Messages in this thread | | | Subject | Re: [PATCH] loop: properly observe rotational flag of underlying device | From | Holger Hoffstätte <> | Date | Wed, 11 Nov 2015 23:08:30 +0100 |
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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@googlemail.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.
phew :)
thanks, Holger
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