[PATCH] loop: properly observe rotational flag of underlying device

[Benjamin Gordon <bmgordon@chromium.org>]

From: Holger Hoffstätte <holger.hoffstaette@googlemail.com>

The loop driver always declares the rotational flag of its device as
rotational, even when the device of the mapped file is nonrotational,
as is the case with SSDs or on tmpfs. This can confuse filesystem tools
which are SSD-aware; in my case I frequently forget to tell mkfs.btrfs
that my loop device on tmpfs is nonrotational, and that I really don't
need any automatic metadata redundancy.

The attached patch fixes this by introspecting the rotational flag of the
mapped file's underlying block device, if it exists. If the mapped file's
filesystem has no associated block device - as is the case on e.g. tmpfs -
we assume nonrotational storage. If there is a better way to identify such
non-devices I'd love to hear them.

Cc: Jens Axboe <axboe@kernel.dk>
Cc: linux-block@vger.kernel.org
Cc: holger@applied-asynchrony.com
Signed-off-by: Holger Hoffstätte <holger.hoffstaette@googlemail.com>
Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Signed-off-by: Benjamin Gordon <bmgordon@chromium.org>
Reviewed-by: Guenter Roeck <groeck@chromium.org>
---
 This is a resend of Holger's original patch from
 https://lkml.org/lkml/2015/11/11/288 with the _unlocked functions
 updated.  We keep running into the same problem on Chrome OS that this
 originally solved; any chance it can go in?

 drivers/block/loop.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index cf5538942834..6c0fc0d49dc0 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -900,6 +900,24 @@ static int loop_prepare_queue(struct loop_device *lo)
 	return 0;
 }
 
+static void loop_update_rotational(struct loop_device *lo)
+{
+	struct file *file = lo->lo_backing_file;
+	struct inode *file_inode = file->f_mapping->host;
+	struct block_device *file_bdev = file_inode->i_sb->s_bdev;
+	struct request_queue *q = lo->lo_queue;
+	bool nonrot = true;
+
+	/* not all filesystems (e.g. tmpfs) have a sb->s_bdev */
+	if (file_bdev)
+		nonrot = blk_queue_nonrot(bdev_get_queue(file_bdev));
+
+	if (nonrot)
+		blk_queue_flag_set(QUEUE_FLAG_NONROT, q);
+	else
+		blk_queue_flag_clear(QUEUE_FLAG_NONROT, q);
+}
+
 static int loop_set_fd(struct loop_device *lo, fmode_t mode,
 		       struct block_device *bdev, unsigned int arg)
 {
@@ -963,6 +981,7 @@ static int loop_set_fd(struct loop_device *lo, fmode_t mode,
 	if (!(lo_flags & LO_FLAGS_READ_ONLY) && file->f_op->fsync)
 		blk_queue_write_cache(lo->lo_queue, true, false);
 
+	loop_update_rotational(lo);
 	loop_update_dio(lo);
 	set_capacity(lo->lo_disk, size);
 	bd_set_size(bdev, size << 9);
-- 
2.20.1.791.gb4d0f1c61a-goog