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

[Evan Green <evgreen@chromium.org>]

On Mon, Dec 10, 2018 at 9:31 AM Evan Green <evgreen@chromium.org> wrote:
>
> On Wed, Dec 5, 2018 at 7:15 PM Martin K. Petersen
> <martin.petersen@oracle.com> wrote:
> >
> >
> > Evan,
> >
> > > Ah, I see. But I think it's useful to reflect max_discard_sectors,
> > > max_write_zeroes_sectors, discard_granularity, and discard_alignment
> > > from the block device to the loop device. With the exception of
> > > discard_alignment, these parameters are visible via sysfs,
> >
> > discard_alignment is visible in sysfs, just not in the queue directory
> > since alignment can be different on a per-partition basis. So there is
> > one discard_alignment at the root of each /sys/block/foo directory and
> > one for each partition subdirectory. This mirrors the alignment_offset
> > variable which indicates a partitions alignment wrt. the underlying
> > physical block size.
> >
> > That said, there are not many devices that actually report a non-zero
> > discard alignment so it's not as useful as the device manufacturers
> > (that were looking for an implementation shortcut) envisioned.
>
> Ah ok, thanks.
>
> >
> > > I'm not totally sure about discard_alignment, that seems to be useful
> > > in cases of merging blk requests. So I can stop mirroring that one if
> > > it's harmful or not helpful. But unless it's a nak, I'd really love to
> > > keep most of the mirroring. In which case the bool doesn't do a whole
> > > lot of simplifying.
> >
> > I think it's fine to export these. The block device topology was
> > explicitly designed to be stackable like this.
>
> Yeah, it seemed to fall in pretty naturally, which is why I was hoping
> it might not be so controversial. Thanks Martin.

Any other thoughts about this series?

Ming, I did go back and experiment a little. The pseudocode you had
proposed earlier would work, and solve the error prints I was seeing.
But I still would like to keep the reflection of the block device
properties, since that allows discard to be used on block devices that
do support it.
-Evan