> On 24 Nov 2025, at 6:52 PM, Kevin Brodsky <[email protected]> wrote:
>
> When the lazy MMU mode was introduced eons ago, it wasn't made clear
> whether such a sequence was legal:
>
> arch_enter_lazy_mmu_mode()
> ...
> arch_enter_lazy_mmu_mode()
> ...
> arch_leave_lazy_mmu_mode()
> ...
> arch_leave_lazy_mmu_mode()
>
> It seems fair to say that nested calls to
> arch_{enter,leave}_lazy_mmu_mode() were not expected, and most
> architectures never explicitly supported it.
>
> Nesting does in fact occur in certain configurations, and avoiding it
> has proved difficult. This series therefore enables lazy_mmu sections to
> nest, on all architectures.
>
> Nesting is handled using a counter in task_struct (patch 8), like other
> stateless APIs such as pagefault_{disable,enable}(). This is fully
> handled in a new generic layer in <linux/pgtable.h>; the arch_* API
> remains unchanged. A new pair of calls, lazy_mmu_mode_{pause,resume}(),
> is also introduced to allow functions that are called with the lazy MMU
> mode enabled to temporarily pause it, regardless of nesting.
>
> An arch now opts in to using the lazy MMU mode by selecting
> CONFIG_ARCH_LAZY_MMU; this is more appropriate now that we have a
> generic API, especially with state conditionally added to task_struct.
>
> ---
>
> Background: Ryan Roberts' series from March [1] attempted to prevent
> nesting from ever occurring, and mostly succeeded. Unfortunately, a
> corner case (DEBUG_PAGEALLOC) may still cause nesting to occur on arm64.
> Ryan proposed [2] to address that corner case at the generic level but
> this approach received pushback; [3] then attempted to solve the issue
> on arm64 only, but it was deemed too fragile.
>
> It feels generally difficult to guarantee that lazy_mmu sections don't
> nest, because callers of various standard mm functions do not know if
> the function uses lazy_mmu itself.
>
> The overall approach in v3/v4 is very close to what David Hildenbrand
> proposed on v2 [4].
>
> Unlike in v1/v2, no special provision is made for architectures to
> save/restore extra state when entering/leaving the mode. Based on the
> discussions so far, this does not seem to be required - an arch can
> store any relevant state in thread_struct during arch_enter() and
> restore it in arch_leave(). Nesting is not a concern as these functions
> are only called at the top level, not in nested sections.
>
> The introduction of a generic layer, and tracking of the lazy MMU state
> in task_struct, also allows to streamline the arch callbacks - this
> series removes 67 lines from arch/.
>
> Patch overview:
>
> * Patch 1: cleanup - avoids having to deal with the powerpc
> context-switching code
>
> * Patch 2-4: prepare arch_flush_lazy_mmu_mode() to be called from the
> generic layer (patch 8)
>
> * Patch 5-6: new API + CONFIG_ARCH_LAZY_MMU
>
> * Patch 7: ensure correctness in interrupt context
>
> * Patch 8: nesting support
>
> * Patch 9-12: replace arch-specific tracking of lazy MMU mode with
> generic API
>
> This series has been tested by running the mm kselftests on arm64 with
> DEBUG_VM, DEBUG_PAGEALLOC, KFENCE and KASAN. It was also build-tested on
> other architectures (with and without XEN_PV on x86).
>
> - Kevin
>
> [1] https://lore.kernel.org/all/[email protected]/
> [2] https://lore.kernel.org/all/[email protected]/
> [3] https://lore.kernel.org/all/[email protected]/
> [4]
> https://lore.kernel.org/all/[email protected]/
> ---
> Changelog
>
> v4..v5:
>
> - Rebased on mm-unstable
> - Patch 3: added missing radix_enabled() check in arch_flush()
> [Ritesh Harjani]
> - Patch 6: declare arch_flush_lazy_mmu_mode() as static inline on x86
> [Ryan Roberts]
> - Patch 7 (formerly 12): moved before patch 8 to ensure correctness in
> interrupt context [Ryan]. The diffs in in_lazy_mmu_mode() and
> queue_pte_barriers() are moved to patch 8 and 9 resp.
