| Message ID | 20260922-rcu-pagetable-freeing-v4-4-fe1ad1f1e303@kernel.org |
|---|---|
| State | New |
| Headers | show |
| Series | mm: make userland page table freeing RCU-safe | expand |
On Tue, Sep 22, 2026 at 04:35:35PM +0100, Lorenzo Stoakes (ARM) wrote: >Commit a0ad5496b2b3 ("arm: mm: enable HAVE_RCU_TABLE_FREE logic") enabled >CONFIG_MMU_GATHER_RCU_TABLE_FREE (then named HAVE_RCU_TABLE_FREE) for SMP >arm architectures with LPAE enabled. > >Regardless of whether CONFIG_ARM_LPAE is enabled or not, the same page >table freeing functions __pte_free_tlb() and __pmd_free_tlb() are used. > >Non-LPAE PMD page tables are folded into the PGD and freed by >pgd_free() (PGD freeing is not part of mmu_gather page table freeing in any >case), so this is a noop in this case. > >Since commit 358d1c39c82a ("arm: convert various functions to use ptdescs") >both LPAE and non-LPAE PTE page table freeing uses tlb_remove_ptdesc(). > >Thus all page table freeing is performed under RCU with >CONFIG_MMU_GATHER_RCU_TABLE_FREE enabled for LPAE and non-LPAE and thus it >need not be gated on LPAE. > >A UP arm system can set CONFIG_PREEMPT_RCU, so a future pure RCU page >table walker requires MMU_GATHER_RCU_TABLE_FREE to be enabled on UP as >well, even if concurrent GUP fast is not possible there. > >Therefore, it is both safe and desirable to set >CONFIG_MMU_GATHER_RCU_TABLE_FREE for all MMU arm architectures (nommu does >not perform mmu_gather operations). > >This forms part of an overall effort to switch every architecture to this >mode. > >Acked-by: Kiryl Shutsemau (Meta) <kas@kernel.org> >Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org> >--- Tested under heavy page-table teardown load on qemu-system-arm, with and without ARM_LPAE. No issues seen :D Tested-by: Lance Yang <lance.yang@linux.dev> Also, feel free to add: Acked-by: Lance Yang <lance.yang@linux.dev> > arch/arm/Kconfig | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > >diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig >index 408aa58a2a5b..72b9afc6ae10 100644 >--- a/arch/arm/Kconfig >+++ b/arch/arm/Kconfig >@@ -134,7 +134,7 @@ config ARM > select HAVE_PERF_REGS > select HAVE_PERF_USER_STACK_DUMP > select HAVE_POSIX_CPU_TIMERS_TASK_WORK >- select MMU_GATHER_RCU_TABLE_FREE if SMP && ARM_LPAE >+ select MMU_GATHER_RCU_TABLE_FREE if MMU > select HAVE_REGS_AND_STACK_ACCESS_API > select HAVE_RSEQ > select HAVE_RUST if CPU_LITTLE_ENDIAN && CPU_32v7 && !KASAN > >-- >2.55.0 > >
On Wed, Sep 23, 2026 at 04:05:23PM +0800, Lance Yang wrote: > > On Tue, Sep 22, 2026 at 04:35:35PM +0100, Lorenzo Stoakes (ARM) wrote: > >Commit a0ad5496b2b3 ("arm: mm: enable HAVE_RCU_TABLE_FREE logic") enabled > >CONFIG_MMU_GATHER_RCU_TABLE_FREE (then named HAVE_RCU_TABLE_FREE) for SMP > >arm architectures with LPAE enabled. > > > >Regardless of whether CONFIG_ARM_LPAE is enabled or not, the same page > >table freeing functions __pte_free_tlb() and __pmd_free_tlb() are used. > > > >Non-LPAE PMD page tables are folded into the PGD and freed by > >pgd_free() (PGD freeing is not part of mmu_gather page table freeing in any > >case), so this is a noop in this case. > > > >Since commit 358d1c39c82a ("arm: convert various functions to use ptdescs") > >both LPAE and non-LPAE PTE page table freeing uses tlb_remove_ptdesc(). > > > >Thus all page table freeing is performed under RCU with > >CONFIG_MMU_GATHER_RCU_TABLE_FREE enabled for LPAE and non-LPAE and thus it > >need