pipe: Allow pipes to have kernel-reserved slots

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>

1 files changed, 5 insertions, 1 deletions

diff --git a/include/linux/pipe_fs_i.h b/include/linux/pipe_fs_i.h
index 96158ca80456..44f2245debda 100644
--- a/include/linux/pipe_fs_i.h
+++ b/include/linux/pipe_fs_i.h
@@ -32,6 +32,7 @@ struct pipe_buffer {
  *	@wait: reader/writer wait point in case of empty/full pipe
  *	@head: The point of buffer production
  *	@tail: The point of buffer consumption
+ *	@max_usage: The maximum number of slots that may be used in the ring
  *	@ring_size: total number of buffers (should be a power of 2)
  *	@tmp_page: cached released page
  *	@readers: number of current readers of this pipe
@@ -50,6 +51,7 @@ struct pipe_inode_info {
 	wait_queue_head_t wait;
 	unsigned int head;
 	unsigned int tail;
+	unsigned int max_usage;
 	unsigned int ring_size;
 	unsigned int readers;
 	unsigned int writers;
@@ -150,9 +152,11 @@ static inline unsigned int pipe_space_for_user(unsigned int head, unsigned int t
 	unsigned int p_occupancy, p_space;
 
 	p_occupancy = pipe_occupancy(head, tail);
-	if (p_occupancy >= pipe->ring_size)
+	if (p_occupancy >= pipe->max_usage)
 		return 0;
 	p_space = pipe->ring_size - p_occupancy;
+	if (p_space > pipe->max_usage)
+		p_space = pipe->max_usage;
 	return p_space;
 }