linux-kernel - Re: [v5 PATCH] arm64: mm: force write fault for atomic RMW instructions

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Date: Fri, 28 Jun 2024 09:57:37 -0700 (PDT)
From: "Christoph Lameter (Ampere)" <cl@...two.org>
To: Catalin Marinas <catalin.marinas@....com>
cc: Yang Shi <yang@...amperecomputing.com>, will@...nel.org, 
    anshuman.khandual@....com, david@...hat.com, scott@...amperecomputing.com, 
    linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [v5 PATCH] arm64: mm: force write fault for atomic RMW
 instructions

On Fri, 28 Jun 2024, Catalin Marinas wrote:

> On Wed, Jun 26, 2024 at 12:18:30PM -0700, Yang Shi wrote:
>> @@ -568,6 +596,12 @@ static int __kprobes do_page_fault(unsigned long far, unsigned long esr,
>>  	if (!vma)
>>  		goto lock_mmap;
>>
>> +	if ((vm_flags & VM_READ) && (vma->vm_flags & VM_WRITE) &&
>> +	    is_el0_atomic_instr(regs)) {
>> +		vm_flags = VM_WRITE;
>> +		mm_flags |= FAULT_FLAG_WRITE;
>> +	}
>
> The patch looks fine now and AFAICT there's no ABI change.
>
> However, before deciding whether to merge this patch, I'd like to
> understand why OpenJDK cannot use madvise(MADV_POPULATE_WRITE). This
> would be the portable (Linux) solution that works better on
> architectures without such atomic instructions (e.g. arm64 without LSE
> atomics). So fixing user-space would be my preferred solution.

Doing so would be requesting application code changes that are linux 
and ARM64 specific from applications running on Linux. A lot of these are 
proprietary.