Subject: Re: [PATCH 1/1] dm raid: fix compat_features validation
To: Mike Snitzer <snitzer@redhat.com>
References: <20161011142835.11620-1-apw@canonical.com>
 <1c517f14-1234-7844-fc6a-cd1b9698fb8b@redhat.com>
 <20161011153808.nmyf6hafjaadcemw@brain>
 <591b9d8d-2036-2d0f-14f2-af176b5beaea@redhat.com>
 <20161011174421.GA25738@redhat.com>
Cc: Andy Whitcroft <apw@canonical.com>, linux-kernel@vger.kernel.org,
        linux-raid@vger.kernel.org, dm-devel@redhat.com,
        Shaohua Li <shli@kernel.org>, Alasdair Kergon <agk@redhat.com>
From: Heinz Mauelshagen <heinzm@redhat.com>
Message-ID: <dc788138-ba1f-339f-52e5-0cc763462496@redhat.com>
Date: Fri, 14 Oct 2016 19:14:32 +0200
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On 10/11/2016 07:44 PM, Mike Snitzer wrote:
> On Tue, Oct 11 2016 at 11:44am -0400,
> Heinz Mauelshagen <heinzm@redhat.com> wrote:
>
>>
>> On 10/11/2016 05:38 PM, Andy Whitcroft wrote:
>>> On Tue, Oct 11, 2016 at 05:04:34PM +0200, Heinz Mauelshagen wrote:
>>>> Andy,
>>>>
>>>> good catch.
>>>>
>>>> We should rather check for  V190 support only in case any
>>>> compat feature flags are actually set.
>>>>
>>>> {
>>>> +       if (le32_to_cpu(sb->compat_features) &&
>>>> +           le32_to_cpu(sb->compat_features) != FEATURE_FLAG_SUPPORTS_V190)
>>>> {
>>>>                  rs->ti->error = "Unable to assemble array: Unknown flag(s)
>>>> in compatible feature flags";
>>>>                  return -EINVAL;
>>>>          }
>>> If the feature flags are single bit combinations then I believe the
>>> below does check exactly that.  Checking for no 1s outside of the
>>> expected features, caring not for the value of the valid bits:
>>>
>>> +     if (le32_to_cpu(sb->compat_features) & ~(FEATURE_FLAG_SUPPORTS_V190)) {
>>>
>>> with the possibilty to or in additional feature bits as they are added.
>> Thanks,
>> I prefer this to be easier readable.
> Readable or not, the code with the != is _not_ future-proof.  Whereas
> Andy's solution is.  If/when a new compat feature comes along then
> FEATURE_FLAG_SUPPORTS_V190 would be replaced to be a macro that ORs all
> the new compat features together (e.g. FEATURE_FLAG_COMPAT).  E.g. how
> dm-thin-metadata.c:__check_incompat_features() does.

If we'll have to introduce more feature flags in the future (e.g. for 
clustered raid1
support), this is going to be based on the test_bit() API for consistency
with any other flag processing we do in the target.

Heinz

> We can go with the != code for now, since any future changes would
> likely cause this test to be changed.  Or we could fix it now _for
> real_.
>
> Mike