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.
Signed-off-by: Holger Hoffstätte <[email protected]>
---
drivers/block/loop.c | 19 +++++++++++++++++++
1 file changed, 19 insertions(+)
diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 423f4ca..2984aca 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -843,6 +843,24 @@ static void loop_config_discard(struct loop_device *lo)
queue_flag_set_unlocked(QUEUE_FLAG_DISCARD, q);
}
+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)
+ queue_flag_set_unlocked(QUEUE_FLAG_NONROT, q);
+ else
+ queue_flag_clear_unlocked(QUEUE_FLAG_NONROT, q);
+}
+
static void loop_unprepare_queue(struct loop_device *lo)
{
flush_kthread_worker(&lo->worker);
@@ -939,6 +957,7 @@ 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_flush(lo->lo_queue, REQ_FLUSH);
+ loop_update_rotational(lo);
loop_update_dio(lo);
set_capacity(lo->lo_disk, size);
bd_set_size(bdev, size << 9);
--
2.6.3
On 11/11/2015 08:21 AM, Holger Hoffstätte wrote:
>
> 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.
>
> Signed-off-by: Holger Hoffstätte <[email protected]>
> ---
> drivers/block/loop.c | 19 +++++++++++++++++++
> 1 file changed, 19 insertions(+)
>
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index 423f4ca..2984aca 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -843,6 +843,24 @@ static void loop_config_discard(struct loop_device *lo)
> queue_flag_set_unlocked(QUEUE_FLAG_DISCARD, q);
> }
>
> +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)
> + queue_flag_set_unlocked(QUEUE_FLAG_NONROT, q);
> + else
> + queue_flag_clear_unlocked(QUEUE_FLAG_NONROT, q);
> +}
Are we sure we want to change the default from rot to nonrot?
Apart from that, looks good.
--
Jens Axboe
On 11/11/15 22:29, Jens Axboe wrote:
> On 11/11/2015 08:21 AM, Holger Hoffstätte wrote:
>>
>> 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.
>>
>> Signed-off-by: Holger Hoffstätte <[email protected]>
>> ---
>> drivers/block/loop.c | 19 +++++++++++++++++++
>> 1 file changed, 19 insertions(+)
>>
>> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
>> index 423f4ca..2984aca 100644
>> --- a/drivers/block/loop.c
>> +++ b/drivers/block/loop.c
>> @@ -843,6 +843,24 @@ static void loop_config_discard(struct loop_device *lo)
>> queue_flag_set_unlocked(QUEUE_FLAG_DISCARD, q);
>> }
>>
>> +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)
>> + queue_flag_set_unlocked(QUEUE_FLAG_NONROT, q);
>> + else
>> + queue_flag_clear_unlocked(QUEUE_FLAG_NONROT, q);
>> +}
>
> Are we sure we want to change the default from rot to nonrot?
Well, that's why I asked for a better way to identify tmpfs. It took
me several hours to figure out that tmpfs doesn't have an s_bdev, and
could not find a better way than to assume that a superblock without
backing device is probably something virtual/nonrotational/nvm etc.
Alternatively I could look at sb->s_type and set nonrot for known fs
types, but that seemed too ugly - not to mention conceptually weird.
> Apart from that, looks good.
phew :)
thanks,
Holger