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

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Message-ID: <71803dce-3fa6-494c-a4b1-55d98fc4aadb@suse.cz>
Date: Mon, 13 Oct 2025 15:07:18 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 David Hildenbrand <david@...hat.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>,
 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 10/13/25 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).
> 
> But also 'we do X in the kernel' doesn't mean doing X is right :)

I think the example to follow could be GFP flags. Nowadays there's an enum
below it, and a layer that adds (__force gfp_t), so you could do similar
thing with vm_flags_t.

However I'm not sure how compatible is that with Lorenzo's plans.