Re: [PATCH v7 2/2] loop: Better discard support for block devices

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Wed, Nov 20, 2019 at 11:25:48AM -0800, Evan Green wrote:
> On Wed, Nov 20, 2019 at 11:13 AM Darrick J. Wong
> <darrick.wong@oracle.com> wrote:
> >
> > On Wed, Nov 20, 2019 at 10:56:30AM -0800, Evan Green wrote:
> > > On Tue, Nov 19, 2019 at 6:25 PM Darrick J. Wong <darrick.wong@oracle.com> wrote:
> > > >
> > > > On Thu, Nov 14, 2019 at 03:50:08PM -0800, Evan Green wrote:
> > > > > If the backing device for a loop device is itself 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.
> 
> (marking this spot for below)
> 
> > > > >
> > > > > 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@chromium.org>
> > > > > Reviewed-by: Gwendal Grignou <gwendal@chromium.org>
> > > > > Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
> > > > > ---
> > > > >
> > > > > Changes in v7:
> > > > > - Rebase on top of Darrick's patch
> > > > > - Tweak opening line of commit description (Darrick)
> > > > >
> > > > > 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 | 40 +++++++++++++++++++++++++++++-----------
> > > > >  1 file changed, 29 insertions(+), 11 deletions(-)
> > > > >
> > > > > diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> > > > > index 6a9fe1f9fe84..e8f23e4b78f7 100644
> > > > > --- a/drivers/block/loop.c
> > > > > +++ b/drivers/block/loop.c
> > > > > @@ -427,11 +427,12 @@ static int lo_fallocate(struct loop_device *lo, struct request *rq, loff_t pos,
> > > > >        * information.
> > > > >        */
> > > > >       struct file *file = lo->lo_backing_file;
> > > > > +     struct request_queue *q = lo->lo_queue;
> > > > >       int ret;
> > > > >
> > > > >       mode |= FALLOC_FL_KEEP_SIZE;
> > > > >
> > > > > -     if ((!file->f_op->fallocate) || lo->lo_encrypt_key_size) {
> > > > > +     if (!blk_queue_discard(q)) {
> > > > >               ret = -EOPNOTSUPP;
> > > > >               goto out;
> > > > >       }
> > > > > @@ -862,6 +863,21 @@ 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);
> > > > > +             blk_queue_max_discard_sectors(q,
> > > > > +                     backingq->limits.max_write_zeroes_sectors);
> > > >
> > > > max_discard_sectors?
> > >
> > > I didn't plumb max_discard_sectors because for my scenario it never
> > > ends up hitting the block device that way.
> > >
> > > The loop device either uses FL_ZERO_RANGE or FL_PUNCH_HOLE. When
> > > backed by a block device, that ends up in blkdev_fallocate(), which
> > > always translates both of those into blkdev_issue_zeroout(), not
> > > blkdev_issue_discard(). So it's really the zeroing capabilities of the
> > > block device that matters, even for loop discard operations. It seems
> > > weird, but I think this is the right thing because it presents a
> > > consistent interface to loop device users whether backed by a file
> > > system file, or directly by a block device. That is, a previously
> > > discarded range will read back as zeroes.
> >
> > Ah, right.  Could you add this paragraph as a comment explaining why
> > we're setting max_discard_sectors from max_write_zeroes_sectors?
> 
> Sure. I put an explanation in the commit description (see spot I
> marked above), but I agree a comment is probably also worthwhile.

<nod> Sorry about the churn here.

I have a strong preference towards documenting decisions like these
directly in the code because (a) I suck at reading patch prologues, (b)
someone reading the code after this gets committed will see it
immediately and right next to the relevant code, and (c) spelunking
through the git history of a file for commit messages is kind of clunky.

Dunno if that's just my age showing (mmm, pre-bk linux) or what. :/

--D

> >
> > --D
> >
> > > -Evan