Page table constructors and destructors were originally introduced
specifically for user PTE pages. Over time, their use became more
widespread, and starting from v6.16 they are called at all levels
and for both user and kernel page tables [1]. Or so I thought. Turns
out, we still have various special kernel page tables where the
ctor/dtor are not called; vmemmap page tables is one such case.
Looking closer, we do not actually need per-level ctors for kernel
page tables. In fact we do not need ctors for them at all, because
nothing is specific to kernel page tables in the ctor/dtor and
nowadays they should be allocated with pagetable_alloc().
Here's the plan: all the generic page table handling moves to
pagetable_{alloc,free}(). Whatever is left is specific to user page
tables; the ctor/dtor are only called for user page tables at the
required levels (PTE/PMD), and renamed to make that clear.
Doing this will allow us to remove a good amount of special casing,
but there's quite a bit of churn to get there. Here's what this
series does:
1. Introduce the concept of "kernel mm" and replace most comparisons
with &init_mm with a flag check (patch 1-12); no functional change
so far.
2. Move the page table accounting/type setting to pagetable_{alloc,free}
(patch 13-15). Kernel mm's do not need to call the ctor/dtor any
more.
3. Mark special mm's (efi_mm, tboot_mm on x86) as kernel mm's (patch
16-19). They now abide by the same rules as init_mm, which means
in particular that split page table locks are no longer used.
4. Remove the ctor/dtor calls (now no-ops) for special kernel page
tables on arm/arm64/riscv (patch 20-22).
[1] https://lore.kernel.org/all/[email protected]/
---
I stopped there for part 1 to keep it digestible. Part 2 will then
ensure that the ctor/dtor are only called for user PTE/PMD pages and
rename them accordingly. This is comparatively simpler but also
involves a lot of churn.
This series was suggested in reaction to [2]; removing dtor calls
entirely for kernel page tables systematically solves such issues.
Worth noting that some comparisons against &init_mm have been left
untouched, as they are truly specific to init_mm. Reviewers are
encouraged to take a look at the remaining cases:
$ git grep -nE '([=!]= *&init_mm|&init_mm *[=!]=)'
Documentation/mm/active_mm.rst:78: if (current->mm == &init_mm)
Documentation/translations/zh_CN/mm/active_mm.rst:76: if (current->mm ==
&init_mm)
arch/arm/include/asm/mmu_context.h:144: if (mm != &init_mm)
arch/arm64/include/asm/mmu_context.h:100: if (mm != &init_mm &&
!system_uses_ttbr0_pan())
arch/arm64/include/asm/mmu_context.h:210: if (mm == &init_mm)
arch/arm64/include/asm/mmu_context.h:242: if (next == &init_mm) {
arch/arm64/mm/fault.c:121: if (mm == &init_mm)
arch/arm64/mm/fault.c:139: if (mm == &init_mm) {
arch/arm64/mm/fault.c:154: mm == &init_mm ? "swapper" : "user",
PAGE_SIZE / SZ_1K,
arch/arm64/mm/mmu.c:997: BUG_ON(mm == &init_mm);
arch/powerpc/xmon/xmon.c:3314: if (mm == &init_mm)
arch/riscv/mm/fault.c:42: mm == &init_mm ?
(u64)__pa_symbol(mm->pgd) : virt_to_phys(mm->pgd));
kernel/cpu.c:917: WARN_ON(mm != &init_mm);
kernel/fork.c:725: BUG_ON(mm == &init_mm);
kernel/sched/core.c:8390: if (mm != &init_mm) {
mm/ptdump.c:182: if (mm != &init_mm)
mm/ptdump.c:191: if (mm != &init_mm)
Testing:
* Built all modified architectures
* arm64
- mm kselftests (FVP)
- Hot adding/removing memory + memory-hotplug kselftests (QEMU)
- Reading /sys/kernel/debug/{efi,kernel}_page_tables
- Reading from /sys/firmware/efi/efivars/
*riscv
- memory-hotplug kselftests (QEMU)
* x86
- mm kselftests (lkvm)
- tboot page table setup using a fake tboot shared page (QEMU)
- CONFIG_EFI_PGT_DUMP=y (QEMU)
Patches based on mm-unstable.
