != -EEXIST) {
+		pr_err("Couldn't create lagacy flash dev ->bcache");
+		goto err;
+	}
+
 	bcache_device_link(d, c, "volume");
 
 	if (bch_has_feature_obso_large_bucket(&c->cache->sb)) {
@@ -2370,12 +2393,19 @@ static int register_cache(struct cache_sb *sb, struct cache_sb_disk *sb_disk,
 		goto err;
 	}
 
-	if (kobject_add(&ca->kobj, bdev_kobj(bdev), "bcache")) {
+	if (kobject_add(&ca->kobj, bdev_kobj(bdev), "bcache_cache")) {
 		err = "error calling kobject_add";
 		ret = -ENOMEM;
 		goto out;
 	}
 
+	ret = sysfs_create_link_nowarn(bdev_kobj(bdev), &ca->kobj, "bcache");
+	if (ret && ret != -EEXIST) {
+		pr_err("Couldn't create lagacy disk sysfs ->cache symlinks\n");
+		goto out;
+	} else
+		ret = 0;
+
 	mutex_lock(&bch_register_lock);
 	err = register_cache_set(ca);
 	mutex_unlock(&bch_register_lock);
-- 
2.17.1