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

[David Laight <david.laight.linux@gmail.com>]

On Tue, 11 Aug 2026 11:00:09 +0200
Kevin Brodsky <kevin.brodsky@arm.com> wrote:

> On 11/08/2026 10:10, David Laight wrote:
> > On Wed, 5 Aug 2026 14:07:01 +0200
> > "David Hildenbrand (Arm)" <david@kernel.org> wrote:
> >  
> >> 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?  
> > The cost of the read is likely to matter most if the branch gets mispredicted.
> > I'd also guess the mm isn't usually NULL - so that branch needs to statically
> > predicted correctly as well.
> > I'd also not assume that the mm is in the cache (unless the surrounding code
> > has already accessed it), the L1 data caches are small.  
> 
> You're right, I didn't think about this the right way round - in general
> it may not be easy to predict that mm points to init_mm, and an
> arbitrary user mm isn't guaranteed to be cached. Repeated calls to
> mm_is_kernel(mm) in the same function shouldn't cost much though.

Depends.
If it is real function call the code will be in the I-cache and is likely
to get predicted correctly by the third call.
OTOH if inlined you've all the I-cache loads and that static branch
prediction might get them all wrong.

Oh, and saving the result in a local might cause a register spill.

	David

> 
> Would some (micro)benchmark be particularly relevant to measure the
> overhead, if any?
> 
> - Kevin