Re: [PATCH] arm64: mm: force write fault for atomic RMW instructions

Anshuman Khandual
2024-05-08T06:45:16Z

Hello Yang,

On 5/8/24 04:05, Yang Shi wrote:
> The atomic RMW instructions, for example, ldadd, actually does load +
> add + store in one instruction, it may trigger two page faults, the
> first fault is a read fault, the second fault is a write fault.

It may or it will definitely create two consecutive page faults. What
if the second write fault never came about. In that case an writable
page table entry would be created unnecessarily (or even wrongfully),
thus breaking the CoW.

Just trying to understand, is the double page fault a possibility or
a certainty. Does that depend on architecture (please do provide some
links) or is it implementation defined.

>
> Some applications use atomic RMW instructions to populate memory, for
> example, openjdk uses atomic-add-0 to do pretouch (populate heap memory

But why cannot normal store operation is sufficient for pre-touching
the heap memory, why read-modify-write (RMW) is required instead ?

> at launch time) between v18 and v22.

V18, V22 ?

>
> But the double page fault has some problems:
>
> 1. Noticeable TLB overhead. The kernel actually installs zero page with
> readonly PTE for the read fault. The write fault will trigger a
> write-protection fault (CoW). The CoW will allocate a new page and
> make the PTE point to the new page, this needs TLB invalidations. The
> tlb invalidation and the mandatory memory barriers may incur
> significant overhead, particularly on the machines with many cores.
>
> 2. Break up huge pages. If THP is on the read fault will install huge
> zero pages. The later CoW will break up the huge page and allocate
> base pages instead of huge page. The applications have to rely on
> khugepaged (kernel thread) to collapse huge pages asynchronously.
> This also incurs noticeable performance penalty.
>
> 3. 512x page faults with huge page. Due to #2, the applications have to
> have page faults for every 4K area for the write, this makes the speed
> up by using huge page actually gone.

The problems mentioned above are reasonable and expected.

If the memory address has some valid data, it must have already reached there
via a previous write access, which would have caused initial CoW transition ?
If the memory address has no valid data to begin with, why even use RMW ?

>
> So it sounds pointless to have two page faults since we know the memory
> will be definitely written very soon. Forcing write fault for atomic RMW
> instruction makes some sense and it can solve the aforementioned problems:
>
> Firstly, it just allocates zero'ed page, no tlb invalidation and memory
> barriers anymore.
> Secondly, it can populate writable huge pages in the first place and
> don't break them up. Just one page fault is needed for 2M area instrad
> of 512 faults and also save cpu time by not using khugepaged.
>
> A simple micro benchmark which populates 1G memory shows the number of
> page faults is reduced by half and the time spent by system is reduced
> by 60% on a VM running on Ampere Altra platform.
>
> And the benchmark for anonymous read fault on 1G memory, file read fault
> on 1G file (cold page cache and warm page cache) don't show noticeable
> regression.
>
> Some other architectures also have code inspection in page fault path,
> for example, SPARC and x86.

Okay, I was about to ask, but is not calling get_user() for all data
read page faults increase the cost for a hot code path in general for
some potential savings for a very specific use case. Not sure if that
is worth the trade-off.

>
> Signed-off-by: Yang Shi <yang@os.amperecomputing.com>
> ---
> arch/arm64/include/asm/insn.h | 1 +
> arch/arm64/mm/fault.c | 19 +++++++++++++++++++
> 2 files changed, 20 insertions(+)
>
> diff --git a/arch/arm64/include/asm/insn.h b/arch/arm64/include/asm/insn.h
> index db1aeacd4cd9..5d5a3fbeecc0 100644
> --- a/arch/arm64/include/asm/insn.h
> +++ b/arch/arm64/include/asm/insn.h
> @@ -319,6 +319,7 @@ static __always_inline u32 aarch64_insn_get_##abbr##_value(void) \
> * "-" means "don't care"
> */
> __AARCH64_INSN_FUNCS(class_branch_sys, 0x1c000000, 0x14000000)
> +__AARCH64_INSN_FUNCS(class_atomic, 0x3b200c00, 0x38200000)
>
> __AARCH64_INSN_FUNCS(adr, 0x9F000000, 0x10000000)
> __AARCH64_INSN_FUNCS(adrp, 0x9F000000, 0x90000000)
> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> index 8251e2fea9c7..f7bceedf5ef3 100644
> --- a/arch/arm64/mm/fault.c
> +++ b/arch/arm64/mm/fault.c
> @@ -529,6 +529,7 @@ static int __kprobes do_page_fault(unsigned long far, unsigned long esr,
> unsigned int mm_flags = FAULT_FLAG_DEFAULT;
> unsigned long addr = untagged_addr(far);
> struct vm_area_struct *vma;
> + unsigned int insn;
>
> if (kprobe_page_fault(regs, esr))
> return 0;
> @@ -586,6 +587,24 @@ static int __kprobes do_page_fault(unsigned long far, unsigned long esr,
> if (!vma)
> goto lock_mmap;
>
> + if (mm_flags & (FAULT_FLAG_WRITE | FAULT_FLAG_INSTRUCTION))
> + goto continue_fault;
> +
> + pagefault_disable();
> +
> + if (get_user(insn, (unsigned int __user *) instruction_pointer(regs))) {
> + pagefault_enable();
> + goto continue_fault;
> + }
> +
> + if (aarch64_insn_is_class_atomic(insn)) {
> + vm_flags = VM_WRITE;
> + mm_flags |= FAULT_FLAG_WRITE;
> + }
> +
> + pagefault_enable();
> +
> +continue_fault:
> if (!(vma->vm_flags & vm_flags)) {
> vma_end_read(vma);
> goto lock_mmap;