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SubjectRe: [PATCH 5/7] FUSE: implement ioctl support
On Thu, 28 Aug 2008, Tejun Heo wrote:
> Miklos Szeredi wrote:
> >> If you're worried about the double copying due to performance reasons,
> >> what we can do is implementing userspace to userspace copying. I just
> >> took easier path of copying twice as u-u copy can't be done using the
> >> existing FUSE mechanics.
> >
> > How about not copying anything in the kernel, just passing the virtual
> > address to the filesystem? It can get/modify the data of the calling
> > task by mapping /proc/pid/mem. Ugly, yes, but ioctls are inherently
> > ugly.
> Hmmm... I was trying to stay within similar operation mechanics as other
> ops. Directly accessing the caller's memory has performance benefits
> but that benefit can also be used by reads and writes. So, if we're
> gonna do direct memory access, maybe doing it in more generic way is a
> better idea?

On the contrary: playing VM games is going to be slow. I think this
approach is best suited for generic ioctl support because it
simplifies the kernel part. I'd hate to add all that complexity to
the kernel if not absolutely necessary.


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