not be gated on LPAE. > > > >A UP arm system can set CONFIG_PREEMPT_RCU, so a future pure RCU page > >table walker requires MMU_GATHER_RCU_TABLE_FREE to be enabled on UP as > >well, even if concurrent GUP fast is not possible there. > > > >Therefore, it is both safe and desirable to set > >CONFIG_MMU_GATHER_RCU_TABLE_FREE for all MMU arm architectures (nommu does > >not perform mmu_gather operations). > > > >This forms part of an overall effort to switch every architecture to this > >mode. > > > >Acked-by: Kiryl Shutsemau (Meta) <kas@kernel.org> > >Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org> > >--- > > Tested under heavy page-table teardown load on qemu-system-arm, > with and without ARM_LPAE. No issues seen :D > > Tested-by: Lance Yang <lance.yang@linux.dev> > > Also, feel free to add: > > Acked-by: Lance Yang <lance.yang@linux.dev> Thanks, much appreciated! :) > > > > arch/arm/Kconfig | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > >diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig > >index 408aa58a2a5b..72b9afc6ae10 100644 > >--- a/arch/arm/Kconfig > >+++ b/arch/arm/Kconfig > >@@ -134,7 +134,7 @@ config ARM > > select HAVE_PERF_REGS > > select HAVE_PERF_USER_STACK_DUMP > > select HAVE_POSIX_CPU_TIMERS_TASK_WORK > >- select MMU_GATHER_RCU_TABLE_FREE if SMP && ARM_LPAE > >+ select MMU_GATHER_RCU_TABLE_FREE if MMU > > select HAVE_REGS_AND_STACK_ACCESS_API > > select HAVE_RSEQ > > select HAVE_RUST if CPU_LITTLE_ENDIAN && CPU_32v7 && !KASAN > > > >-- > >2.55.0 > > > > -- Cheers, Lorenzo
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig index 408aa58a2a5b..72b9afc6ae10 100644 --- a/arch/arm/Kconfig +++ b/arch/arm/Kconfig @@ -134,7 +134,7 @@ config ARM select HAVE_PERF_REGS select HAVE_PERF_USER_STACK_DUMP select HAVE_POSIX_CPU_TIMERS_TASK_WORK - select MMU_GATHER_RCU_TABLE_FREE if SMP && ARM_LPAE + select MMU_GATHER_RCU_TABLE_FREE if MMU select HAVE_REGS_AND_STACK_ACCESS_API select HAVE_RSEQ select HAVE_RUST if CPU_LITTLE_ENDIAN && CPU_32v7 && !KASAN
Commit a0ad5496b2b3 ("arm: mm: enable HAVE_RCU_TABLE_FREE logic") enabled CONFIG_MMU_GATHER_RCU_TABLE_FREE (then named HAVE_RCU_TABLE_FREE) for SMP arm architectures with LPAE enabled. Regardless of whether CONFIG_ARM_LPAE is enabled or not, the same page table freeing functions __pte_free_tlb() and __pmd_free_tlb() are used. Non-LPAE PMD page tables are folded into the PGD and freed by pgd_free() (PGD freeing is not part of mmu_gather page table freeing in any case), so this is a noop in this case. Since commit 358d1c39c82a ("arm: convert various functions to use ptdescs") both LPAE and non-LPAE PTE page table freeing uses tlb_remove_ptdesc(). Thus all page table freeing is performed under RCU with CONFIG_MMU_GATHER_RCU_TABLE_FREE enabled for LPAE and non-LPAE and thus it need not be gated on LPAE. A UP arm system can set CONFIG_PREEMPT_RCU, so a future pure RCU page table walker requires MMU_GATHER_RCU_TABLE_FREE to be enabled on UP as well, even if concurrent GUP fast is not possible there. Therefore, it is both safe and desirable to set CONFIG_MMU_GATHER_RCU_TABLE_FREE for all MMU arm architectures (nommu does not perform mmu_gather operations). This forms part of an overall effort to switch every architecture to this mode. Acked-by: Kiryl Shutsemau (Meta) <kas@kernel.org> Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org> --- arch/arm/Kconfig | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)