Re: [PATCH 1/4] fs/splice: enhance direct pipe & splice for moving pages in kernel

[Miklos Szeredi <miklos@szeredi.hu>]

On Wed, 15 Feb 2023 at 01:11, Ming Lei <ming.lei@redhat.com> wrote:
>
> On Tue, Feb 14, 2023 at 04:39:01PM +0100, Miklos Szeredi wrote:
> > On Tue, 14 Feb 2023 at 15:35, Ming Lei <ming.lei@redhat.com> wrote:
> >
> > > I understand it isn't one issue from block device driver viewpoint at
> > > least, since the WRITE to buffer in sink end can be thought as DMA
> > > to buffer from device, and it is the upper layer(FS)'s responsibility
> > > for updating page flag. And block driver needn't to handle page
> > > status update.
> >
> > The block device still needs to know when the DMA is finished, right?
>
> Yeah, for normal in-kernel device driver, the completion is triggered by
> interrupt or io polling.
>
> For ublk, io handling is done by userspace, here we use io_uring to
> handle the IO in aio style. When the aio is completed, the userspace
> gets notified of the completion.

I think it might be better if the write completion was directly
signaled to the original pipe buffer.  There are several advantages:

 - the kernel can guarantee (modulo bugs) that the buffer was
initialized, this is important if the userspace server is unprivileged

 - the server process does not need to be woken up on I/O completion

 - there could be a chain of splices involving various entities, yet
the original pipe buffer should always get the completion

I'm not sure what a good implementation would look like.  When a pipe
buffer is split, things get complicated.  Maybe just disallow
splitting on such buffers...

Thanks,
Miklos