[PATCH 2/3] pipe: enable handling of IOCB_NOWAIT

[Jens Axboe <axboe@kernel.dk>]

In preparation for enabling FMODE_NOWAIT for pipes, ensure that the read
and write path handle it correctly. This includes the pipe locking,
page allocation for writes, and confirming pipe buffers.

Acked-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 fs/pipe.c | 39 +++++++++++++++++++++++++++++++--------
 1 file changed, 31 insertions(+), 8 deletions(-)

diff --git a/fs/pipe.c b/fs/pipe.c
index 340f253913a2..dc00b20e56c8 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -108,6 +108,16 @@ static inline void __pipe_unlock(struct pipe_inode_info *pipe)
 	mutex_unlock(&pipe->mutex);
 }
 
+static inline bool __pipe_trylock(struct pipe_inode_info *pipe, bool nonblock)
+{
+	if (!nonblock) {
+		__pipe_lock(pipe);
+		return true;
+	}
+
+	return mutex_trylock(&pipe->mutex);
+}
+
 void pipe_double_lock(struct pipe_inode_info *pipe1,
 		      struct pipe_inode_info *pipe2)
 {
@@ -234,6 +244,7 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
 	struct file *filp = iocb->ki_filp;
 	struct pipe_inode_info *pipe = filp->private_data;
 	bool was_full, wake_next_reader = false;
+	const bool nonblock = iocb->ki_flags & IOCB_NOWAIT;
 	ssize_t ret;
 
 	/* Null read succeeds. */
@@ -241,7 +252,8 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
 		return 0;
 
 	ret = 0;
-	__pipe_lock(pipe);
+	if (!__pipe_trylock(pipe, nonblock))
+		return -EAGAIN;
 
 	/*
 	 * We only wake up writers if the pipe was full when we started
@@ -297,7 +309,7 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
 				chars = total_len;
 			}
 
-			error = pipe_buf_confirm(pipe, buf, false);
+			error = pipe_buf_confirm(pipe, buf, nonblock);
 			if (error) {
 				if (!ret)
 					ret = error;
@@ -342,7 +354,7 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
 			break;
 		if (ret)
 			break;
-		if (filp->f_flags & O_NONBLOCK) {
+		if (filp->f_flags & O_NONBLOCK || nonblock) {
 			ret = -EAGAIN;
 			break;
 		}
@@ -423,12 +435,14 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
 	ssize_t chars;
 	bool was_empty = false;
 	bool wake_next_writer = false;
+	const bool nonblock = iocb->ki_flags & IOCB_NOWAIT;
 
 	/* Null write succeeds. */
 	if (unlikely(total_len == 0))
 		return 0;
 
-	__pipe_lock(pipe);
+	if (!__pipe_trylock(pipe, nonblock))
+		return -EAGAIN;
 
 	if (!pipe->readers) {
 		send_sig(SIGPIPE, current, 0);
@@ -461,7 +475,7 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
 
 		if ((buf->flags & PIPE_BUF_FLAG_CAN_MERGE) &&
 		    offset + chars <= PAGE_SIZE) {
-			ret = pipe_buf_confirm(pipe, buf, false);
+			ret = pipe_buf_confirm(pipe, buf, nonblock);
 			if (ret)
 				goto out;
 
@@ -493,9 +507,18 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
 			int copied;
 
 			if (!page) {
-				page = alloc_page(GFP_HIGHUSER | __GFP_ACCOUNT);
+				gfp_t gfp = __GFP_HIGHMEM | __GFP_ACCOUNT |
+					    __GFP_HARDWALL;
+				int this_ret = -EAGAIN;
+
+				if (!nonblock) {
+					this_ret = -ENOMEM;
+					gfp |= GFP_USER;
+				}
+				page = alloc_page(gfp);
 				if (unlikely(!page)) {
-					ret = ret ? : -ENOMEM;
+					if (!ret)
+						ret = this_ret;
 					break;
 				}
 				pipe->tmp_page = page;
@@ -547,7 +570,7 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
 			continue;
 
 		/* Wait for buffer space to become available. */
-		if (filp->f_flags & O_NONBLOCK) {
+		if (filp->f_flags & O_NONBLOCK || nonblock) {
 			if (!ret)
 				ret = -EAGAIN;
 			break;
-- 
2.39.2