Re: [PATCH 3/4] btrfs: preallocate anon_dev for subvolume and snapshot creation

[Qu Wenruo <wqu@suse.com>]

On 2020/6/16 下午11:10, David Sterba wrote:
> On Tue, Jun 16, 2020 at 10:17:36AM +0800, Qu Wenruo wrote:
>> [BUG]
>> When a lot of subvolumes are created, there is a user report about
>> transaction aborted:
>>
>>   ------------[ cut here ]------------
>>   BTRFS: Transaction aborted (error -24)
>>   WARNING: CPU: 17 PID: 17041 at fs/btrfs/transaction.c:1576 create_pending_snapshot+0xbc4/0xd10 [btrfs]
>>   RIP: 0010:create_pending_snapshot+0xbc4/0xd10 [btrfs]
>>   Call Trace:
>>    create_pending_snapshots+0x82/0xa0 [btrfs]
>>    btrfs_commit_transaction+0x275/0x8c0 [btrfs]
>>    btrfs_mksubvol+0x4b9/0x500 [btrfs]
>>    btrfs_ioctl_snap_create_transid+0x174/0x180 [btrfs]
>>    btrfs_ioctl_snap_create_v2+0x11c/0x180 [btrfs]
>>    btrfs_ioctl+0x11a4/0x2da0 [btrfs]
>>    do_vfs_ioctl+0xa9/0x640
>>    ksys_ioctl+0x67/0x90
>>    __x64_sys_ioctl+0x1a/0x20
>>    do_syscall_64+0x5a/0x110
>>    entry_SYSCALL_64_after_hwframe+0x44/0xa9
>>   ---[ end trace 33f2f83f3d5250e9 ]---
>>   BTRFS: error (device sda1) in create_pending_snapshot:1576: errno=-24 unknown
>>   BTRFS info (device sda1): forced readonly
>>   BTRFS warning (device sda1): Skipping commit of aborted transaction.
>>   BTRFS: error (device sda1) in cleanup_transaction:1831: errno=-24 unknown
>>
>> [CAUSE]
>> When the global anonymous block device pool is exhausted, the following
>> call chain will fail, and lead to transaction abort:
>>
>>  btrfs_ioctl_snap_create_v2()
>>  |- btrfs_ioctl_snap_create_transid()
>>     |- btrfs_mksubvol()
>>        |- btrfs_commit_transaction()
>>           |- create_pending_snapshot()
>>              |- btrfs_get_fs_root()
>>                 |- btrfs_init_fs_root()
>>                    |- get_anon_bdev()
>>
>> [FIX]
>> Although we can't enlarge the anonymous block device pool, at least we
>> can preallocate anon_dev for subvolume/snapshot creation.
>> So that when the pool is exhausted, user will get an error other than
>> aborting transaction later.
>>
>> Reported-by: Greed Rong <greedrong@gmail.com>
>> Link: https://lore.kernel.org/linux-btrfs/CA+UqX+NTrZ6boGnWHhSeZmEY5J76CTqmYjO2S+=tHJX7nb9DPw@mail.gmail.com/
>> Signed-off-by: Qu Wenruo <wqu@suse.com>
>> ---
>>  fs/btrfs/disk-io.c     | 51 ++++++++++++++++++++++++++++++++++++------
>>  fs/btrfs/disk-io.h     |  2 ++
>>  fs/btrfs/ioctl.c       | 21 ++++++++++++++++-
>>  fs/btrfs/transaction.c |  3 ++-
>>  fs/btrfs/transaction.h |  2 ++
>>  5 files changed, 70 insertions(+), 9 deletions(-)
>>
>> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
>> index cfc0ff288238..14fd69b71cb8 100644
>> --- a/fs/btrfs/disk-io.c
>> +++ b/fs/btrfs/disk-io.c
>> @@ -1395,7 +1395,7 @@ struct btrfs_root *btrfs_read_tree_root(struct btrfs_root *tree_root,
>>  	goto out;
>>  }
>>  
>> -static int btrfs_init_fs_root(struct btrfs_root *root)
>> +static int btrfs_init_fs_root(struct btrfs_root *root, dev_t anon_dev)
>>  {
>>  	int ret;
>>  	unsigned int nofs_flag;
>> @@ -1435,9 +1435,13 @@ static int btrfs_init_fs_root(struct btrfs_root *root)
>>  	 */
>>  	if (is_fstree(root->root_key.objectid) &&
>>  	    btrfs_root_refs(&root->root_item)) {
>> -		ret = get_anon_bdev(&root->anon_dev);
>> -		if (ret)
>> -			goto fail;
>> +		if (!anon_dev) {
>> +			ret = get_anon_bdev(&root->anon_dev);
>> +			if (ret)
>> +				goto fail;
>> +		} else {
>> +			root->anon_dev = anon_dev;
>> +		}
>>  	}
>>  
>>  	mutex_lock(&root->objectid_mutex);
>> @@ -1542,8 +1546,27 @@ void btrfs_free_fs_info(struct btrfs_fs_info *fs_info)
>>  }
>>  
>>  
>> -struct btrfs_root *btrfs_get_fs_root(struct btrfs_fs_info *fs_info,
>> -				     u64 objectid, bool check_ref)
>> +/*
>> + * Get a fs root.
>> + *
>> + * For essential trees like root/extent tree, we grab it from fs_info directly.
>> + * For subvolume trees, we check the cached fs roots first. If miss then
>> + * read it from disk and add it to cached fs roots.
>> + *
>> + * Caller should release the root by calling btrfs_put_root() after the usage.
>> + *
>> + * NOTE: Reloc and log trees can't be read by this function as they share the
>> + *	 same root objectid.
>> + *
>> + * @objectid:	Root (subvolume) id
>> + * @anon_dev:	Preallocated anonymous block device number for new roots.
>> + * 		Pass 0 for automatic allocation.
>> + * @check_ref:	Whether to check root refs. If true, return -ENOENT for orphan
>> + * 		roots.
>> + */
>> +static struct btrfs_root *__get_fs_root(struct btrfs_fs_info *fs_info,
>> +					u64 objectid, dev_t anon_dev,
>> +					bool check_ref)
> 
> 
>> +struct btrfs_root *btrfs_get_fs_root(struct btrfs_fs_info *fs_info,
>> +				     u64 objectid, bool check_ref)
>> +{
>> +	return __get_fs_root(fs_info, objectid, 0, check_ref);
>> +}
>> +
>> +struct btrfs_root *btrfs_get_new_fs_root(struct btrfs_fs_info *fs_info,
>> +					 u64 objectid, dev_t anon_dev)
>> +{
>> +	return __get_fs_root(fs_info, objectid, anon_dev, true);
>> +}
> 
> This does not look like a good API, we should keep btrfs_get_fs_root and
> add the anon_bdev initialization to the callers, there are only a few.
> 
OK, I'd go that direction.

Thanks,
Qu