Re: [PATCH v2 2/2] btrfs: relocation: review the call sites which can be interruped by signal

[Qu Wenruo <wqu@suse.com>]

On 2020/7/9 下午5:54, David Sterba wrote:
> On Thu, Jul 09, 2020 at 04:33:33PM +0800, Qu Wenruo wrote:
>> Since most metadata reservation calls can return -EINTR when get
>> interruped by fatal signal, we need to review the all the metadata
>> reservation call sites.
>>
>> In relocation code, the metadata reservation happens in the following
>> sites:
>> - btrfs_block_rsv_refill() in merge_reloc_root()
>>   merge_reloc_root() is a pretty critial section, we don't want get
>>   interrupted by signal, so change the flush status to
>>   BTRFS_RESERVE_FLUSH_LIMIT, so it won't get interrupted by signal.
>>   Since such change can be ENPSPC-prone, also shrink the amount of
>>   metadata to reserve a little to avoid deadly ENOSPC there.
>>
>> - btrfs_block_rsv_refill() in reserve_metadata_space()
>>   It calls with BTRFS_RESERVE_FLUSH_LIMIT, which won't get interrupred
>>   by signal.
> 
> This semantics of BTRFS_RESERVE_FLUSH_LIMIT regarding signals should be
> documented, right now there's a comment but says something about avoidig
> deadlocks.
> 
>> - btrfs_block_rsv_refill() in prepare_to_relocate()
>> - btrfs_block_rsv_add() in prepare_to_relocate()
>> - btrfs_block_rsv_refill() in relocate_block_group()
>> - btrfs_delalloc_reserve_metadata() in relocate_file_extent_cluster()
>> - btrfs_start_transaction() in relocate_block_group()
>> - btrfs_start_transaction() in create_reloc_inode()
>>   Can be interruped by fatal signal and we can handle it easily.
>>   For these call sites, just catch the -EINTR value in btrfs_balance()
>>   and count them as canceled.
>>
>> Signed-off-by: Qu Wenruo <wqu@suse.com>
>> ---
>>  fs/btrfs/relocation.c | 13 +++++++++++--
>>  fs/btrfs/volumes.c    |  2 +-
>>  2 files changed, 12 insertions(+), 3 deletions(-)
>>
>> diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
>> index 2b869fb2e62c..23914edd4710 100644
>> --- a/fs/btrfs/relocation.c
>> +++ b/fs/btrfs/relocation.c
>> @@ -1686,12 +1686,21 @@ static noinline_for_stack int merge_reloc_root(struct reloc_control *rc,
>>  		btrfs_unlock_up_safe(path, 0);
>>  	}
>>  
>> -	min_reserved = fs_info->nodesize * (BTRFS_MAX_LEVEL - 1) * 2;
>> +	/*
>> +	 * In merge_reloc_root(), we modify the upper level pointer to swap
>> +	 * the tree blocks between reloc tree and subvolume tree.
>> +	 * Thus for tree block COW, we COW at most from level 1 to root level
>> +	 * for each tree.
>> +	 *
>> +	 * Thus the needed metadata space is at most root_level * nodesize,
>> +	 * and * 2 since we have two trees to COW.
>> +	 */
>> +	min_reserved = fs_info->nodesize * btrfs_root_level(root_item) * 2;
>>  	memset(&next_key, 0, sizeof(next_key));
>>  
>>  	while (1) {
>>  		ret = btrfs_block_rsv_refill(root, rc->block_rsv, min_reserved,
>> -					     BTRFS_RESERVE_FLUSH_ALL);
>> +					     BTRFS_RESERVE_FLUSH_LIMIT);
>>  		if (ret) {
>>  			err = ret;
>>  			goto out;
>> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
>> index aabc6c922e04..d60df30bdc47 100644
>> --- a/fs/btrfs/volumes.c
>> +++ b/fs/btrfs/volumes.c
>> @@ -4135,7 +4135,7 @@ int btrfs_balance(struct btrfs_fs_info *fs_info,
>>  	mutex_lock(&fs_info->balance_mutex);
>>  	if (ret == -ECANCELED && atomic_read(&fs_info->balance_pause_req))
>>  		btrfs_info(fs_info, "balance: paused");
>> -	else if (ret == -ECANCELED && atomic_read(&fs_info->balance_cancel_req))
>> +	else if (ret == -ECANCELED  || ret == -EINTR)
> 
> Why do you remove atomic_read(&fs_info->balance_cancel_req) ?

Because now btrfs_should_cancel_balance() can return ECANCELED without
balance_cancel_req increased due to pending fatal signal.

> 
>>  		btrfs_info(fs_info, "balance: canceled");
> 
> I'm not sure if it would be useful to print the reason, like
> 
> - 'canceled: user request'
> - 'canceled: interrupted'

To me, if user interrupt the balance progress, it's obvious they want to
cancel it.
Thus no need to distinguish btrfs balance cancel and signal cancel.

Thanks,
Qu

> 
>>  	else
>>  		btrfs_info(fs_info, "balance: ended with status: %d", ret);
>> -- 
>> 2.27.0