Re: [PATCH 0/5] splice: locking changes and code refactoring

[Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>]

On Fri, Jan 17, 2014 at 11:46 PM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> On Fri, Jan 17, 2014 at 11:22:04PM -0800, Linus Torvalds wrote:
>> On Fri, Jan 17, 2014 at 10:40 PM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
>> >
>> > Objections, comments?
>>
>> I certainly object to the "map, then unmap" approach. No VM games.
>
> Um...
>
> If we are going to copy that data (and all users of 
> generic_file_splice_write()
> do that memcpy() to page cache), we have to kmap the source ;-/

Yeah, the kmap/kunmap we have to do. But that's a no-op on 64-bit, and
has to be done one page at a time (well, I guess you could do a
couple).

But you can't do that *around* the default_file_splice_write(), so I
thought you meant some kind of "map into user space". And I absolutely
*detest* that kind of approach.

                  Linus