On 8/3/26 15:59, Christophe Leroy (CS GROUP) wrote:
Le 14/07/2026 à 16:03, Kevin Brodsky a écrit :
quoted
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