Make pipe buffer handling more generic.

Use a "pipe_buf_operations" structure to describe the ops that
can be done on a pipe buffer, so that pipe buffers from different
sources can have their own rules.

Right now the rules are just about how you map the buffers into
kernel virtual memory space, and how to release them (and whether
you can append new data to the end of an existing buffer).

fs/pipe.c

@@ -86,7 +86,7 @@ pipe_iov_copy_to_user(struct iovec *iov, const void *from, unsigned long len)
 	return 0;
 }
 
-static void release_pipe_buf(struct pipe_inode_info *info, struct pipe_buffer *buf)
+static void anon_pipe_buf_release(struct pipe_inode_info *info, struct pipe_buffer *buf)
 {
 	struct page *page = buf->page;
 
@@ -97,6 +97,23 @@ static void release_pipe_buf(struct pipe_inode_info *info, struct pipe_buffer *b
 	info->tmp_page = page;
 }
 
+static void *anon_pipe_buf_map(struct file *file, struct pipe_inode_info *info, struct pipe_buffer *buf)
+{
+	return kmap(buf->page);
+}
+
+static void anon_pipe_buf_unmap(struct pipe_inode_info *info, struct pipe_buffer *buf)
+{
+	kunmap(buf->page);
+}
+
+static struct pipe_buf_operations anon_pipe_buf_ops = {
+	.can_merge = 1,
+	.map = anon_pipe_buf_map,
+	.unmap = anon_pipe_buf_unmap,
+	.release = anon_pipe_buf_release,
+};
+
 static ssize_t
 pipe_readv(struct file *filp, const struct iovec *_iov,
 	   unsigned long nr_segs, loff_t *ppos)
@@ -122,14 +139,17 @@ pipe_readv(struct file *filp, const struct iovec *_iov,
 		if (bufs) {
 			int curbuf = info->curbuf;
 			struct pipe_buffer *buf = info->bufs + curbuf;
+			struct pipe_buf_operations *ops = buf->ops;
+			void *addr;
 			size_t chars = buf->len;
 			int error;
 
 			if (chars > total_len)
 				chars = total_len;
 
-			error = pipe_iov_copy_to_user(iov, kmap(buf->page) + buf->offset, chars);
-			kunmap(buf->page);
+			addr = ops->map(filp, info, buf);
+			error = pipe_iov_copy_to_user(iov, addr + buf->offset, chars);
+			ops->unmap(info, buf);
 			if (unlikely(error)) {
 				if (!ret) ret = -EFAULT;
 				break;
@@ -138,8 +158,8 @@ pipe_readv(struct file *filp, const struct iovec *_iov,
 			buf->offset += chars;
 			buf->len -= chars;
 			if (!buf->len) {
-				release_pipe_buf(info, buf);
-				buf->page = NULL;
+				buf->ops = NULL;
+				ops->release(info, buf);
 				curbuf = (curbuf + 1) & (PIPE_BUFFERS-1);
 				info->curbuf = curbuf;
 				info->nrbufs = --bufs;
@@ -219,11 +239,12 @@ pipe_writev(struct file *filp, const struct iovec *_iov,
 	if (info->nrbufs && total_len < PAGE_SIZE) {
 		int lastbuf = (info->curbuf + info->nrbufs - 1) & (PIPE_BUFFERS-1);
 		struct pipe_buffer *buf = info->bufs + lastbuf;
+		struct pipe_buf_operations *ops = buf->ops;
 		int offset = buf->offset + buf->len;
-		if (offset + total_len <= PAGE_SIZE) {
-			struct page *page = buf->page;
-			int error = pipe_iov_copy_from_user(offset + kmap(page), iov, total_len);
-			kunmap(page);
+		if (ops->can_merge && offset + total_len <= PAGE_SIZE) {
+			void *addr = ops->map(filp, info, buf);
+			int error = pipe_iov_copy_from_user(offset + addr, iov, total_len);
+			ops->unmap(info, buf);
 			ret = error;
 			do_wakeup = 1;
 			if (error)
@@ -251,7 +272,7 @@ pipe_writev(struct file *filp, const struct iovec *_iov,
 			int error;
 
 			if (!page) {
-				page = alloc_page(GFP_KERNEL);
+				page = alloc_page(GFP_HIGHUSER);
 				if (unlikely(!page)) {
 					ret = ret ? : -ENOMEM;
 					break;
@@ -278,6 +299,7 @@ pipe_writev(struct file *filp, const struct iovec *_iov,
 
 			/* Insert it into the buffer array */
 			buf->page = page;
+			buf->ops = &anon_pipe_buf_ops;
 			buf->offset = 0;
 			buf->len = chars;
 			info->nrbufs = ++bufs;
@@ -612,8 +634,8 @@ void free_pipe_info(struct inode *inode)
 		__free_page(info->tmp_page);
 	for (i = 0; i < PIPE_BUFFERS; i++) {
 		struct pipe_buffer *buf = info->bufs + i;
-		if (buf->page)
-			release_pipe_buf(info, buf);
+		if (buf->ops)
+			buf->ops->release(info, buf);
 	}
 	kfree(info);
 }

include/linux/pipe_fs_i.h

@@ -7,7 +7,15 @@
 
 struct pipe_buffer {
 	struct page *page;
-	unsigned short offset, len;
+	unsigned int offset, len;
+	struct pipe_buf_operations *ops;
+};
+
+struct pipe_buf_operations {
+	int can_merge;
+	void * (*map)(struct file *, struct pipe_inode_info *, struct pipe_buffer *);
+	void (*unmap)(struct pipe_inode_info *, struct pipe_buffer *);
+	void (*release)(struct pipe_inode_info *, struct pipe_buffer *);
 };
 
 struct pipe_inode_info {