target/file: Fix 32-bit highmem breakage for SGL -> iovec mapping

commit 40ff2c3b upstream.

This patch changes vectored file I/O to use kmap + kunmap when mapping
incoming SGL memory -> struct iovec in order to properly support 32-bit
highmem configurations.  This is because an extra bounce buffer may be
required when processing scatterlist pages allocated with GFP_KERNEL.
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>
[bwh: Backported to 3.2: use task->task_sg{,_nents} for iteration]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/target/target_core_file.c

@@ -300,7 +300,7 @@ static int fd_do_readv(struct se_task *task)
 
 	for_each_sg(task->task_sg, sg, task->task_sg_nents, i) {
 		iov[i].iov_len = sg->length;
-		iov[i].iov_base = sg_virt(sg);
+		iov[i].iov_base = kmap(sg_page(sg)) + sg->offset;
 	}
 
 	old_fs = get_fs();
@@ -308,6 +308,8 @@ static int fd_do_readv(struct se_task *task)
 	ret = vfs_readv(fd, &iov[0], task->task_sg_nents, &pos);
 	set_fs(old_fs);
 
+	for_each_sg(task->task_sg, sg, task->task_sg_nents, i)
+		kunmap(sg_page(sg));
 	kfree(iov);
 	/*
 	 * Return zeros and GOOD status even if the READ did not return
@@ -353,7 +355,7 @@ static int fd_do_writev(struct se_task *task)
 
 	for_each_sg(task->task_sg, sg, task->task_sg_nents, i) {
 		iov[i].iov_len = sg->length;
-		iov[i].iov_base = sg_virt(sg);
+		iov[i].iov_base = kmap(sg_page(sg)) + sg->offset;
 	}
 
 	old_fs = get_fs();
@@ -361,6 +363,9 @@ static int fd_do_writev(struct se_task *task)
 	ret = vfs_writev(fd, &iov[0], task->task_sg_nents, &pos);
 	set_fs(old_fs);
 
+	for_each_sg(task->task_sg, sg, task->task_sg_nents, i)
+		kunmap(sg_page(sg));
+
 	kfree(iov);
 
 	if (ret < 0 || ret != task->task_size) {