Signed-off-by: Kevin Brodsky <[email protected]>
[2] https://lore.kernel.org/all/[email protected]/
---
To: [email protected]
Cc: Andrew Morton <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Lorenzo Stoakes <[email protected]>
Cc: "Liam R. Howlett" <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Mike Rapoport <[email protected]>
Cc: Suren Baghdasaryan <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Pasha Tatashin <[email protected]>
Cc: Russell King <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: Ryan Roberts <[email protected]>
Cc: [email protected]
Cc: Huacai Chen <[email protected]>
Cc: [email protected]
Cc: "James E.J. Bottomley" <[email protected]>
Cc: Helge Deller <[email protected]>
Cc: [email protected]
Cc: Madhavan Srinivasan <[email protected]>
Cc: Michael Ellerman <[email protected]>
Cc: [email protected]
Cc: Paul Walmsley <[email protected]>
Cc: Palmer Dabbelt <[email protected]>
Cc: Albert Ou <[email protected]>
Cc: [email protected]
Cc: Heiko Carstens <[email protected]>
Cc: Vasily Gorbik <[email protected]>
Cc: Alexander Gordeev <[email protected]>
Cc: Gerald Schaefer <[email protected]>
Cc: [email protected]
Cc: "David S. Miller" <[email protected]>
Cc: Andreas Larsson <[email protected]>
Cc: [email protected]
Cc: Richard Weinberger <[email protected]>
Cc: Anton Ivanov <[email protected]>
Cc: Johannes Berg <[email protected]>
Cc: [email protected]
Cc: Thomas Gleixner <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Ning Sun <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: Ard Biesheuvel <[email protected]>
Cc: Ilias Apalodimas <[email protected]>
Cc: [email protected]
Cc: Vishal Moola <[email protected]>
Cc: Alistair Popple <[email protected]>
Cc: "Matthew Wilcox (Oracle)" <[email protected]>
Cc: [email protected]
Cc: [email protected]
---
Kevin Brodsky (22):
mm: drop unused __mm_flags_set_mask_bits_word()
mm: move mm_flags helpers to mm_types.h
mm: introduce MMF_KERNEL flag and set it for init_mm
mm: use mm_is_kernel() in generic page table code
arm64: mm: use mm_is_kernel() for kernel mm checks
loongarch: mm: use mm_is_kernel() in switch_mm_irqs_off()
parisc: mm: use mm_is_kernel() for kernel mm checks
powerpc: mm: use mm_is_kernel() for kernel mm checks
s390: mm: use mm_is_kernel() for kernel mm checks
sparc: mm: use mm_is_kernel() for kernel mm checks
um: mm: use mm_is_kernel() in TLB sync
x86/mm: use mm_is_kernel() for kernel mm checks
mm: account page table pages when allocated
mm: set page table page type when allocated
mm: only initialise pt_share_count for user pgtables
efi: mark efi_mm as a kernel mm
mm: pagewalk: drop redundant address check for kernel mm walks
arm64: mm: drop explicit mm_is_efi() check in contpte
x86/tboot: mark tboot_mm as a kernel mm
arm64: mm: drop ctor/dtor calls for kernel page tables
arm: mm: drop ctor call for kernel page tables
riscv: mm: drop ctor/dtor calls for kernel page tables
arch/arm/mm/mmu.c | 3 +-
arch/arm64/include/asm/pgalloc.h | 10 ++--
arch/arm64/mm/contpte.c | 26 ++++------
arch/arm64/mm/hugetlbpage.c | 2 +-
arch/arm64/mm/mmu.c | 82 +++++++++-----------------------
arch/loongarch/include/asm/mmu_context.h | 2 +-
arch/parisc/include/asm/mmu_context.h | 2 +-
arch/parisc/include/asm/pgalloc.h | 2 +-
arch/parisc/include/asm/tlbflush.h | 2 +-
arch/powerpc/include/asm/pgalloc.h | 2 +-
arch/powerpc/mm/book3s64/pgtable.c | 2 +-
arch/powerpc/mm/mmu_context.c | 2 +-
arch/powerpc/mm/pgtable.c | 2 +-
arch/riscv/mm/init.c | 63 ++++++------------------
arch/s390/include/asm/mmu_context.h | 2 +-
arch/s390/include/asm/pgtable.h | 6 +--
arch/s390/mm/pgalloc.c | 4 +-
arch/sparc/include/asm/mmu_context_64.h | 2 +-
arch/sparc/include/asm/pgtable_64.h | 6 +--
arch/sparc/mm/init_64.c | 2 +-
arch/sparc/mm/tlb.c | 2 +-
arch/um/kernel/tlb.c | 2 +-
arch/x86/include/asm/tlbflush.h | 8 ++--
arch/x86/kernel/tboot.c | 4 +-
arch/x86/mm/pgtable.c | 2 +-
arch/x86/mm/tlb.c | 9 ++--
drivers/firmware/efi/efi.c | 1 +
include/asm-generic/pgalloc.h | 16 +++----
include/linux/mm.h | 69 +++++++--------------------
include/linux/mm_types.h | 52 +++++++++++++++-----
mm/init-mm.c | 1 +
mm/memory.c | 6 +--
mm/page_table_check.c | 14 +++---
mm/pagewalk.c | 7 +--
34 files changed, 163 insertions(+), 254 deletions(-)
---
base-commit: a531f0cf705af3374388eafe4115f651c625d8db
change-id: 20260611-remove_pgtable_cdtor-3b123938f8cd
