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

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Message-ID: <dfc18351-bb77-4099-bcdd-eb2bd4f1bea5@redhat.com>
Date: Mon, 13 Oct 2025 14:31:35 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Ye Liu <ye.liu@...ux.dev>, Andrew Morton <akpm@...ux-foundation.org>,
 Ye Liu <liuye@...inos.cn>, "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
 Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC RFC PATCH] mm: convert VM flags from macros to enum

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@...inos.cn>
>>>>
>>>> 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