Re: zero-copy read() interface

[Andrew Morton <akpm@osdl.org>]

Folkert van Heusden <folkert@vanheusden.com> wrote:
>
> > > What about a zero-copy read-interface?
> > > An ioctl (or something) which enables the kernel to do dma directly to
> > > the userspace. Of course this should be limited to the root-user or a
> > > user with special capabilities (rights) since if a drive screws up, data
> > > from a different sector (or so) might end up in the proces' memory. Of
> > > course copying a sector from kernel- to userspace can be done pretty
> > > fast but i.m.h.o. all possible speedimprovements should be made unless
> > > unclean.
> > Just use mmap().  Unlike your proposal, it cooperates with the page
> > cache.
> 
> Doesn't that one also use copying? I've also heard that using mmap is
> expensive due to pagefaulting. I've found, for example, that copying a
> 1.3GB file using read/write instead of mmap & memcpy is seconds faster.
> 

You can use remap_file_pages() to read all the pages into pagecache and
then instantiate all their pte's in a single syscall.