Incorrect epoch_id after restoring snapshot entries

Issue:

If remounting NOVA after creating a snapshot, the NOVA's epoch id will be as same the last snapshot's epoch id. After that, we cannot create a new snapshot since the epoch id has existed in the radix tree. Correctly, the system's epoch id should be one bigger than the last snapshot's epoch id.

Reproduction:

mount -t NOVA -o init,dax /dev/pmem0 /mnt/pmem0
# create a snapshot
echo 1 > /proc/fs/NOVA/pmem0/create_snapshot
umount /mnt/pmem0
# remount NOVA
mount -t NOVA -o dax /dev/pmem0 /mnt/pmem0
# try to create a new snapshot after the recovery
echo 1 > /proc/fs/NOVA/pmem0/create_snapshot 
# get the error msg
dmesg
# the msg will show "nova: nova_insert_snapshot_info ERROR -17" since the radix tree insertion is failed.

Reason:

In recovery phase, nova_sb_info->s_epoch_id will be set as nova_snapshot_info_entry->epoch_id, as Line 870 shows.

https://github.com/NVSL/linux-nova/blob/b817ca322e6fc61f532174e7effc4b6c81528e3f/fs/nova/snapshot.c#L869-L870

When the recovery is done, nova_sb_info->s_epoch_id will be set as nova_super_block->epoch_id to obtain the correct epoch.

https://github.com/NVSL/linux-nova/blob/b817ca322e6fc61f532174e7effc4b6c81528e3f/fs/nova/bbuild.c#L1603

However, nova_super_block->epoch_id is incorrect which was updated in the nova_create_snapshot function. As the below code shows, Line 926 gets the current epoch id and increases nova_sb_info->s_epoch_id by one. Then, the got epoch id is assigned to nova_super_block, as Line 966 says. Indeed, this epoch id is one smaller than the correct one, a.k.a. nova_sb_info->s_epoch_id.

https://github.com/NVSL/linux-nova/blob/b817ca322e6fc61f532174e7effc4b6c81528e3f/fs/nova/snapshot.c#L925-L926 https://github.com/NVSL/linux-nova/blob/b817ca322e6fc61f532174e7effc4b6c81528e3f/fs/nova/snapshot.c#L965-L967

Potential Correction:

// for Line 869 - 870, nova_restore_snapshot_entry(), linux-nova/fs/nova/snapshot.c
// ensure sbi's epoch id is always one bigger than the epoch id of the restored snapshot.
if (epoch_id >= sbi->s_epoch_id) 
    sbi->s_epoch_id = epoch_id + 1;

// for Line 966, nova_create_snapshot(), linux-nova/fs/nova/snapshot.c
// correct the epoch id is one bigger than the epoch id of the new created snapshot.
sbi->nova_sb->s_wtime = cpu_to_le32(get_seconds()); 
sbi->nova_sb->s_epoch_id = cpu_to_le64(epoch_id) + 1; 
nova_update_super_crc(sb);

NVSL / linux-nova