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Code Issues Releases Wiki Activity

btrfs: document some invariants of seed code

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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 <anand.jain@oracle.com>
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
Nikolay Borisov 2020-08-12 17:04:36 +03:00 committed by David Sterba
parent 944d3f9fac
commit 427c8fddb1
1 changed files with 15 additions and 0 deletions
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15
fs/btrfs/volumes.c
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@ -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;