proc: unify mem_read() and mem_write()

No functional changes, cleanup and preparation.

mem_read() and mem_write() are very similar. Move this code into the
new common helper, mem_rw(), which takes the additional "int write"
argument.

Cc: stable@kernel.org
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/base.c

@@ -718,13 +718,13 @@ static int mem_open(struct inode* inode, struct file* file)
 	return 0;
 }
 
-static ssize_t mem_read(struct file * file, char __user * buf,
-			size_t count, loff_t *ppos)
+static ssize_t mem_rw(struct file *file, char __user *buf,
+			size_t count, loff_t *ppos, int write)
 {
-	int ret;
-	char *page;
-	unsigned long src = *ppos;
 	struct mm_struct *mm = file->private_data;
+	unsigned long addr = *ppos;
+	ssize_t copied;
+	char *page;
 
 	if (!mm)
 		return 0;
@@ -733,74 +733,48 @@ static ssize_t mem_read(struct file * file, char __user * buf,
 	if (!page)
 		return -ENOMEM;
 
-	ret = 0;
- 
+	copied = 0;
 	while (count > 0) {
-		int this_len, retval;
+		int this_len = min_t(int, count, PAGE_SIZE);
 
-		this_len = (count > PAGE_SIZE) ? PAGE_SIZE : count;
-		retval = access_remote_vm(mm, src, page, this_len, 0);
-		if (!retval) {
-			if (!ret)
-				ret = -EIO;
+		if (write && copy_from_user(page, buf, this_len)) {
+			copied = -EFAULT;
 			break;
 		}
 
-		if (copy_to_user(buf, page, retval)) {
-			ret = -EFAULT;
+		this_len = access_remote_vm(mm, addr, page, this_len, write);
+		if (!this_len) {
+			if (!copied)
+				copied = -EIO;
 			break;
 		}
 
-		ret += retval;
-		src += retval;
-		buf += retval;
-		count -= retval;
+		if (!write && copy_to_user(buf, page, this_len)) {
+			copied = -EFAULT;
+			break;
 		}
-	*ppos = src;
+
+		buf += this_len;
+		addr += this_len;
+		copied += this_len;
+		count -= this_len;
+	}
+	*ppos = addr;
 
 	free_page((unsigned long) page);
-	return ret;
+	return copied;
 }
 
-static ssize_t mem_write(struct file * file, const char __user *buf,
+static ssize_t mem_read(struct file *file, char __user *buf,
 			size_t count, loff_t *ppos)
 {
-	int copied;
-	char *page;
-	unsigned long dst = *ppos;
-	struct mm_struct *mm = file->private_data;
-
-	if (!mm)
-		return 0;
-
-	page = (char *)__get_free_page(GFP_TEMPORARY);
-	if (!page)
-		return -ENOMEM;
-
-	copied = 0;
-	while (count > 0) {
-		int this_len, retval;
-
-		this_len = (count > PAGE_SIZE) ? PAGE_SIZE : count;
-		if (copy_from_user(page, buf, this_len)) {
-			copied = -EFAULT;
-			break;
-		}
-		retval = access_remote_vm(mm, dst, page, this_len, 1);
-		if (!retval) {
-			if (!copied)
-				copied = -EIO;
-			break;
-		}
-		copied += retval;
-		buf += retval;
-		dst += retval;
-		count -= retval;			
-	}
-	*ppos = dst;
+	return mem_rw(file, buf, count, ppos, 0);
+}
 
-	free_page((unsigned long) page);
-	return copied;
+static ssize_t mem_write(struct file *file, const char __user *buf,
+			 size_t count, loff_t *ppos)
+{
+	return mem_rw(file, (char __user*)buf, count, ppos, 1);
 }
 
 loff_t mem_lseek(struct file *file, loff_t offset, int orig)