From: "Mika Penttilä" <mika.penttila@nextfour.com>
To: Qi Zheng <zhengqi.arch@bytedance.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 17:17:44 +0300 [thread overview]
Message-ID: <ad00037e-cddb-1de5-bd77-1ade053952ed@nextfour.com> (raw)
In-Reply-To: <5ffff30c-59d7-fbb0-e3c2-fe9e47fc4658@bytedance.com>
On 19.7.2021 17.12, Qi Zheng wrote:
> 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
Ok I didn't notice the added pte_put()'s at the end of these funcs.
Thanks,
Mika
>
>>
>>> 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.
>>
next prev parent reply other threads:[~2021-07-19 14:17 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
2021-07-19 13:55 ` Mika Penttilä
2021-07-19 14:12 ` Qi Zheng
2021-07-19 14:17 ` Mika Penttilä [this message]
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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