On 10/11/2016 07:44 PM, Mike Snitzer wrote:
On Tue, Oct 11 2016 at 11:44am -0400,
Heinz Mauelshagen <hei...@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:

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.
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.


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


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