[Ocfs2-devel] [PATCH] ocfs2: fall back to buffer IO when append dio is disabled with file hole existing

[Changwei Ge]

On 2017/12/19 9:24, Joseph Qi wrote:
> 
> On 17/12/19 05:53, Andrew Morton wrote:
>> On Mon, 18 Dec 2017 12:06:21 +0000 Changwei Ge <ge.changwei at h3c.com> wrote:
>>
>>> Before ocfs2 supporting allocating clusters while doing append-dio, all append
>>> dio will fall back to buffer io to allocate clusters firstly. Also, when it
>>> steps on a file hole, it will fall back to buffer io, too. But for current
>>> code, writing to file hole will leverage dio to allocate clusters. This is not
>>> right, since whether append-io is enabled tells the capability whether ocfs2 can
>>> allocate space while doing dio.
>>> So introduce file hole check function back into ocfs2.
>>> Once ocfs2 is doing dio upon a file hole with append-dio disabled, it will fall
>>> back to buffer IO to allocate clusters.
>>>
>>
>> hm, that's a bit hard to understand.  Hopefully reviewers will know
>> what's going on ;)>
> Also suggest rearrange the description:)

Hi Joseph,
OK, I will elaborate the intention of this patch further in the next version.

> 
>>> --- a/fs/ocfs2/aops.c
>>> +++ b/fs/ocfs2/aops.c
>>> @@ -2414,6 +2414,44 @@ static int ocfs2_dio_end_io(struct kiocb *iocb,
>>>    	return ret;
>>>    }
>>>    
>>> +/*
>>> + * Will look for holes and unwritten extents in the range starting at
>>> + * pos for count bytes (inclusive).
>>> + */
>>> +static int ocfs2_check_range_for_holes(struct inode *inode, loff_t pos,
>>> +				       size_t count)
>>> +{
>>> +	int ret = 0;
>>> +	unsigned int extent_flags;
>>> +	u32 cpos, clusters, extent_len, phys_cpos;
>>> +	struct super_block *sb = inode->i_sb;
>>> +
>>> +	cpos = pos >> OCFS2_SB(sb)->s_clustersize_bits;
>>> +	clusters = ocfs2_clusters_for_bytes(sb, pos + count) - cpos;
>>> +
>>> +	while (clusters) {
>>> +		ret = ocfs2_get_clusters(inode, cpos, &phys_cpos, &extent_len,
>>> +					 &extent_flags);
>>> +		if (ret < 0) {
>>> +			mlog_errno(ret);
>>> +			goto out;
>>> +		}
>>> +
>>> +		if (phys_cpos == 0 || (extent_flags & OCFS2_EXT_UNWRITTEN)) {
>>> +			ret = 1;
>>> +			break;
>>> +		}
>>> +
>>> +		if (extent_len > clusters)
>>> +			extent_len = clusters;
>>> +
>>> +		clusters -= extent_len;
>>> +		cpos += extent_len;
>>> +	}
>>> +out:
>>> +	return ret;
>>> +}
>>
>> A few thoughts:
>>
>> - a function which does "check_foo" isn't well named.  Because the
>>    reader cannot determine whether the return value means "foo is true"
>>    or "foo is false".
>>
>>    So a better name for this function is ocfs2_range_has_holes(), so
>>    the reader immediately understand what its return value *means".
>>
>>    Also a bool return value is more logical.
>>
>> - Mixing "goto out" with "break" as above is a bit odd.
>>
>> So...
>>
>>
>> --- a/fs/ocfs2/aops.c~ocfs2-fall-back-to-buffer-io-when-append-dio-is-disabled-with-file-hole-existing-fix
>> +++ a/fs/ocfs2/aops.c
>> @@ -2469,10 +2469,9 @@ static int ocfs2_dio_end_io(struct kiocb
>>    * Will look for holes and unwritten extents in the range starting at
>>    * pos for count bytes (inclusive).
>>    */
>> -static int ocfs2_check_range_for_holes(struct inode *inode, loff_t pos,
>> -				       size_t count)
>> +static bool ocfs2_range_has_holes(struct inode *inode, loff_t pos, size_t count)
>>   {
>> -	int ret = 0;
>> +	bool ret = false;
> 
> I have a different opinion here. If we change return value from int to
> bool, the error returned by ocfs2_get_clusters cannot be reflected. So
> I'd prefer the original version.

Yes, it's my mistake here.
You have to forgive me.:)

Thanks,
Changwei

> 
> Thanks,
> Joseph
> 
>>   	unsigned int extent_flags;
>>   	u32 cpos, clusters, extent_len, phys_cpos;
>>   	struct super_block *sb = inode->i_sb;
>> @@ -2489,8 +2488,8 @@ static int ocfs2_check_range_for_holes(s
>>   		}
>>   
>>   		if (phys_cpos == 0 || (extent_flags & OCFS2_EXT_UNWRITTEN)) {
>> -			ret = 1;
>> -			break;
>> +			ret = true;
>> +			goto out;
>>   		}
>>   
>>   		if (extent_len > clusters)
>> _
>>
>