Re: [PATCH 03/22] mm: introduce MMF_KERNEL flag and set it for init_mm

From: "David Hildenbrand (Arm)" <david@kernel.org>
To: "Christophe Leroy (CS GROUP)" <chleroy@kernel.org>,
	Kevin Brodsky <kevin.brodsky@arm.com>,
	linux-mm@kvack.org
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Subject: Re: [PATCH 03/22] mm: introduce MMF_KERNEL flag and set it for init_mm
Date: Wed, 5 Aug 2026 14:07:01 +0200	[thread overview]
Message-ID: <33fc9d31-2866-4315-9510-0004deb93994@kernel.org> (raw)
In-Reply-To: <01f07ea2-8bdd-4450-8a49-548f8b646cfd@kernel.org>

On 8/3/26 15:59, Christophe Leroy (CS GROUP) wrote:
> 
> 
> Le 14/07/2026 à 16:03, Kevin Brodsky a écrit :
>> mm code often needs to know whether some mm represents a kernel or
>> user address space. This is currently done by comparing the mm
>> pointer with &init_mm; besides not being particularly elegant, this
>> ignores the fact that other mm's (e.g. efi_mm) may also represent
>> parts of the kernel address space.
>>
>> Introduce a new mm flag MMF_KERNEL and set it for init_mm.
>> Subsequent patches will use this flag to replace comparisons with
>> &init_mm. No functional change is introduced for now.
> 
> Did you consider performance impact ? This test is usually done in quite
> critical memory handling functions.
> 
> init_mm is known at link time. Before your patch 08/22 there is just a
> comparison of mm (r3) with a constant (loaded in r10):
> 
> c0014048 <assert_pte_locked>:
> c0014048:    3d 40 c1 09     lis     r10,-16119
> c001404c:    39 4a 03 98     addi    r10,r10,920
> c0014050:    7c 03 50 00     cmpw    r3,r10
> c0014054:    4d 82 00 20     beqlr
> ...
> 
> After patch 08/22 we have, it first checks that mm is not 0, then it loads the
> word located at mm+528 then AND it with 0x1. This load might be costly.
> 
> c0014048 <assert_pte_locked>:
> c0014048:    2c 03 00 00     cmpwi   r3,0
> c001404c:    7c 85 23 78     mr      r5,r4
> c0014050:    41 82 00 10     beq     c0014060 <assert_pte_locked+0x18>
> c0014054:    81 23 02 10     lwz     r9,528(r3)
> c0014058:    71 29 00 01     andi.   r9,r9,1
> c001405c:    4c 82 00 20     bnelr

Is that a real problem, though?

What we could do is to just keep the == &init_mm check internally on configs
where we know that there is only a single such MM context.

Just a thought.

-- 
Cheers,

David

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