Re: [PATCH v5 8/9] vfs: Add vfs_copy_file_range() support for pagecache copies

["Pádraig Brady" <P@draigBrady.com>]

On 08/10/15 02:40, Neil Brown wrote:
> Anna Schumaker <Anna.Schumaker@netapp.com> writes:
> 
>> @@ -1338,34 +1362,26 @@ ssize_t vfs_copy_file_range(struct file *file_in, loff_t pos_in,
>>  			    struct file *file_out, loff_t pos_out,
>>  			    size_t len, unsigned int flags)
>>  {
>> -	struct inode *inode_in;
>> -	struct inode *inode_out;
>>  	ssize_t ret;
>>  
>> -	if (flags)
>> +	/* Flags should only be used exclusively. */
>> +	if ((flags & COPY_FR_COPY) && (flags & ~COPY_FR_COPY))
>> +		return -EINVAL;
>> +	if ((flags & COPY_FR_REFLINK) && (flags & ~COPY_FR_REFLINK))
>> +		return -EINVAL;
>> +	if ((flags & COPY_FR_DEDUP) && (flags & ~COPY_FR_DEDUP))
>>  		return -EINVAL;
>>  
> 
> Do you also need:
> 
>    if (flags & ~(COPY_FR_COPY | COPY_FR_REFLINK | COPY_FR_DEDUP))
>    	return -EINVAL;
> 
> so that future user-space can test if the kernel supports new flags?

Seems like a good idea, yes.

Also that got me thinking about COPY_FR_SPARSE.
What's the current behavior when copying a sparse range?
Is the hole propagated by default (good), or is it expanded?

Note cp(1) has --sparse={never,auto,always}. Auto is the default,
so it would be good I think if that was the default mode for copy_file_range().
With other sparse modes, we'd have to avoid copy_file_range() unless
there was control possible with COPY_FR_SPARSE_{AUTO,NONE,ALWAYS}.
Note currently cp --sparse=always will detect runs of zeros and also
avoid speculative preallocation by using fallocate (fd, FALLOC_FL_PUNCH_HOLE, ...)

thanks,
Pádraig.