On Fri, May 7, 2021 at 11:21 AM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
> So the question is likely, "do we want this for /dev/zero?"
Well, /dev/zero should at least be safe, and I guess it's actually
interesting from a performance testing standpoint (ie useful for some
kind of "what is the overhead of the splice code with no data copy").
So I'll happily take a sane patch for /dev/zero, although I think it
probably only makes sense if it's made to use the zero page explicitly
(ie exactly for that "no data copy testing" case).
So very much *not* using generic_file_splice_read(), even if that
might be the one-liner.
/dev/zero should probably also use the (already existing)
splice_write_null() function for the .splice_write case.
Anybody willing to look into this? My gu feel is that it *should* be easy to do.
That said - looking at the current 'pipe_zero()', it uses
'push_pipe()' to actually allocation regular pages, and then clear
them.
Which is basically what a generic_file_splice_read() would do, and it
feels incredibly pointless and stupid to me.
I *think* we should be able to just do something like
len = size;
while (len > 0) {
struct pipe_buffer *buf;
unsigned int tail = pipe->tail;
unsigned int head = pipe->head;
unsigned int mask = pipe->ring_size - 1;
if (pipe_full(head, tail, pipe->max_usage))
break;
buf = &pipe->bufs[iter_head & p_mask];
buf->ops = &zero_pipe_buf_ops;
buf->page = ZERO_PAGE(0);
buf->offset = 0;
buf->len = min_t(ssize_t, len, PAGE_SIZE);
len -= buf->len;
pipe->head = head+1;
}
return size - len;
but honestly, I haven't thought a lot about it.
Al? This is another of those "right up your alley" things.
Maybe it's not worth it, and just using generic_file_splice_read() is
the way to go, but I do get the feeling that if we are splicing
/dev/null, the whole _point_ of it is about benchmarking, not "make it
work".
Linus