> - Patch 8:
> * Removed all restrictions regarding lazy_mmu_mode_{pause,resume}().
> They may now be called even when lazy MMU isn't enabled, and
> any call to lazy_mmu_mode_* may be made while paused (such calls
> will be ignored). [David, Ryan]
> * lazy_mmu_state.{nesting_level,active} are replaced with
> {enable_count,pause_count} to track arbitrary nesting of both
> enable/disable and pause/resume [Ryan]
> * Added __task_lazy_mmu_mode_active() for use in patch 12 [David]
> * Added documentation for all the functions [Ryan]
> - Patch 9: keep existing test + set TIF_LAZY_MMU_PENDING instead of
> atomic RMW [David, Ryan]
> - Patch 12: use __task_lazy_mmu_mode_active() instead of accessing
> lazy_mmu_state directly [David]
> - Collected R-b/A-b tags
>
> v4:
> https://lore.kernel.org/all/[email protected]/
>
> v3..v4:
>
> - Patch 2: restored ordering of preempt_{disable,enable}() [Dave Hansen]
> - Patch 5 onwards: s/ARCH_LAZY_MMU/ARCH_HAS_LAZY_MMU_MODE/ [Mike Rapoport]
> - Patch 7: renamed lazy_mmu_state members, removed VM_BUG_ON(),
> reordered writes to lazy_mmu_state members [David Hildenbrand]
> - Dropped patch 13 as it doesn't seem justified [David H]
> - Various improvements to commit messages [David H]
>
> v3:
> https://lore.kernel.org/all/[email protected]/
>
> v2..v3:
>
> - Full rewrite; dropped all Acked-by/Reviewed-by.
> - Rebased on v6.18-rc1.
>
> v2:
> https://lore.kernel.org/all/[email protected]/
>
> v1..v2:
> - Rebased on mm-unstable.
> - Patch 2: handled new calls to enter()/leave(), clarified how the "flush"
> pattern (leave() followed by enter()) is handled.
> - Patch 5,6: removed unnecessary local variable [Alexander Gordeev's
> suggestion].
> - Added Mike Rapoport's Acked-by.
>
> v1:
> https://lore.kernel.org/all/[email protected]/
> ---
> Cc: Alexander Gordeev <[email protected]>
> Cc: Andreas Larsson <[email protected]>
> Cc: Andrew Morton <[email protected]>
> Cc: Boris Ostrovsky <[email protected]>
> Cc: Borislav Petkov <[email protected]>
> Cc: Catalin Marinas <[email protected]>
> Cc: Christophe Leroy <[email protected]>
> Cc: Dave Hansen <[email protected]>
> Cc: David Hildenbrand <[email protected]>
> Cc: "David S. Miller" <[email protected]>
> Cc: David Woodhouse <[email protected]>
> Cc: "H. Peter Anvin" <[email protected]>
> Cc: Ingo Molnar <[email protected]>
> Cc: Jann Horn <[email protected]>
> Cc: Juergen Gross <[email protected]>
> Cc: "Liam R. Howlett" <[email protected]>
> Cc: Lorenzo Stoakes <[email protected]>
> Cc: Madhavan Srinivasan <[email protected]>
> Cc: Michael Ellerman <[email protected]>
> Cc: Michal Hocko <[email protected]>
> Cc: Mike Rapoport <[email protected]>
> Cc: Nicholas Piggin <[email protected]>
> Cc: Peter Zijlstra <[email protected]>
> Cc: Ritesh Harjani (IBM) <[email protected]>
> Cc: Ryan Roberts <[email protected]>
> Cc: Suren Baghdasaryan <[email protected]>
> Cc: Thomas Gleixner <[email protected]>
> Cc: Venkat Rao Bagalkote <[email protected]>
> Cc: Vlastimil Babka <[email protected]>
> Cc: Will Deacon <[email protected]>
> Cc: Yeoreum Yun <[email protected]>
> Cc: [email protected]
> Cc: [email protected]
> Cc: [email protected]
