>>>> - * We use break_ksm to break COW on a ksm page: it's a stripped down
>>>> + * We use break_ksm to break COW on a ksm page or KSM-placed zero page (only
>>>> + * happen when enabling use_zero_pages): it's a stripped down
>>>> *
>>>> * if (get_user_pages(addr, 1, FOLL_WRITE, &page, NULL) == 1)
>>>> * put_page(page);
>>>> @@ -434,7 +428,8 @@ static inline bool ksm_test_exit(struct mm_struct *mm)
>>>> * of the process that owns 'vma'. We also do not want to enforce
>>>> * protection keys here anyway.
>>>> */
>>>> -static int break_ksm(struct vm_area_struct *vma, unsigned long addr)
>>>> +static int break_ksm(struct vm_area_struct *vma, unsigned long addr,
>>>> + bool ksm_check_bypass)
>>>> {
>>>> struct page *page;
>>>> vm_fault_t ret = 0;
>>>> @@ -449,6 +444,16 @@ static int break_ksm(struct vm_area_struct *vma, unsigned long addr)
>>>> ret = handle_mm_fault(vma, addr,
>>>> FAULT_FLAG_WRITE | FAULT_FLAG_REMOTE,
>>>> NULL);
>>>> + else if (ksm_check_bypass && is_zero_pfn(page_to_pfn(page))) {
>>>> + /*
>>>> + * Although it's not ksm page, it's zero page as placed by
>>>> + * KSM use_zero_page, so we should unshare it when
>>>> + * ksm_check_bypass is true.
>>>> + */
>>>> + ret = handle_mm_fault(vma, addr,
>>>> + FAULT_FLAG_WRITE | FAULT_FLAG_REMOTE,
>>>> + NULL);
>>>> + }
>>>
>>> Please don't duplicate that page fault triggering code.
>>>
>>> Also, please be aware that this collides with
>>>
>>> https://lkml.kernel.org/r/20221021101141.84170-1-david@xxxxxxxxxx
>>>
>>> Adjustments should be comparatively easy.
>>
>> ... except that I'm still working on FAULT_FLAG_UNSHARE support for the
>> shared zeropage. That will be posted soonish (within next 2 weeks).
>>
>
>Posted: https://lkml.kernel.org/r/20221107161740.144456-1-david@xxxxxxxxxx
>
>With that, we can use FAULT_FLAG_UNSHARE also to break COW on the shared
>zeropage.
Sounds a better way for breaking COW working with reliable R/O long-tern pinning.
>--
>Thanks,
>
>David / dhildenb
