linux-kernel - Re: [PATCH] loop: properly observe rotational flag of underlying device

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Date:	Tue, 12 Jan 2016 00:20:27 +0100
From:	Holger Hoffstätte 
	<holger.hoffstaette@...glemail.com>
To:	Jens Axboe <axboe@...com>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] loop: properly observe rotational flag of underlying
 device

On 11/11/15 23:08, Holger Hoffstätte wrote:
> 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 <holger.hoffstaette@...glemail.com>
>>> ---
>>>   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.

Jens,

I haven't seen this merged in any trees yet and was wondering if there's
any chance to get this into 4.5? If there's something left to fix up please
let me know.

Thanks,
Holger