> Cc: [email protected]
> Cc: [email protected]
> Cc: [email protected]
> ---
> Alexander Gordeev (1):
> powerpc/64s: Do not re-activate batched TLB flush
>
> Kevin Brodsky (11):
> x86/xen: simplify flush_lazy_mmu()
> powerpc/mm: implement arch_flush_lazy_mmu_mode()
> sparc/mm: implement arch_flush_lazy_mmu_mode()
> mm: introduce CONFIG_ARCH_HAS_LAZY_MMU_MODE
> mm: introduce generic lazy_mmu helpers
> mm: bail out of lazy_mmu_mode_* in interrupt context
> mm: enable lazy_mmu sections to nest
> arm64: mm: replace TIF_LAZY_MMU with in_lazy_mmu_mode()
> powerpc/mm: replace batch->active with in_lazy_mmu_mode()
> sparc/mm: replace batch->active with in_lazy_mmu_mode()
> x86/xen: use lazy_mmu_state when context-switching
>
> arch/arm64/Kconfig | 1 +
> arch/arm64/include/asm/pgtable.h | 41 +----
> arch/arm64/include/asm/thread_info.h | 3 +-
> arch/arm64/mm/mmu.c | 4 +-
> arch/arm64/mm/pageattr.c | 4 +-
> .../include/asm/book3s/64/tlbflush-hash.h | 20 ++-
> arch/powerpc/include/asm/thread_info.h | 2 -
> arch/powerpc/kernel/process.c | 25 ---
> arch/powerpc/mm/book3s64/hash_tlb.c | 10 +-
> arch/powerpc/mm/book3s64/subpage_prot.c | 4 +-
> arch/powerpc/platforms/Kconfig.cputype | 1 +
> arch/sparc/Kconfig | 1 +
> arch/sparc/include/asm/tlbflush_64.h | 5 +-
> arch/sparc/mm/tlb.c | 14 +-
> arch/x86/Kconfig | 1 +
> arch/x86/boot/compressed/misc.h | 1 +
> arch/x86/boot/startup/sme.c | 1 +
> arch/x86/include/asm/paravirt.h | 1 -
> arch/x86/include/asm/pgtable.h | 1 +
> arch/x86/include/asm/thread_info.h | 4 +-
> arch/x86/xen/enlighten_pv.c | 3 +-
> arch/x86/xen/mmu_pv.c | 6 +-
> fs/proc/task_mmu.c | 4 +-
> include/linux/mm_types_task.h | 5 +
> include/linux/pgtable.h | 147 +++++++++++++++++-
> include/linux/sched.h | 45 ++++++
> mm/Kconfig | 3 +
> mm/kasan/shadow.c | 8 +-
> mm/madvise.c | 18 +--
> mm/memory.c | 16 +-
> mm/migrate_device.c | 8 +-
> mm/mprotect.c | 4 +-
> mm/mremap.c | 4 +-
> mm/userfaultfd.c | 4 +-
> mm/vmalloc.c | 12 +-
> mm/vmscan.c | 12 +-
> 36 files changed, 282 insertions(+), 161 deletions(-)
Tested this patch series by applying on top of mm-unstable, on both HASH and
RADIX MMU, and all tests are passed on both MMU’s.
Ran: cache_shape, copyloops, mm from linux source, selftests/powerpc/ and ran
memory-hotplug from selftests/. Also ran below tests from avocado misc-test
repo.
Link to repo: https://github.com/avocado-framework-tests/avocado-misc-tests
avocado-misc-tests/memory/stutter.py
avocado-misc-tests/memory/eatmemory.py
avocado-misc-tests/memory/hugepage_sanity.py
avocado-misc-tests/memory/fork_mem.py
avocado-misc-tests/memory/memory_api.py
avocado-misc-tests/memory/mprotect.py
avocado-misc-tests/memory/vatest.py
avocado-misc-tests/memory/vatest.py.data/vatest.yaml
avocado-misc-tests/memory/transparent_hugepages.py
avocado-misc-tests/memory/transparent_hugepages_swapping.py
avocado-misc-tests/memory/transparent_hugepages_defrag.py
avocado-misc-tests/memory/ksm_poison.py
If its good enough, please add below tag for PowerPC changes.
Tested-by: Venkat Rao Bagalkote <[email protected]>
Regards,
Venkat.
>
>
> base-commit: 1f1edd95f9231ba58a1e535b10200cb1eeaf1f67
> --
> 2.51.2
>
