linux-kernel - Re: [PATCH v2 0/9] Free user PTE page table pages

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Message-ID: <a8b3c8b1-43da-d2ad-505b-46ca7624238f@bytedance.com>
Date:   Thu, 16 Sep 2021 16:41:57 +0800
From:   Qi Zheng <zhengqi.arch@...edance.com>
To:     David Hildenbrand <david@...hat.com>,
        Jason Gunthorpe <jgg@...pe.ca>
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 0/9] Free user PTE page table pages



On 9/16/21 4:30 PM, David Hildenbrand wrote:
> On 16.09.21 07:32, Qi Zheng wrote:
>>
>>
>> On 9/15/21 10:59 PM, Jason Gunthorpe wrote:
>>> On Wed, Sep 15, 2021 at 10:52:40PM +0800, Qi Zheng wrote:
>>>> I am going to split this patch series as follows:
>>>>
>>>> 1. Introduce the new dummy APIs, which is an empty implementation.
>>>>      But I will explain its semantics.
>>>> 2. Merge #6, #7 and #8, and call these dummy APIs in any necessary
>>>>      location, and split some special cases into single patches, 
>>>> such as
>>>>      pagefault and gup, etc. So that we can explain in more detail the
>>>>      concurrency in these cases. For example, we don't need to hold any
>>>>      pte_refcount in the fast path in gup on the x86_64 platform. 
>>>> Because
>>>>      the PTE page can't be freed after the local CPU interrupt is 
>>>> closed
>>>>      in the fast path in gup.
>>>> 3. Introduce CONFIG_FREE_USER_PTE and implement these empty dummy APIs.
>>>> 4. Add a description document.
>>>>
>>>> And I try to add a function that combines pte_offset_map() and
>>>> pte_try_get(). Maybe the func name is pte_try_map() recommended by
>>>> Jason, or keep the pte_offset_map() unchanged?
>>>
>>> It is part of the transformation, add a
>>> pte_try_map()/pte_undo_try_map() and replace all the pte_offset_map()
>>> callsites that can use the new API with it. The idea was that try_map
>>> would incorporate the pmd_trans_unstable/etc mess so searching for
>>> trans_unstable is a good place to start finding candidates. Some are
>>> simple, some are tricky.
>>
>> Yes, I will search pte_offset_map()/pmd_trans_unstable/etc, and then
>> analyze the specific situation.
> 
> Maybe propose the new API first, before doing the actual implementation. 
> Might safe you from doing some additional back-and-forth
> work eventually.
> 

OK, I prepare to propose all the dummy APIs in the step 1.

Thanks,
Qi

>