Re: [PATCH] btrfs: Document some invariants of seed code

Anand Jain
2020-08-26T09:13:34Z

On 12/8/20 10:04 pm, Nikolay Borisov wrote:
> Without good understanding of how seed devices works it's hard to grok
> some of what the code in open_seed_devices or btrfs_prepare_sprout does.
>
> Add comments hopefully reducing some of the cognitive load.
>
> Signed-off-by: Nikolay Borisov <nborisov@suse.com>
> ---
> fs/btrfs/volumes.c | 15 +++++++++++++++
> 1 file changed, 15 insertions(+)
>
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index 631cb03b3513..f7ee7837e6bc 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -2361,10 +2361,20 @@ static int btrfs_prepare_sprout(struct btrfs_fs_info *fs_info)
> if (!fs_devices->seeding)
> return -EINVAL;
>
> + /*
> + * Private copy of the seed devices, anchored at
> + * fs_info->fs_devices->seed_list
> + */
> seed_devices = alloc_fs_devices(NULL, NULL);
> if (IS_ERR(seed_devices))
> return PTR_ERR(seed_devices);
>
> + /*
> + * It's necessary to retain a copy of the original seed fs_devices in
> + * fs_uuids so that filesystems which have been seeded can be
> + * successfully reference the seed device from open_seed_devices. This
> + * also supports multiple fs seed.
> + */
>
> old_devices = clone_fs_devices(fs_devices);
> if (IS_ERR(old_devices)) {
> kfree(seed_devices);
> @@ -6713,6 +6723,7 @@ static struct btrfs_fs_devices *open_seed_devices(struct btrfs_fs_info *fs_info,
> lockdep_assert_held(&uuid_mutex);
> ASSERT(fsid);
>
> + /* This will match only for multi-device seed fs */
> list_for_each_entry(fs_devices, &fs_info->fs_devices->seed_list, seed_list)
> if (!memcmp(fs_devices->fsid, fsid, BTRFS_FSID_SIZE))
> return fs_devices;
> @@ -6732,6 +6743,10 @@ static struct btrfs_fs_devices *open_seed_devices(struct btrfs_fs_info *fs_info,
> return fs_devices;
> }
>
> + /*
> + * Upon first call for a seed fs fsid just create a private copy of the
> + * respective fs_devices and anchor it at fs_info->fs_devices->seed_list
> + */
> fs_devices = clone_fs_devices(fs_devices);
> if (IS_ERR(fs_devices))
> return fs_devices;
>

Looks good. Thanks

Reviewed-by: Anand Jain <anand.jain@oracle.com>