[Drbd-dev] [PATCH 25/27] block: remove the discard_zeroes_data flag

[Nicholas A. Bellinger]

On Wed, 2017-05-10 at 16:06 +0200, hch at lst.de wrote:
> On Mon, May 08, 2017 at 11:46:14PM -0700, Nicholas A. Bellinger wrote:
> > That said, simply propagating up q->limits.max_write_zeroes_sectors as
> > dev_attrib->unmap_zeroes_data following existing code still looks like
> > the right thing to do.
> 
> It is not.  Martin has decoupled write same/zeroes support from discard
> support.  Any device will claim to support it initially, and we'll
> only clear the flag if a Write Same command fails.
> 
> So even if LBPRZ is not set you can trivially get into a situation
> where discard is supported through UNMAP, and you'll incorrectly
> set LBPRZ and will cause data corruption.

In that case, there are two choices.

1) Expose a block_device or request_queue bit to signal 'real LBPRZ'
support up to IBLOCK, in order to maintain SCSI target feature
compatibility.

2) Or drop the LBPRZ bit usage for IBLOCK all-together.

Since I happen happen to support a block driver that has 'real LBPRZ'
support for all discards, I'd prefer the latter so this doesn't have to
be carried out-of-tree.

So what are the options for this in post v4.12..?