Re: [PATCH] splice: add support for SPLICE_F_MOVE flag

[Andrew Morton]

Andrew Morton <akpm@xxxxxxxx> wrote:
>
>   static void page_cache_pipe_buf_unmap(struct pipe_inode_info *info,
>  >  				      struct pipe_buffer *buf)
>  >  {
>  > -	unlock_page(buf->page);
>  > +	if (!buf->stolen)
>  > +		unlock_page(buf->page);
>  >  	kunmap(buf->page);
>  >  }
> 
>  There go our chances of ever getting rid of kmap().  Is it not feasible to
>  use atomic kmaps throughout this code?

What are the kmaps for, anyway?  afaict they're doing the
kmap-the-page-while-we-run-some-a_ops thing which ceased being a
requirement 3-4 years ago.

The general approach we should take is that the code which actually
modifies a page's contents is the code which is responsible for kmapping
that page.  Use an atomic kmap, memcpy-or-memset, atomic kunmap.  Just four
or five lines.

If we can do that, pipe_buf_operations.map/unmap can be removed.
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