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 22:12:04 +0800 [thread overview]
Message-ID: <5ffff30c-59d7-fbb0-e3c2-fe9e47fc4658@bytedance.com> (raw)
In-Reply-To: <7fe2dd75-9b48-9685-8986-27a4cecc840f@nextfour.com>
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.
>
next prev parent reply other threads:[~2021-07-19 14:12 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 [this message]
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=5ffff30c-59d7-fbb0-e3c2-fe9e47fc4658@bytedance.com \
--to=zhengqi.arch@bytedance.com \
--cc=akpm@linux-foundation.org \
--cc=hannes@cmpxchg.org \
--cc=linux-doc@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=mhocko@kernel.org \
--cc=mika.penttila@nextfour.com \
--cc=songmuchun@bytedance.com \
--cc=tglx@linutronix.de \
--cc=vdavydov.dev@gmail.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).