Date:   Thu, 1 Nov 2018 12:48:47 +0000
From:   Changwei Ge <ge.changwei@....com>
To:     Larry Chen <lchen@...e.com>, Joseph Qi <jiangqi903@...il.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "ocfs2-devel@....oracle.com" <ocfs2-devel@....oracle.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [Ocfs2-devel] [PATCH V3] ocfs2: fix dead lock caused by
 ocfs2_defrag_extent

Hi Larry,

On 2018/11/1 20:39, Larry Chen wrote:
> Hi Joseph,
> 
> On 11/1/18 7:52 PM, Changwei Ge wrote:
>> Hello Joseph,
>>
>> On 2018/11/1 17:01, Joseph Qi wrote:
>>> Hi Larry,
>>>
>>> On 18/11/1 15:14, Larry Chen wrote:
>>>> ocfs2_defrag_extent may fall into deadlock.
>>>>
>>>> ocfs2_ioctl_move_extents
>>>>        ocfs2_ioctl_move_extents
>>>>          ocfs2_move_extents
>>>>            ocfs2_defrag_extent
>>>>              ocfs2_lock_allocators_move_extents
>>>>
>>>>                ocfs2_reserve_clusters
>>>>                  inode_lock GLOBAL_BITMAP_SYSTEM_INODE
>>>>
>>>> 	  __ocfs2_flush_truncate_log
>>>>                  inode_lock GLOBAL_BITMAP_SYSTEM_INODE
>>>>
>>>> As backtrace shows above, ocfs2_reserve_clusters() will call inode_lock
>>>> against the global bitmap if local allocator has not sufficient cluters.
>>>> Once global bitmap could meet the demand, ocfs2_reserve_cluster will
>>>> return success with global bitmap locked.
>>>>
>>>> After ocfs2_reserve_cluster(), if truncate log is full,
>>>> __ocfs2_flush_truncate_log() will definitely fall into deadlock because it
>>>> needs to inode_lock global bitmap, which has already been locked.
>>>>
>>>> To fix this bug, we could remove from ocfs2_lock_allocators_move_extents()
>>>> the code which intends to lock global allocator, and put the removed code
>>>> after __ocfs2_flush_truncate_log().
>>>>
>>>> ocfs2_lock_allocators_move_extents() is referred by 2 places, one is here,
>>>> the other does not need the data allocator context, which means this patch
>>>> does not affect the caller so far.
>>>>
>>>> Change log:
>>>> 1. Correct the function comment.
>>>> 2. Remove unused argument from ocfs2_lock_meta_allocator_move_extents.
>>>>
>>>> Signed-off-by: Larry Chen <lchen@...e.com>
>>>> ---
>>>>     fs/ocfs2/move_extents.c | 47 ++++++++++++++++++++++++++---------------------
>>>>     1 file changed, 26 insertions(+), 21 deletions(-)
>>>>
>>
>>> IMO, here clusters_to_move is only for data_ac, since we change this
>>> function to only handle meta_ac, I'm afraid clusters_to_move related
>>> logic has to been changed correspondingly.
>>
>> I think we can't remove *clusters_to_move* from here as clusters can be reserved latter outsides this function, but we
>> still have to reserve metadata(extents) in advance.
>> So we need that argument.
>>
> 
> Yeah, I think clusters_to_move should be reserved, in order to keep the
> original logic as it was.
> 
> But I'm curious about why max_recs_needed should be equal to
> 2 * extents_to_split + cluster_to_move?
> Does that mean that each cluster might form an extent??

It's because in the worst case that the file is of great fragment and one extent record can only represent one cluster.
And we might encounter extents split during merging and rotation so we also need _2 * extents_to_split_.

Thanks,
Changwei

> 
> Thanks,
> Larry
> 
> 
>> Thanks,
>> Changwei
>>
>>>
>>> Thanks,
>>> Joseph
>>>>     					u32 extents_to_split,
>>>>     					struct ocfs2_alloc_context **meta_ac,
>>>> -					struct ocfs2_alloc_context **data_ac,
>>>>     					int extra_blocks,
>>>>     					int *credits)
>>>>     {
>>>> @@ -192,13 +188,6 @@ static int ocfs2_lock_allocators_move_extents(struct inode *inode,
>>>>     		goto out;
>>>>     	}
>>>>     
>>>> -	if (data_ac) {
>>>> -		ret = ocfs2_reserve_clusters(osb, clusters_to_move, data_ac);
>>>> -		if (ret) {
>>>> -			mlog_errno(ret);
>>>> -			goto out;
>>>> -		}
>>>> -	}
>>>>     
>>>>     	*credits += ocfs2_calc_extend_credits(osb->sb, et->et_root_el);
>>>>     
>>>> @@ -257,10 +246,10 @@ static int ocfs2_defrag_extent(struct ocfs2_move_extents_context *context,
>>>>     		}
>>>>     	}
>>>>     
>>>> -	ret = ocfs2_lock_allocators_move_extents(inode, &context->et, *len, 1,
>>>> -						 &context->meta_ac,
>>>> -						 &context->data_ac,
>>>> -						 extra_blocks, &credits);
>>>> +	ret = ocfs2_lock_meta_allocator_move_extents(inode, &context->et,
>>>> +						*len, 1,
>>>> +						&context->meta_ac,
>>>> +						extra_blocks, &credits);
>>>>     	if (ret) {
>>>>     		mlog_errno(ret);
>>>>     		goto out;
>>>> @@ -283,6 +272,21 @@ static int ocfs2_defrag_extent(struct ocfs2_move_extents_context *context,
>>>>     		}
>>>>     	}
>>>>     
>>>> +	/*
>>>> +	 * Make sure ocfs2_reserve_cluster is called after
>>>> +	 * __ocfs2_flush_truncate_log, otherwise, dead lock may happen.
>>>> +	 *
>>>> +	 * If ocfs2_reserve_cluster is called
>>>> +	 * before __ocfs2_flush_truncate_log, dead lock on global bitmap
>>>> +	 * may happen.
>>>> +	 *
>>>> +	 */
>>>> +	ret = ocfs2_reserve_clusters(osb, *len, &context->data_ac);
>>>> +	if (ret) {
>>>> +		mlog_errno(ret);
>>>> +		goto out_unlock_mutex;
>>>> +	}
>>>> +
>>>>     	handle = ocfs2_start_trans(osb, credits);
>>>>     	if (IS_ERR(handle)) {
>>>>     		ret = PTR_ERR(handle);
>>>> @@ -600,9 +604,10 @@ static int ocfs2_move_extent(struct ocfs2_move_extents_context *context,
>>>>     		}
>>>>     	}
>>>>     
>>>> -	ret = ocfs2_lock_allocators_move_extents(inode, &context->et, len, 1,
>>>> -						 &context->meta_ac,
>>>> -						 NULL, extra_blocks, &credits);
>>>> +	ret = ocfs2_lock_meta_allocator_move_extents(inode, &context->et,
>>>> +						len, 1,
>>>> +						&context->meta_ac,
>>>> +						extra_blocks, &credits);
>>>>     	if (ret) {
>>>>     		mlog_errno(ret);
>>>>     		goto out;
>>>>
>>>
>>> _______________________________________________
>>> Ocfs2-devel mailing list
>>> Ocfs2-devel@....oracle.com
>>> https://oss.oracle.com/mailman/listinfo/ocfs2-devel
>>>
>>
> 
> 

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