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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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        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>
From:   Qi Zheng <zhengqi.arch@bytedance.com>
Message-ID: <80b7d7fc-9d6d-0d1b-a333-b0ccd856e7c1@bytedance.com>
Date:   Mon, 19 Jul 2021 20:56:01 +0800
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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.

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.