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From: Qi Zheng <zhengqi.arch@bytedance.com>
To: "Mika Penttilä" <mika.penttila@nextfour.com>,
	akpm@linux-foundation.org, tglx@linutronix.de,
	hannes@cmpxchg.org, mhocko@kernel.org, vdavydov.dev@gmail.com
Cc: linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-mm@kvack.org, songmuchun@bytedance.com
Subject: Re: [PATCH 5/7] mm: free user PTE page table pages
Date: Mon, 19 Jul 2021 20:56:01 +0800	[thread overview]
Message-ID: <80b7d7fc-9d6d-0d1b-a333-b0ccd856e7c1@bytedance.com> (raw)
In-Reply-To: <9c3c87d5-e64e-f13f-ef36-b438e4de1e66@nextfour.com>

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.


  reply	other threads:[~2021-07-19 12:56 UTC|newest]

Thread overview: 27+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-18  4:30 [PATCH 0/7] Free user PTE page table pages Qi Zheng
2021-07-18  4:30 ` [PATCH 1/7] mm: fix the deadlock in finish_fault() Qi Zheng
2021-07-18 21:28   ` Kirill A. Shutemov
2021-07-19  9:53     ` Qi Zheng
2021-07-20 23:14       ` Andrew Morton
2021-07-21  2:21         ` Qi Zheng
2021-07-18  4:30 ` [PATCH 2/7] mm: introduce pte_install() helper Qi Zheng
2021-07-18 21:31   ` Kirill A. Shutemov
2021-07-19 10:20     ` Qi Zheng
2021-07-18  4:30 ` [PATCH 3/7] mm: remove redundant smp_wmb() Qi Zheng
2021-07-27 13:39   ` Muchun Song
2021-07-18  4:30 ` [PATCH 4/7] mm: rework the parameter of lock_page_or_retry() Qi Zheng
2021-07-18  4:30 ` [PATCH 5/7] mm: free user PTE page table pages Qi Zheng
2021-07-18  6:19   ` Mika Penttilä
2021-07-19 12:56     ` Qi Zheng [this message]
2021-07-19 13:55       ` Mika Penttilä
2021-07-19 14:12         ` Qi Zheng
2021-07-19 14:17           ` Mika Penttilä
2021-07-18 22:01   ` Kirill A. Shutemov
2021-07-19 13:55     ` Qi Zheng
2021-07-18  4:30 ` [PATCH 6/7] mm: defer freeing PTE page table for a grace period Qi Zheng
2021-07-18  4:30 ` [PATCH 7/7] mm: use mmu_gather to free PTE page table Qi Zheng
2021-07-19  7:34 ` [PATCH 0/7] Free user PTE page table pages David Hildenbrand
2021-07-19 11:28   ` David Hildenbrand
2021-07-19 12:42     ` Muchun Song
2021-07-19 13:30       ` Muchun Song
2021-07-20  4:00     ` Qi Zheng

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