Re: [PATCH v6 2/2] loop: Better discard support for block devices

[Evan Green]

Thanks for replying and taking a look Darrick. I didn't see your patch
in Jens tree when I looked just before sending it, but maybe I missed
it.

On Mon, Nov 11, 2019 at 5:37 PM Darrick J. Wong <darrick.wong@xxxxxxxxxx> wrote:
>
> On Mon, Nov 11, 2019 at 10:50:30AM -0800, Evan Green wrote:
> > If the backing device for a loop device is a block device,
> > then mirror the "write zeroes" capabilities of the underlying
> > block device into the loop device. Copy this capability into both
> > max_write_zeroes_sectors and max_discard_sectors of the loop device.
> >
> > The reason for this is that REQ_OP_DISCARD on a loop device translates
> > into blkdev_issue_zeroout(), rather than blkdev_issue_discard(). This
> > presents a consistent interface for loop devices (that discarded data
> > is zeroed), regardless of the backing device type of the loop device.
> > There should be no behavior change for loop devices backed by regular
> > files.
> >
> > While in there, differentiate between REQ_OP_DISCARD and
> > REQ_OP_WRITE_ZEROES, which are different for block devices,
> > but which the loop device had just been lumping together, since
> > they're largely the same for files.
> >
> > This change fixes blktest block/003, and removes an extraneous
> > error print in block/013 when testing on a loop device backed
> > by a block device that does not support discard.
> >
> > Signed-off-by: Evan Green <evgreen@xxxxxxxxxxxx>
> > Reviewed-by: Gwendal Grignou <gwendal@xxxxxxxxxxxx>
> > Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@xxxxxxx>
> > ---
> >
> > Changes in v6: None
> > Changes in v5:
> > - Don't mirror discard if lo_encrypt_key_size is non-zero (Gwendal)
> >
> > Changes in v4:
> > - Mirror blkdev's write_zeroes into loopdev's discard_sectors.
> >
> > Changes in v3:
> > - Updated commit description
> >
> > Changes in v2: None
> >
> >  drivers/block/loop.c | 57 ++++++++++++++++++++++++++++----------------
> >  1 file changed, 37 insertions(+), 20 deletions(-)
> >
> > diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> > index d749156a3d88..236f6deb0772 100644
> > --- a/drivers/block/loop.c
> > +++ b/drivers/block/loop.c
> > @@ -417,19 +417,14 @@ static int lo_read_transfer(struct loop_device *lo, struct request *rq,
> >       return ret;
> >  }
> >
> > -static int lo_discard(struct loop_device *lo, struct request *rq, loff_t pos)
> > +static int lo_discard(struct loop_device *lo, struct request *rq,
> > +             int mode, loff_t pos)
> >  {
> > -     /*
> > -      * We use punch hole to reclaim the free space used by the
> > -      * image a.k.a. discard. However we do not support discard if
> > -      * encryption is enabled, because it may give an attacker
> > -      * useful information.
> > -      */
> >       struct file *file = lo->lo_backing_file;
> > -     int mode = FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE;
> > +     struct request_queue *q = lo->lo_queue;
> >       int ret;
> >
> > -     if ((!file->f_op->fallocate) || lo->lo_encrypt_key_size) {
> > +     if (!blk_queue_discard(q)) {
> >               ret = -EOPNOTSUPP;
> >               goto out;
> >       }
> > @@ -599,8 +594,13 @@ static int do_req_filebacked(struct loop_device *lo, struct request *rq)
> >       case REQ_OP_FLUSH:
> >               return lo_req_flush(lo, rq);
> >       case REQ_OP_DISCARD:
> > +             return lo_discard(lo, rq,
> > +                     FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE, pos);
> > +
> >       case REQ_OP_WRITE_ZEROES:
> > -             return lo_discard(lo, rq, pos);
> > +             return lo_discard(lo, rq,
> > +                     FALLOC_FL_ZERO_RANGE | FALLOC_FL_KEEP_SIZE, pos);
>
> Yes, this more or less reimplements what's already in -next...

Agree, this part would disappear if I rebased on top of your patch.
This series has been around for awhile, you see :)

>
> > +
> >       case REQ_OP_WRITE:
> >               if (lo->transfer)
> >                       return lo_write_transfer(lo, rq, pos);
> > @@ -854,6 +854,21 @@ static void loop_config_discard(struct loop_device *lo)
> >       struct file *file = lo->lo_backing_file;
> >       struct inode *inode = file->f_mapping->host;
> >       struct request_queue *q = lo->lo_queue;
> > +     struct request_queue *backingq;
> > +
> > +     /*
> > +      * If the backing device is a block device, mirror its zeroing
> > +      * capability. REQ_OP_DISCARD translates to a zero-out even when backed
> > +      * by block devices to keep consistent behavior with file-backed loop
> > +      * devices.
> > +      */
> > +     if (S_ISBLK(inode->i_mode) && !lo->lo_encrypt_key_size) {
> > +             backingq = bdev_get_queue(inode->i_bdev);
>
> What happens if the inode is from a filesystem that can have multiple
> backing devices (like btrfs)?

Then I would expect S_ISBLK(inode->i_mode) would not be true. This is
only for when you've created a loop device directly on top of a block
device (ie you pointed the loop device at /dev/sda). We use this in
our Chrome OS installer because it makes the logic simple whether
you're installing to a real disk or a file image.

>
> > +             blk_queue_max_discard_sectors(q,
> > +                     backingq->limits.max_write_zeroes_sectors);
> > +
> > +             blk_queue_max_write_zeroes_sectors(q,
> > +                     backingq->limits.max_write_zeroes_sectors);
>
> Also, seeing as filesystems tend to implement PUNCH_HOLE and ZERO_RANGE
> on their own independent of the hardware capabilities of the underlying
> device, it doesn't make much sense to forward the blockdev limits to the
> loop device.
>
> (Put another way, XFS's ZERO_RANGE implementation can zero hundreds of
> gigabytes at a time even if the underlying device is a spinning rust.)

Hopefully my comment above addresses this too (there is no file system
in the scenario I'm coding for).