On 11/08/2026 15:25, David Laight wrote:
On Mon, 3 Aug 2026 15:59:35 +0200
"Christophe Leroy (CS GROUP)" [off-list ref] wrote:
quoted
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
...
Thought...
Could the check be replaced by one that checks for kernel static data?
That would require that other mm that represent part of the kernel
address space be static (and probably not in modules).
ISTR there is an address range check that can be used - and might be
cheaper than the explicit test for init_mm on many 64bit archs.
That is actually an interesting idea. Finding those "kernel mms" boils
down to looking for global mm_struct, so checking if mm points to global
data would achieve essentially the same thing without a flag (and likely
faster). I'm not sure we want to bake in the assumption that "kernel mm"
is equivalent to "mm is global" though.
- Kevin