Re: [RFC RFC PATCH] mm: convert VM flags from macros to enum

[David Hildenbrand]

On 13.10.25 13:33, Lorenzo Stoakes wrote:
> On Mon, Oct 13, 2025 at 01:12:20PM +0200, David Hildenbrand wrote:
> > On 13.10.25 13:04, Lorenzo Stoakes wrote:
> > > On Sat, Oct 11, 2025 at 05:30:52PM +0800, Ye Liu wrote:
> > > > From: Ye Liu <liuye@xxxxxxxxxx>
> > > >
> > > > Hello MM maintainers and drgn community,
> > > >
> > > > This RFC proposes to convert VM_* flags from #define macros to enum
> > > > vm_flags. The motivation comes from recent drgn development where we
> > > > encountered difficulties in implementing VM flag parsing due to the
> > > > current macro-based approach.
> > >
> > > This isn't going to work sorry, it's not valid to have flag values as an enum
> >
> > I don't follow, can you elaborate? IIRC, the compiler will use an integer
> > type to back the enum that will fit all values.
>
> switch (flags) {
> 	case VAL1:
> 	case VAL2:
> 	etc.
> }
>
> Is broken (compiler will say you cover all cases when you don't...)

I assume you mean theoretically, because there is no such code, right?

> An enum implies independent values that exhaustively describe all state, however
> these flag values are not that - they're intended to be bit fields.

Observe how we use an enum for FOLL_* flags, vm_fault_reason, fault_flag 
and probably other things.

But more importantly,

enum pageflags { ... :)

--
Cheers

David / dhildenb