Re: [PATCH] btrfs: Use btrfs_add_unused_bgs() to replace open code

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

On 2018年05月22日 15:37, Nikolay Borisov wrote:
> 
> 
> On 22.05.2018 10:29, Qu Wenruo wrote:
>> Introduce a small helper, btrfs_add_unused_bgs(), to accquire needed
> 
> This function name sounds a bit awkard, mainly because you use the
> plural form. How about btrfs_mark_bg_unused() ? The name seems more
> unambiguous.

Sounds much better.

> 
>> locks and add a block group to unused_bgs list.
>>
>> No functional modification, and only 3 callers are involved.
>>
>> Signed-off-by: Qu Wenruo <wqu@suse.com>
>> ---
>> This patch should provide the basis for later block group auto-removal
>> to get more info (mostly transid) to determine should one block group
>> being removed in current trans.
>> ---
>>  fs/btrfs/ctree.h       |  1 +
>>  fs/btrfs/extent-tree.c | 35 ++++++++++++++++-------------------
>>  fs/btrfs/scrub.c       |  9 +--------
>>  3 files changed, 18 insertions(+), 27 deletions(-)
>>
>> diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
>> index bbb358143ded..701a52034ec6 100644
>> --- a/fs/btrfs/ctree.h
>> +++ b/fs/btrfs/ctree.h
>> @@ -2827,6 +2827,7 @@ void check_system_chunk(struct btrfs_trans_handle *trans,
>>  			struct btrfs_fs_info *fs_info, const u64 type);
>>  u64 add_new_free_space(struct btrfs_block_group_cache *block_group,
>>  		       u64 start, u64 end);
>> +void btrfs_add_unused_bgs(struct btrfs_block_group_cache *bg);
>>  
>>  /* ctree.c */
>>  int btrfs_bin_search(struct extent_buffer *eb, const struct btrfs_key *key,
>> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
>> index ccf2690f7ca1..484c9d11e5b6 100644
>> --- a/fs/btrfs/extent-tree.c
>> +++ b/fs/btrfs/extent-tree.c
>> @@ -6312,16 +6312,8 @@ static int update_block_group(struct btrfs_trans_handle *trans,
>>  		 * dirty list to avoid races between cleaner kthread and space
>>  		 * cache writeout.
>>  		 */
>> -		if (!alloc && old_val == 0) {
>> -			spin_lock(&info->unused_bgs_lock);
>> -			if (list_empty(&cache->bg_list)) {
>> -				btrfs_get_block_group(cache);
>> -				trace_btrfs_add_unused_block_group(cache);
>> -				list_add_tail(&cache->bg_list,
>> -					      &info->unused_bgs);
>> -			}
>> -			spin_unlock(&info->unused_bgs_lock);
>> -		}
>> +		if (!alloc && old_val == 0)
>> +			btrfs_add_unused_bgs(cache);
>>  
>>  		btrfs_put_block_group(cache);
>>  		total -= num_bytes;
>> @@ -10144,15 +10136,7 @@ int btrfs_read_block_groups(struct btrfs_fs_info *info)
>>  		if (btrfs_chunk_readonly(info, cache->key.objectid)) {
>>  			inc_block_group_ro(cache, 1);
>>  		} else if (btrfs_block_group_used(&cache->item) == 0) {
>> -			spin_lock(&info->unused_bgs_lock);
>> -			/* Should always be true but just in case. */
>> -			if (list_empty(&cache->bg_list)) {
>> -				btrfs_get_block_group(cache);
>> -				trace_btrfs_add_unused_block_group(cache);
>> -				list_add_tail(&cache->bg_list,
>> -					      &info->unused_bgs);
>> -			}
>> -			spin_unlock(&info->unused_bgs_lock);
>> +			btrfs_add_unused_bgs(cache);
>>  		}
>>  	}
>>  
>> @@ -11071,3 +11055,16 @@ void btrfs_wait_for_snapshot_creation(struct btrfs_root *root)
>>  			       !atomic_read(&root->will_be_snapshotted));
>>  	}
>>  }
>> +
>> +void btrfs_add_unused_bgs(struct btrfs_block_group_cache *bg)
>> +{
>> +	struct btrfs_fs_info *fs_info = bg->fs_info;
>> +
>> +	spin_lock(&fs_info->unused_bgs_lock);
>> +	if (list_empty(&bg->bg_list)) {
> 
> Given the comment in btrfs_read_block_groups:
> 
> /* Should always be true but just in case. */
> 
> How about you make it ASSERT(list_empty(&bg->bg_list));
> 
> /* code to add the bg */
> 
> So right now either :
> 
> a) The comment is bogus and it is indeed required to check if this bg
> has already been marked unused.
> 
> or
> 
> b) The comment is correct and it's in fact a bug to try and mark a bg as
> unused twice.

Not exactly.

1) bg_list is kind of abused.
   Not only fs_info->unused_bgs, but also transaction->deleted_bgs, and
   even transaction->new_bgs could use bg_cache->bg_list.
   So it's not only used to detect unused bgs.
   And it's possible some bg get moved to deleted_bgs list.

2) That is comment only works for caller in btrfs_read_block_groups().
   As at that timing, there is no race at all since we're still mounting
   the fs.
   But may not work for other callers.

Thus I just kept the code while removed the comment, since in the
extracted function, it may no longer be the case.
(And my focus is later auto-removal generation check, so I just left
code as is)

Thanks,
Qu

> 
>> +		btrfs_get_block_group(bg);
>> +		trace_btrfs_add_unused_block_group(bg);
>> +		list_add_tail(&bg->bg_list, &fs_info->unused_bgs);
>> +	}
>> +	spin_unlock(&fs_info->unused_bgs_lock);
>> +}
>> diff --git a/fs/btrfs/scrub.c b/fs/btrfs/scrub.c
>> index a59005862010..1044ab2fc71c 100644
>> --- a/fs/btrfs/scrub.c
>> +++ b/fs/btrfs/scrub.c
>> @@ -3981,14 +3981,7 @@ int scrub_enumerate_chunks(struct scrub_ctx *sctx,
>>  		if (!cache->removed && !cache->ro && cache->reserved == 0 &&
>>  		    btrfs_block_group_used(&cache->item) == 0) {
>>  			spin_unlock(&cache->lock);
>> -			spin_lock(&fs_info->unused_bgs_lock);
>> -			if (list_empty(&cache->bg_list)) {
>> -				btrfs_get_block_group(cache);
>> -				trace_btrfs_add_unused_block_group(cache);
>> -				list_add_tail(&cache->bg_list,
>> -					      &fs_info->unused_bgs);
>> -			}
>> -			spin_unlock(&fs_info->unused_bgs_lock);
>> +			btrfs_add_unused_bgs(cache);
>>  		} else {
>>  			spin_unlock(&cache->lock);
>>  		}
>>
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