[dm-devel] Why don't return -EOPNOTSUPP?

[Lukáš Czerner]

On Tue, 2 Feb 2016, jiangyiwen wrote:

> Date: Tue, 2 Feb 2016 19:57:37 +0800
> From: jiangyiwen <jiangyiwen at huawei.com>
> To: lczerner at redhat.com
> Cc: dm-devel at redhat.com, "Qijiang (Joseph, Euler)" <joseph.qi at huawei.com>,
>     xuejiufei at huawei.com
> Subject: [dm-devel] Why don't return -EOPNOTSUPP?
> 
> Hi Lukas,
> I have a question about commit 8af1954d172a("blkdev: Do not return
> -EOPNOTSUPP if discard is supported"). That is we issue a
> WRITE SAME request to device and return failed with EOPNOTSUPP,
> but blkdev_issue_write_same ignore this error and return
> success to caller. In this way, this will cause an inconsistent state
> between upper layer and bottom layer.
> 
> I don't know why don't return EOPNOTSUPP if part of devices support
> discard, because I think that blkdev_issue_discard should return
> EOPNOTSUPP to caller even if some of devices support discard.
> 
> Thanks,
> Yiwen Jiang.

Hi,

I think that the reasoning is very well explained in the commit
description. Simply put, if the device does not support the discard
at all then there is:

if (!blk_queue_discard(q))
	return -EOPNOTSUPP;

however if the device itself (possibly because it's dm device)
advertise that it indeed does support discard and only parts of it
does not (again possibly because it's a dm device) then there is no
reason to return EOPNOTSUPP because that would only confuse the
upper layers.

Now the question is, why is this a problem ? It's very much a
advisory interface so if we send a discard request and it's not
supported on the part of the device, what harm will be done by
returning success ?

On the other hand if we did return EOPNOTSUPP, upper layer may stop
issuing discard whatsoever which will be harmful for the parts of
the device where discard is actually supported. Moreover it might
also be a problem of alignment or granularity, not just that it's
not supported.

However it's possible that situation changed since this commit and
the lower layers, mainly probably dm, is doing the right thing, or
is configurable in some way...I do not know.

-Lukas