From 427c8fddb1296dd013d8af5a4957d9d88fb8ab4e Mon Sep 17 00:00:00 2001 From: Nikolay Borisov Date: Wed, 12 Aug 2020 17:04:36 +0300 Subject: [PATCH] btrfs: document some invariants of seed code 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. Reviewed-by: Anand Jain Signed-off-by: Nikolay Borisov Signed-off-by: David Sterba --- fs/btrfs/volumes.c | 15 +++++++++++++++ 1 file changed, 15 insertions(+) diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c index 2d7c845b8e67..7a39f3aebc53 100644 --- a/fs/btrfs/volumes.c +++ b/fs/btrfs/volumes.c @@ -2357,10 +2357,20 @@ 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 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); @@ -6720,6 +6730,7 @@ 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; @@ -6739,6 +6750,10 @@ 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; -- GitLab