[dm-devel] [PATCH 1/4] brd: handle misaligned discard

[Bart Van Assche]

On 10/26/2016 02:46 PM, Mikulas Patocka wrote:
> I don't like the idea of complicating the code by turning discards into
> writes.

That's not what my patch series does. The only writes added by my patch 
series are those for the non-aligned head and tail of the range passed 
to blkdev_issue_zeroout().

> The flag "discard_zeroes_data" is actually misdesigned, because the
> storage stack can change dynamically while bios are in progress. You can
> send a discard bio to a device mapper device that has
> "discard_zeroes_data" - and while the bio is in progress, the device
> mapper stack can be reconfigured to redirect the bio to another device
> that doesn't have "discard_zeroes_data" - and the bio won't zero data and
> the caller that issued it has no way to find it out.
>
> 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.

Sorry but I don't like this proposal. I think that a much better 
solution would be to pause I/O before making any changes to the 
discard_zeroes_data flag.

Bart.