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

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Message-ID: <8e9d92c8-b2f6-4178-8a76-a6db935589ca@redhat.com>
Date: Mon, 13 Oct 2025 15:11:21 +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 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@...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?
> 
> 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