linux-kernel - Re: [PATCH v2 6/9] mm: free user PTE page table pages

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Date:   Wed, 1 Sep 2021 15:57:09 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Jason Gunthorpe <jgg@...dia.com>,
        Qi Zheng <zhengqi.arch@...edance.com>
Cc:     akpm@...ux-foundation.org, tglx@...utronix.de, hannes@...xchg.org,
        mhocko@...nel.org, vdavydov.dev@...il.com,
        kirill.shutemov@...ux.intel.com, mika.penttila@...tfour.com,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, songmuchun@...edance.com
Subject: Re: [PATCH v2 6/9] mm: free user PTE page table pages

On 01.09.21 15:53, Jason Gunthorpe wrote:
> On Thu, Aug 19, 2021 at 11:18:55AM +0800, Qi Zheng wrote:
> 
>> diff --git a/mm/gup.c b/mm/gup.c
>> index 2630ed1bb4f4..30757f3b176c 100644
>> +++ b/mm/gup.c
>> @@ -500,6 +500,9 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
>>   	if (unlikely(pmd_bad(*pmd)))
>>   		return no_page_table(vma, flags);
>>   
>> +	if (!pte_try_get(mm, pmd))
>> +		return no_page_table(vma, flags);
>> +
>>   	ptep = pte_offset_map_lock(mm, pmd, address, &ptl);
> 
> This is not good on a performance path, the pte_try_get() is
> locking/locking the same lock that pte_offset_map_lock() is getting.

Yes, and we really need patch #8, anything else is just confusing reviewers.

> 
> This would be much better if the map_lock infra could manage the
> refcount itself.
> 
> I'm also not really keen on adding ptl level locking to all the
> currently no-lock paths. If we are doing that then the no-lock paths
> should rely on the ptl for alot more of their operations and avoid the
> complicatred no-lock data access we have. eg 'pte_try_get()' should
> also copy the pte_t under the lock.
> 
> Also, I don't really understand how this scheme works with
> get_user_pages_fast.

With the RCU change it in #8 it should work just fine, because RCU 
synchronize has to wait either until all other CPUs have left the RCU 
read section, or re-enabled interrupts.

-- 
Thanks,

David / dhildenb