Linux Virtualization - Sep 2020 - [PATCH v2 3/7] mm/memory_hotplug: prepare passing flags to add_memory() and friends

On Tue, Sep 08, 2020 at 10:10:08PM +0200, David Hildenbrand
wrote:
> We soon want to pass flags, e.g., to mark added System RAM resources.
> mergeable. Prepare for that.
What are these random "flags", and how do we know what should be
passed
to them?

Why not make this an enumerated type so that we know it all works
properly, like the GPF_* flags are?  Passing around a random unsigned
long feels very odd/broken...

thanks,

greg k-h

On 09.09.20 09:17, Greg Kroah-Hartman wrote:
> On Tue, Sep 08, 2020 at 10:10:08PM +0200, David Hildenbrand wrote:
>> We soon want to pass flags, e.g., to mark added System RAM resources.
>> mergeable. Prepare for that.
> 
> What are these random "flags", and how do we know what should be
passed
> to them?
> 
> Why not make this an enumerated type so that we know it all works
> properly, like the GPF_* flags are?  Passing around a random unsigned
> long feels very odd/broken...
Agreed, an enum (mhp_flags) seems to give a better hint what can
actually be passed. Thanks!

-- 
Thanks,

David / dhildenb

David Hildenbrand <david at redhat.com> writes:
> On 09.09.20 09:17, Greg Kroah-Hartman wrote:
>> On Tue, Sep 08, 2020 at 10:10:08PM +0200, David Hildenbrand wrote:
>>> We soon want to pass flags, e.g., to mark added System RAM
resources.
>>> mergeable. Prepare for that.
>> 
>> What are these random "flags", and how do we know what should
be passed
>> to them?
>> 
>> Why not make this an enumerated type so that we know it all works
>> properly, like the GPF_* flags are?  Passing around a random unsigned
>> long feels very odd/broken...
>
> Agreed, an enum (mhp_flags) seems to give a better hint what can
> actually be passed. Thanks!
You probably know this but ...

Just using a C enum doesn't get you any type safety.

You can get some checking via sparse by using __bitwise, which is what
gfp_t does. You don't actually have to use an enum for that, it works
with #defines also.

Or you can wrap the flag in a struct, the way atomic_t does, and then
the compiler will prevent passing plain integers in place of your custom
type.

cheers