linux-kernel - Re: [PATCH v2 13/20] x86/mm: enable page table sharing

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Message-Id: <20250818094402.13385-1-linyongting@bytedance.com>
Date: Mon, 18 Aug 2025 17:44:02 +0800
From: Yongting Lin <linyongting@...edance.com>
To: anthony.yznaga@...cle.com,
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Subject: Re: [PATCH v2 13/20] x86/mm: enable page table sharing

Thank you! Anthony.

Yep, I checked the comments in arch/mm/x86/fault.c file which says as your
advices in previous email.


I changed my code in kernel 5.5 as below:

       if (unlikely(is_shared_vma) && ((fault & VM_FAULT_RETRY) &&
           (flags & FAULT_FLAG_ALLOW_RETRY) || fault_signal_pending(fault, regs)))
               mmap_read_unlock(mm);

BTW: I wrote some selftests in my github repostory, which perform
the basic function of mshare, and I will write some complicated cases
to support the new functions or defect found in mshare. For example,
once you support mshare as a VMA in KVM (just as the defeat viewed by 
Jann Horn), I will add extra test cases to verify its correctiness for 
this scenario.

>From Jann Horn's review:
https://lore.kernel.org/all/CAG48ez3cUZf+xOtP6UkkS2-CmOeo+3K5pvny0AFL_XBkHh5q_g@mail.gmail.com/

Currently, I put my selftest in my github repostory, and you could retrieve it
as below:

    git remote add yongting-mshare-selftests https://github.com/ivanalgo/linux-kernel-develop/
    git fetch yongting-mshare-selftests dev-mshare-v2-selftest-v1
    git cherry-pick a64f2ff6497d13c09badc0fc68c44d9995bc2fef

At this stage, I am not sure what is the best way to proceed:
- Should I send them as part of your next version (v3)?
- Or should I post them separately as [RFC PATCH] for early review?

Please let me know your preference and any sugestion is welcome.
I am happy to rebase and resend in the format that works best for
the community.

Thanks
Yongting

> Anthony
>
>>
>> As a result, needs to release vma->vm_mm.mmap_lock as well.
>>
>> So it is supposed to be like below:
>>
>> -    fault = handle_mm_fault(vma, address, flags, regs);
>> +    fault = handle_mm_fault(vma, addr, flags, regs);
>> +
>> +    if (unlikely(is_shared_vma) && ((fault & VM_FAULT_COMPLETED) ||
>> +        (fault & VM_FAULT_RETRY) || fault_signal_pending(fault, regs))) {
>> +        mmap_read_unlock(vma->vm_mm);
>> +        mmap_read_unlock(mm);
>> +    }
>>
>>>         if (fault_signal_pending(fault, regs)) {
>>>           /*
>>> @@ -1413,6 +1446,8 @@ void do_user_addr_fault(struct pt_regs *regs,
>>>           goto retry;
>>>       }
>>>   +    if (unlikely(is_shared_vma))
>>> +        mmap_read_unlock(vma->vm_mm);
>>>       mmap_read_unlock(mm);
>>>   done:
>>>       if (likely(!(fault & VM_FAULT_ERROR)))
>>> diff --git a/mm/Kconfig b/mm/Kconfig
>>> index e6c90db83d01..8a5a159457f2 100644
>>> --- a/mm/Kconfig
>>> +++ b/mm/Kconfig
>>> @@ -1344,7 +1344,7 @@ config PT_RECLAIM
>>>     config MSHARE
>>>       bool "Mshare"
>>> -    depends on MMU
>>> +    depends on MMU && ARCH_SUPPORTS_MSHARE
>>>       help
>>>         Enable msharefs: A ram-based filesystem that allows multiple
>>>         processes to share page table entries for shared pages. A file 
>>
>> Yongting Lin. 
>
>