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

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Message-ID: <d33a4349-a228-f98a-ab14-e221bf80dd4a@bytedance.com>
Date:   Thu, 16 Sep 2021 13:32:17 +0800
From:   Qi Zheng <zhengqi.arch@...edance.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     David Hildenbrand <david@...hat.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 0/9] Free user PTE page table pages



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.

> 
> When you get to step 3 you just change pte_try_map() and the callsites
> don't need changing.
> 
> Jason
> 

Thanks,
Qi