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Subject: Re: [PATCH 5/7] mm: free user PTE page table pages
To:     =?UTF-8?Q?Mika_Penttil=c3=a4?= <mika.penttila@nextfour.com>,
        akpm@linux-foundation.org, tglx@linutronix.de, hannes@cmpxchg.org,
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Cc:     linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, songmuchun@bytedance.com
References: <20210718043034.76431-1-zhengqi.arch@bytedance.com>
 <20210718043034.76431-6-zhengqi.arch@bytedance.com>
 <9c3c87d5-e64e-f13f-ef36-b438e4de1e66@nextfour.com>
 <80b7d7fc-9d6d-0d1b-a333-b0ccd856e7c1@bytedance.com>
 <7fe2dd75-9b48-9685-8986-27a4cecc840f@nextfour.com>
From:   Qi Zheng <zhengqi.arch@bytedance.com>
Message-ID: <5ffff30c-59d7-fbb0-e3c2-fe9e47fc4658@bytedance.com>
Date:   Mon, 19 Jul 2021 22:12:04 +0800
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On 7/19/21 9:55 PM, Mika Penttilä wrote:
> 
> 
> On 19.7.2021 15.56, Qi Zheng wrote:
>> On 7/18/21 2:19 PM, Mika Penttilä wrote:
>>
>>>> +
>>>> +/*
>>>> + * returns true if the pmd has been populated with PTE page table,
>>>> + * or false for all other cases.
>>>> + */
>>>> +bool pte_install_try_get(struct mm_struct *mm, pmd_t *pmd, 
>>>> pgtable_t *pte)
>>>> +{
>>>> +    spinlock_t *ptl;
>>>> +    bool retval = true;
>>>> +
>>>> +retry:
>>>> +    ptl = pmd_lock(mm, pmd);
>>>> +    if (likely(pmd_none(*pmd))) {
>>>> +        __pte_install(mm, pmd, pte);
>>>> +    } else if (pmd_leaf(*pmd) || !pmd_present(*pmd)) {
>>>> +        retval = false;
>>>> +    } else if (!pte_get_unless_zero(pmd)) {
>>>> +        spin_unlock(ptl);
>>>> +        goto retry;
>>>> +    }
>>>> +    spin_unlock(ptl);
>>>> +    return retval;
>>>> +}
>>>> +
>>>
>>> Can pte_get_unless_zero() return true above? Can the pmd have been by 
>>> populated by others? In that case the ref count is wrongly incremented.
>>>
>>
>> Here we only have mmap_read_lock(mm), so the pmd can be populated with
>> other PTE page table page after a page fault in a different thread B 
>> of this mm. In this case, thread B already hold a pte_refcount of the 
>> PTE page table page populated in the pmd, so pte_get_unless_zero() can
>> return true above.
>>
> 
> Yes but if thread B populates the page table page and pte, then we also 
> increase the refcount with pte_get_unless_zero() , but dont decrease it 
> when notice !pte_none().
> And in the pte_none() case, the refcount is increased again, so double 
> accounting. see finish_fault().

The semantics of this function is to hold the pte_refcount count when it
returns 1, and its caller is responsible for decrease the pte_refcount
by calling pte_put(), like the following pattern:

	do_anonymous_page()

	--> pte_alloc_try_get()

	    do something about pte

	    pte_put()

Similarly in finish_fault().

Thanks,

Qi

> 
>> Similarly, if THP is enabled, the pmd also can be populated with a THP 
>> page, we can see more detail in comment in handle_pte_fault(). The
>> pmd_leaf() above is to detect this situation.
>