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

[David Hildenbrand]

On 13.10.25 14:57, Lorenzo Stoakes wrote:
> On Mon, Oct 13, 2025 at 02:31:35PM +0200, David Hildenbrand wrote:
> > 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?
>
> Right, it's a general point about why enum's are not such a great idea for this.
>
> > > 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.
>
> FOLL_* flags are an anonymous enum, enum fault_flag is not used as a type
> anywhere, nor is vm_fault_reason. So those are both kinda weird as to why we
> even name the type (they're in effect anonymous).

Well, there are of course more, even in MM.

For these cases, we indeed don't declare a type, but I am not really
sure if it makes a big difference here today.

I agree that once the flags are actually an opaque object (what you are
planning on to work on), it will be a different story.

> But also 'we do X in the kernel' doesn't mean doing X is right :)

Right, but it in general works. It's just nasty here in particular.

--
Cheers

David / dhildenb