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pipe: Allow pipes to have kernel-reserved slots

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Split pipe->ring_size into two numbers:

 (1) pipe->ring_size - indicates the hard size of the pipe ring.

 (2) pipe->max_usage - indicates the maximum number of pipe ring slots that
     userspace orchestrated events can fill.

This allows for a pipe that is both writable by the general kernel
notification facility and by userspace, allowing plenty of ring space for
notifications to be added whilst preventing userspace from being able to
pin too much unswappable kernel space.

Signed-off-by: David Howells <dhowells@redhat.com>
This commit is contained in:
David Howells 2019-10-16 16:47:32 +01:00
parent 8cefc107ca
commit 6718b6f855
5 changed files with 30 additions and 24 deletions
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4
lib/iov_iter.c
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@ -395,7 +395,7 @@ static size_t copy_page_to_iter_pipe(struct page *page, size_t offset, size_t by
i_head++;
buf = &pipe->bufs[i_head & p_mask];
}
if (pipe_full(i_head, p_tail, pipe->ring_size))
if (pipe_full(i_head, p_tail, pipe->max_usage))
return 0;
buf->ops = &page_cache_pipe_buf_ops;
@ -528,7 +528,7 @@ static size_t push_pipe(struct iov_iter *i, size_t size,
pipe->bufs[iter_head & p_mask].len = PAGE_SIZE;
iter_head++;
}
while (!pipe_full(iter_head, p_tail, pipe->ring_size)) {
while (!pipe_full(iter_head, p_tail, pipe->max_usage)) {
struct pipe_buffer *buf = &pipe->bufs[iter_head & p_mask];
struct page *page = alloc_page(GFP_USER);
if (!page)