[PATCH RFC v4 02/12] riscv/mm: add untagged_addr_remote_unlocked()

[Suren Baghdasaryan]

On Fri, Jul 24, 2026 at 3:30 PM Rik van Riel <riel at surriel.com> wrote:
>
> __access_remote_vm() reads another task's memory under the mmap lock. That
> lock is contended on large multi-threaded workloads, where an mmap() or
> munmap() stalls readers like /proc/PID/cmdline or process_vm_readv() even
> though the target memory is resident and reachable under the per-VMA lock.
>
> Reaching it under the per-VMA lock means looking up the VMA first, which
> requires untagging the remote address without the mmap lock. riscv's
> untagged_addr_remote() asserts that lock and has no unlocked variant; the
> generic untagged_addr_remote_unlocked() falls back to untagged_addr(),
> which masks against current->mm, not the target mm.
>
> Add untagged_addr_remote_unlocked(), which masks @addr against the target
> mm without asserting the lock, and redefine untagged_addr_remote() on top
> of it so it keeps the assertion.
>
> The mask width is mm->context.pmlen, written while a program is single
> threaded, and frozen with MM_CONTEXT_LOCK_PMLEN once it spawns a second
> thread.
>
> That read need not be stable: pmlen is already read locklessly by
> untagged_addr() and mm_untag_mask(), and a remote untag is best-effort.
> Annotate the lockless reads with READ_ONCE() and the writes with
> WRITE_ONCE(), so a reader sees one whole value and the accesses are
> KCSAN-clean.
>
> Assisted-by: Claude:claude-opus-4.8
> Signed-off-by: Rik van Riel <riel at surriel.com>

Reviewed-by: Suren Baghdasaryan <surenb at google.com>

> ---
>  arch/riscv/include/asm/mmu_context.h |  4 ++--
>  arch/riscv/include/asm/uaccess.h     | 10 +++++++---
>  arch/riscv/kernel/process.c          | 12 +++++++-----
>  3 files changed, 16 insertions(+), 10 deletions(-)
>
> diff --git a/arch/riscv/include/asm/mmu_context.h b/arch/riscv/include/asm/mmu_context.h
> index dbf27a78df6c..3ce16796e5a2 100644
> --- a/arch/riscv/include/asm/mmu_context.h
> +++ b/arch/riscv/include/asm/mmu_context.h
> @@ -21,7 +21,7 @@ static inline void activate_mm(struct mm_struct *prev,
>                                struct mm_struct *next)
>  {
>  #ifdef CONFIG_RISCV_ISA_SUPM
> -       next->context.pmlen = 0;
> +       WRITE_ONCE(next->context.pmlen, 0);
>  #endif
>         switch_mm(prev, next, NULL);
>  }
> @@ -44,7 +44,7 @@ DECLARE_STATIC_KEY_FALSE(use_asid_allocator);
>  #define mm_untag_mask mm_untag_mask
>  static inline unsigned long mm_untag_mask(struct mm_struct *mm)
>  {
> -       return -1UL >> mm->context.pmlen;
> +       return -1UL >> READ_ONCE(mm->context.pmlen);
>  }
>  #endif
>
> diff --git a/arch/riscv/include/asm/uaccess.h b/arch/riscv/include/asm/uaccess.h
> index 5d4ec15584cf..53806e0f7dcf 100644
> --- a/arch/riscv/include/asm/uaccess.h
> +++ b/arch/riscv/include/asm/uaccess.h
> @@ -16,7 +16,7 @@
>  static inline unsigned long __untagged_addr_remote(struct mm_struct *mm, unsigned long addr)
>  {
>         if (riscv_has_extension_unlikely(RISCV_ISA_EXT_SUPM)) {
> -               u8 pmlen = mm->context.pmlen;
> +               u8 pmlen = READ_ONCE(mm->context.pmlen);
>
>                 /* Virtual addresses are sign-extended; physical addresses are zero-extended. */
>                 if (IS_ENABLED(CONFIG_MMU))
> @@ -33,12 +33,16 @@ static inline unsigned long __untagged_addr_remote(struct mm_struct *mm, unsigne
>         (__force __typeof__(addr))__untagged_addr_remote(current->mm, __addr);  \
>  })
>
> -#define untagged_addr_remote(mm, addr) ({                                      \
> +#define untagged_addr_remote_unlocked(mm, addr) ({                             \
>         unsigned long __addr = (__force unsigned long)(addr);                   \
> -       mmap_assert_locked(mm);                                                 \
>         (__force __typeof__(addr))__untagged_addr_remote(mm, __addr);           \
>  })
>
> +#define untagged_addr_remote(mm, addr) ({                                      \
> +       mmap_assert_locked(mm);                                                 \
> +       untagged_addr_remote_unlocked(mm, addr);                                \
> +})
> +
>  #define access_ok(addr, size) likely(__access_ok(untagged_addr(addr), size))
>  #else
>  #define untagged_addr(addr) (addr)
> diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c
> index b2df7f72241a..6ae7552fed09 100644
> --- a/arch/riscv/kernel/process.c
> +++ b/arch/riscv/kernel/process.c
> @@ -357,13 +357,15 @@ long set_tagged_addr_ctrl(struct task_struct *task, unsigned long arg)
>         if (mmap_write_lock_killable(mm))
>                 return -EINTR;
>
> -       if (test_bit(MM_CONTEXT_LOCK_PMLEN, &mm->context.flags) && mm->context.pmlen != pmlen) {
> -               mmap_write_unlock(mm);
> -               return -EBUSY;
> +       if (test_bit(MM_CONTEXT_LOCK_PMLEN, &mm->context.flags)) {
> +               if (READ_ONCE(mm->context.pmlen) != pmlen) {
> +                       mmap_write_unlock(mm);
> +                       return -EBUSY;
> +               }
>         }
>
>         envcfg_update_bits(task, ENVCFG_PMM, pmm);
> -       mm->context.pmlen = pmlen;
> +       WRITE_ONCE(mm->context.pmlen, pmlen);
>
>         mmap_write_unlock(mm);
>
> @@ -394,7 +396,7 @@ long get_tagged_addr_ctrl(struct task_struct *task)
>                 break;
>         }
>
> -       if (task->mm->context.pmlen)
> +       if (READ_ONCE(task->mm->context.pmlen))
>                 ret |= PR_TAGGED_ADDR_ENABLE;
>
>         return ret;
> --
> 2.53.0-Meta
>
>