linux-kernel - Re: [PATCH v3 2/5] ksm: support unsharing zero pages placed by KSM

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Date:   Mon, 14 Nov 2022 03:02:36 +0000
From:   xu xin <xu.xin.sc@...il.com>
To:     david@...hat.com
Cc:     akpm@...ux-foundation.org, imbrenda@...ux.ibm.com,
        jiang.xuexin@....com.cn, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, ran.xiaokai@....com.cn, xu.xin.sc@...il.com,
        xu.xin16@....com.cn, yang.yang29@....com.cn
Subject: Re: [PATCH v3 2/5] ksm: support unsharing zero pages placed by KSM

>>>> - * 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@redhat.com
>>>
>>> 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@redhat.com
>
>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