On Fri, May 07, 2021 at 12:06:31PM -0700, Linus Torvalds wrote:
> 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.
Umm... That would do wonders to anything that used to do
copy_to_user()/clear_user()/copy_to_user() and got converted
to copy_to_iter()/iov_iter_zero()/copy_to_iter()...
Are you sure we can shove zero page into pipe, anyway?
IIRC, get_page()/put_page() on that is not allowed, and
I'm not at all sure that nothing in e.g. fuse splice-related
logics would go ahead an do just that. Or am I confused
about the page refcounting for those?
