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

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Date:   Wed, 15 Sep 2021 11:59:33 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Qi Zheng <zhengqi.arch@...edance.com>
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 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.

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

Jason