[dm-devel] REQ_OP for zeroing, was Re: [PATCH 1/4] brd: handle misaligned discard

[Christoph Hellwig]

On Wed, Oct 26, 2016 at 05:46:11PM -0400, Mikulas Patocka wrote:
> I think the proper thing would be to move "discard_zeroes_data" flag into 
> the bio itself - there would be REQ_OP_DISCARD and REQ_OP_DISCARD_ZERO - 
> and if the device doesn't support REQ_OP_DISCARD_ZERO, it rejects the bio 
> and the caller is supposed to do zeroing manually.

Yes, Martin and I have come to a similar conclusion recently.  An
additional aspect is that NVMe has a Write Zeroes command which is more
limited than what REQ_OP_WRITE_SAME does.

So I think the right way is to add a REQ_OP_WRITE_ZEROES (or
REQ_OP_ZERO) and have modifies if it may discard or not.