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    SubjectRe: [PATCH] riscv: mm: try VMA lock-based page fault handling first
    On Tue, May 23, 2023 at 10:03 PM <guoren@kernel.org> wrote:
    >
    > > Attempt VMA lock-based page fault handling first, and fall back to the
    > > existing mmap_lock-based handling if that fails.
    > >
    > > A simple running the ebizzy benchmark on Lichee Pi 4A shows that
    > > PER_VMA_LOCK can improve the ebizzy benchmark by about 32.68%. In
    > Good improvement, I think VMA lock is worth to support in riscv.
    >
    > Please give more details about ebizzy, Is it
    > https://github.com/linux-test-project/ltp/blob/master/utils/benchmark/ebizzy-0.3/ebizzy.c
    > ?
    >
    > > theory, the more CPUs, the bigger improvement, but I don't have any
    > > HW platform which has more than 4 CPUs.
    > >
    > > This is the riscv variant of "x86/mm: try VMA lock-based page fault
    > > handling first".
    > >
    >
    > How about add Link tag here:
    > Link: https://lwn.net/Articles/906852/
    >
    > > Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
    > > ---
    > > Any performance numbers are welcome! Especially the numbers on HW
    > > platforms with 8 or more CPUs.
    > >
    > > arch/riscv/Kconfig | 1 +
    > > arch/riscv/mm/fault.c | 33 +++++++++++++++++++++++++++++++++
    > > 2 files changed, 34 insertions(+)
    > >
    > > diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
    > > index 62e84fee2cfd..b958f67f9a12 100644
    > > --- a/arch/riscv/Kconfig
    > > +++ b/arch/riscv/Kconfig
    > > @@ -42,6 +42,7 @@ config RISCV
    > > select ARCH_SUPPORTS_DEBUG_PAGEALLOC if MMU
    > > select ARCH_SUPPORTS_HUGETLBFS if MMU
    > > select ARCH_SUPPORTS_PAGE_TABLE_CHECK if MMU
    > > + select ARCH_SUPPORTS_PER_VMA_LOCK if MMU
    > > select ARCH_USE_MEMTEST
    > > select ARCH_USE_QUEUED_RWLOCKS
    > > select ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT if MMU
    > > diff --git a/arch/riscv/mm/fault.c b/arch/riscv/mm/fault.c
    > > index 8685f85a7474..eccdddf26f4b 100644
    > > --- a/arch/riscv/mm/fault.c
    > > +++ b/arch/riscv/mm/fault.c
    > > @@ -286,6 +286,36 @@ void handle_page_fault(struct pt_regs *regs)
    > > flags |= FAULT_FLAG_WRITE;
    > > else if (cause == EXC_INST_PAGE_FAULT)
    > > flags |= FAULT_FLAG_INSTRUCTION;
    > > +#ifdef CONFIG_PER_VMA_LOCK
    > > + if (!(flags & FAULT_FLAG_USER))
    > > + goto lock_mmap;
    > > +
    > > + vma = lock_vma_under_rcu(mm, addr);
    > > + if (!vma)
    > > + goto lock_mmap;
    > > +
    > > + if (unlikely(access_error(cause, vma))) {
    > > + vma_end_read(vma);
    > > + goto lock_mmap;
    > > + }
    > > +
    > > + fault = handle_mm_fault(vma, addr, flags | FAULT_FLAG_VMA_LOCK, regs);
    > > + vma_end_read(vma);
    > > +
    > > + if (!(fault & VM_FAULT_RETRY)) {
    > > + count_vm_vma_lock_event(VMA_LOCK_SUCCESS);
    > > + goto done;
    > > + }
    > > + count_vm_vma_lock_event(VMA_LOCK_RETRY);
    > > +
    > > + if (fault_signal_pending(fault, regs)) {
    > > + if (!user_mode(regs))
    > > + no_context(regs, addr);
    > > + return;
    > > + }
    > > +lock_mmap:
    > > +#endif /* CONFIG_PER_VMA_LOCK */
    > > +
    > > retry:
    > > mmap_read_lock(mm);
    > > vma = find_vma(mm, addr);
    > > @@ -355,6 +385,9 @@ void handle_page_fault(struct pt_regs *regs)
    > >
    > > mmap_read_unlock(mm);
    > >
    > > +#ifdef CONFIG_PER_VMA_LOCK
    > > +done:
    > > +#endif
    > It's very close to cd7f176aea5f ("arm64/mm: try VMA lock-based page fault
    > handling first"), and I didn't find any problem. So:
    >
    > Reviewed-by: Guo Ren <guoren@kernel.org>

    Looks correct to me.

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

    >
    > F.Y.I Huacai Chen, maybe he also would be interesting this new feature.
    >
    >
    > > if (unlikely(fault & VM_FAULT_ERROR)) {
    > > tsk->thread.bad_cause = cause;
    > > mm_fault_error(regs, addr, fault);
    > > --
    > > 2.40.1

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