Messages in this thread |  | | Subject | Re: [PATCH v5 00/12] Nesting support for lazy MMU mode | | From | Venkat <> | | Date | Wed, 3 Dec 2025 21:38:46 +0530 |
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> On 24 Nov 2025, at 6:52 PM, Kevin Brodsky <kevin.brodsky@arm.com> 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/20250303141542.3371656-1-ryan.roberts@arm.com/ > [2] https://lore.kernel.org/all/20250530140446.2387131-1-ryan.roberts@arm.com/ > [3] https://lore.kernel.org/all/20250606135654.178300-1-ryan.roberts@arm.com/ > [4] https://lore.kernel.org/all/ef343405-c394-4763-a79f-21381f217b6c@redhat.com/ > --- > 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/20251029100909.3381140-1-kevin.brodsky@arm.com/ > > 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/20251015082727.2395128-1-kevin.brodsky@arm.com/ > > v2..v3: > > - Full rewrite; dropped all Acked-by/Reviewed-by. > - Rebased on v6.18-rc1. > > v2: https://lore.kernel.org/all/20250908073931.4159362-1-kevin.brodsky@arm.com/ > > 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/20250904125736.3918646-1-kevin.brodsky@arm.com/ > --- > Cc: Alexander Gordeev <agordeev@linux.ibm.com> > Cc: Andreas Larsson <andreas@gaisler.com> > Cc: Andrew Morton <akpm@linux-foundation.org> > Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com> > Cc: Borislav Petkov <bp@alien8.de> > Cc: Catalin Marinas <catalin.marinas@arm.com> > Cc: Christophe Leroy <christophe.leroy@csgroup.eu> > Cc: Dave Hansen <dave.hansen@linux.intel.com> > Cc: David Hildenbrand <david@redhat.com> > Cc: "David S. Miller" <davem@davemloft.net> > Cc: David Woodhouse <dwmw2@infradead.org> > Cc: "H. Peter Anvin" <hpa@zytor.com> > Cc: Ingo Molnar <mingo@redhat.com> > Cc: Jann Horn <jannh@google.com> > Cc: Juergen Gross <jgross@suse.com> > Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com> > Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com> > Cc: Madhavan Srinivasan <maddy@linux.ibm.com> > Cc: Michael Ellerman <mpe@ellerman.id.au> > Cc: Michal Hocko <mhocko@suse.com> > Cc: Mike Rapoport <rppt@kernel.org> > Cc: Nicholas Piggin <npiggin@gmail.com> > Cc: Peter Zijlstra <peterz@infradead.org> > Cc: Ritesh Harjani (IBM) <ritesh.list@gmail.com> > Cc: Ryan Roberts <ryan.roberts@arm.com> > Cc: Suren Baghdasaryan <surenb@google.com> > Cc: Thomas Gleixner <tglx@linutronix.de> > Cc: Venkat Rao Bagalkote <venkat88@linux.ibm.com> > Cc: Vlastimil Babka <vbabka@suse.cz> > Cc: Will Deacon <will@kernel.org> > Cc: Yeoreum Yun <yeoreum.yun@arm.com> > Cc: linux-arm-kernel@lists.infradead.org > Cc: linux-kernel@vger.kernel.org > Cc: linuxppc-dev@lists.ozlabs.org > Cc: sparclinux@vger.kernel.org > Cc: xen-devel@lists.xenproject.org > Cc: x86@kernel.org > --- > 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 <venkat88@linux.ibm.com>
Regards, Venkat. > > > base-commit: 1f1edd95f9231ba58a1e535b10200cb1eeaf1f67 > -- > 2.51.2 >
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