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

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Date:   Wed, 1 Sep 2021 18:19:03 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Qi Zheng <zhengqi.arch@...edance.com>, 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 18:16, Jason Gunthorpe wrote:
> On Wed, Sep 01, 2021 at 06:13:07PM +0200, David Hildenbrand wrote:
>> On 01.09.21 17:32, Jason Gunthorpe wrote:
>>> On Wed, Sep 01, 2021 at 03:57:09PM +0200, David Hildenbrand wrote:
>>>> 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.
>>>
>>> It is a bit better with patch 8, but it is still not optimal, we don't
>>> need to do the atomic work at all if the entire ptep is accessed while
>>> locked. So the above is stil not what I would expect here, even with
>>> RCU.
>>>
>>> eg I would expect that this kind of change would work first with the
>>> existing paired acessors, ie
>>>
>>> 	pte = pte_offset_map(pmd, address);
>>> 	pte_unmap(pte);
>>>
>>> Should handle the refcount under the covers, and same kind of idea for
>>> the _locked/_unlocked varient.
>>
>> See my other mail.
> 
> Do you have a reference?

Reply to the other mail you just send.

> 
>>> Only places that don't already use that pairing should get modified.
>>>
>>> To do this we have to extend the API so that pte_offset_map() can
>>> fail, or very cleverly return some kind of global non-present pte page
>>> (I wonder if the zero page would work?)
>>
>> I explored both ideas (returning NULL, return a specially prepared page) and
>> it didn't work in some cases where we unmap+remap etc.
> 
> I wouldn't think it works everywhere, bit it works in a lot of places,
> and it is a heck of a lot better than what is proposed here. I'd
> rather see the places that can use it be moved, and the few places
> that can't be opencoded.

Well, I used ptep_get_map_lock() and friends. But hacking directly into 
ptep_map_lock() and friends wasn't possible due to all the corner cases.

-- 
Thanks,

David / dhildenb