[PATCH] Hack to get dvd-burning at 8x (instead of 3x) with ide-cd (2.6.11)

[Linh Dang]

Hi,

I'd like to receive  comments/guide-lines about a hack I made to the
2.6.11 kernel to improve DVD-burning speed (using growisofs.)

The basic idea is the 16-pages pipe between mkisofs and growisofs is
too small for DVD burning (typical 4GB of data.)

In the hack, pipe_new will simply check for, if privileges permitted,
the enviroment variable
PIPE_MAX_ORDER to see if a (much) longer pipe is requested.

This hack enable me to burn DVD at 8X (instead of 3X) on my old
P3-450MHz (with growisofs and mkisofs running at SCHED_FIFO.)

-- 
Linh Dang
--- linux-2.6.11/include/linux/pipe_fs_i.h	        2005/03/17 15:39:38
+++ linux-2.6.11-vpipe/include/linux/pipe_fs_i.h	2005/03/23 17:27:12
@@ -21,7 +21,6 @@ struct pipe_buf_operations {
 struct pipe_inode_info {
 	wait_queue_head_t wait;
 	unsigned int nrbufs, curbuf;
-	struct pipe_buffer bufs[PIPE_BUFFERS];
 	struct page *tmp_page;
 	unsigned int start;
 	unsigned int readers;
@@ -31,6 +30,8 @@ struct pipe_inode_info {
 	unsigned int w_counter;
 	struct fasync_struct *fasync_readers;
 	struct fasync_struct *fasync_writers;
+        unsigned max_nr_buffers;
+	struct pipe_buffer bufs[0];
 };
 
 /* Differs from PIPE_BUF in that PIPE_SIZE is the length of the actual
--- linux-2.6.11/fs/pipe.c	        2005/03/17 15:30:32
+++ linux-2.6.11-vpipe/fs/pipe.c	2005/03/23 17:16:46
@@ -160,7 +160,7 @@ pipe_readv(struct file *filp, const stru
 			if (!buf->len) {
 				buf->ops = NULL;
 				ops->release(info, buf);
-				curbuf = (curbuf + 1) & (PIPE_BUFFERS-1);
+				curbuf = (curbuf + 1) & (info->max_nr_buffers - 1);
 				info->curbuf = curbuf;
 				info->nrbufs = --bufs;
 				do_wakeup = 1;
@@ -243,7 +243,7 @@ pipe_writev(struct file *filp, const str
 
 	/* We try to merge small writes */
 	if (info->nrbufs && total_len < PAGE_SIZE) {
-		int lastbuf = (info->curbuf + info->nrbufs - 1) & (PIPE_BUFFERS-1);
+		int lastbuf = (info->curbuf + info->nrbufs - 1) & (info->max_nr_buffers-1);
 		struct pipe_buffer *buf = info->bufs + lastbuf;
 		struct pipe_buf_operations *ops = buf->ops;
 		int offset = buf->offset + buf->len;
@@ -270,9 +270,9 @@ pipe_writev(struct file *filp, const str
 			break;
 		}
 		bufs = info->nrbufs;
-		if (bufs < PIPE_BUFFERS) {
+		if (bufs < info->max_nr_buffers) {
 			ssize_t chars;
-			int newbuf = (info->curbuf + bufs) & (PIPE_BUFFERS-1);
+			int newbuf = (info->curbuf + bufs) & (info->max_nr_buffers-1);
 			struct pipe_buffer *buf = info->bufs + newbuf;
 			struct page *page = info->tmp_page;
 			int error;
@@ -315,7 +315,7 @@ pipe_writev(struct file *filp, const str
 			if (!total_len)
 				break;
 		}
-		if (bufs < PIPE_BUFFERS)
+		if (bufs < info->max_nr_buffers)
 			continue;
 		if (filp->f_flags & O_NONBLOCK) {
 			if (!ret) ret = -EAGAIN;
@@ -382,7 +382,7 @@ pipe_ioctl(struct inode *pino, struct fi
 			nrbufs = info->nrbufs;
 			while (--nrbufs >= 0) {
 				count += info->bufs[buf].len;
-				buf = (buf+1) & (PIPE_BUFFERS-1);
+				buf = (buf+1) & (info->max_nr_buffers-1);
 			}
 			up(PIPE_SEM(*inode));
 			return put_user(count, (int __user *)arg);
@@ -412,7 +412,7 @@ pipe_poll(struct file *filp, poll_table 
 	}
 
 	if (filp->f_mode & FMODE_WRITE) {
-		mask |= (nrbufs < PIPE_BUFFERS) ? POLLOUT | POLLWRNORM : 0;
+		mask |= (nrbufs < info->max_nr_buffers) ? POLLOUT | POLLWRNORM : 0;
 		if (!PIPE_READERS(*inode))
 			mask |= POLLERR;
 	}
@@ -641,7 +641,7 @@ void free_pipe_info(struct inode *inode)
 	struct pipe_inode_info *info = inode->i_pipe;
 
 	inode->i_pipe = NULL;
-	for (i = 0; i < PIPE_BUFFERS; i++) {
+	for (i = 0; i < info->max_nr_buffers; i++) {
 		struct pipe_buffer *buf = info->bufs + i;
 		if (buf->ops)
 			buf->ops->release(info, buf);
@@ -653,13 +653,59 @@ void free_pipe_info(struct inode *inode)
 
 struct inode* pipe_new(struct inode* inode)
 {
+#define ENV "PIPE_MAX_ORDER="
 	struct pipe_inode_info *info;
+        struct mm_struct       *mm = current ? get_task_mm(current) : NULL;
+
+
+        char*                   env            = NULL;
+        unsigned                env_len        = 0;
+        unsigned                max_nr_buffers = PIPE_BUFFERS;
+
+        if (unlikely(capable(CAP_SYS_RESOURCE))) {
+                if (mm) {
+                        env_len = mm->env_end - mm->env_start;
+                        env     = kmalloc(env_len, GFP_KERNEL);
+                        access_process_vm(current, mm->env_start, env, env_len, 0);
+                        mmput(mm);
+                }
+
+                if (env) {
+                        const char* env_end = env + env_len;
+                        char*       p       = env;
+                        char*       dummy;
+
+
+                        while (p < env_end && strncmp(p, ENV, sizeof(ENV)))
+                                p = memscan(p, '\0', env_end - p) + 1;
+
+                        if (p < env_end && strncmp(p, ENV, sizeof(ENV)) == 0) {
+                                unsigned  pipe_nr_buffers_order =
+                                        simple_strtoul(strchr(p, '='), &dummy, 10);
+
+                                if (pipe_nr_buffers_order < 4)
+                                        pipe_nr_buffers_order = 4;
+
+                                if (pipe_nr_buffers_order > 16)
+                                        pipe_nr_buffers_order = 16;
+
+                                max_nr_buffers = 1 << pipe_nr_buffers_order;
+                        }
+
+                        kfree(env);
+                }
+        }
+
+
+	info = kcalloc(1, sizeof(struct pipe_inode_info) +
+                       sizeof(struct pipe_buffer) * max_nr_buffers,
+                                       GFP_KERNEL);
 
-	info = kmalloc(sizeof(struct pipe_inode_info), GFP_KERNEL);
 	if (!info)
-		goto fail_page;
-	memset(info, 0, sizeof(*info));
-	inode->i_pipe = info;
+                goto fail_page;
+
+        info->max_nr_buffers = max_nr_buffers;
+	inode->i_pipe        = info;
 
 	init_waitqueue_head(PIPE_WAIT(*inode));
 	PIPE_RCOUNTER(*inode) = PIPE_WCOUNTER(*inode) = 1;