linux-kernel - Re: [PATCH v3 0/3] Encapsulate PTE contents from non-arch code

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Message-ID: <d38dc237-6093-d4c5-993e-e8ffdd6cb6fa@arm.com>
Date:   Tue, 13 Jun 2023 09:52:55 +0100
From:   Ryan Roberts <ryan.roberts@....com>
To:     Muchun Song <muchun.song@...ux.dev>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        SeongJae Park <sj@...nel.org>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Mike Rapoport <rppt@...nel.org>, Yu Zhao <yuzhao@...gle.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Dimitri Sivanich <dimitri.sivanich@....com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Ian Rogers <irogers@...gle.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Alexander Potapenko <glider@...gle.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Shakeel Butt <shakeelb@...gle.com>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Pasha Tatashin <pasha.tatashin@...een.com>,
        Uladzislau Rezki <urezki@...il.com>,
        Christoph Hellwig <hch@...radead.org>,
        Lorenzo Stoakes <lstoakes@...il.com>,
        linux-kernel@...r.kernel.org,
        Linux Memory Management List <linux-mm@...ck.org>,
        damon@...ts.linux.dev
Subject: Re: [PATCH v3 0/3] Encapsulate PTE contents from non-arch code

On 13/06/2023 03:16, Muchun Song wrote:
> 
> 
>> On Jun 12, 2023, at 23:15, Ryan Roberts <ryan.roberts@....com> wrote:
>>
>> Hi All,
>>
>> (Including wider audience this time since changes touch a fair few subsystems)
>>
>> This is the second half of v3 of a series to improve the encapsulation of pte
>> entries by disallowing non-arch code from directly dereferencing pte_t pointers.
>> Based on earlier feedback, I split the series in 2; the first part, fixes for
>> existing bugs, was already posted at [3] and merged into mm-stable. This second
>> part contains the conversion from direct dereferences to instead use
>> ptep_get()/ptep_get_lockless().
>>
>> See the v1 cover letter at [1] for rationale for this work.
>>
>> Based on feedback at v2, I've removed the new ptep_deref() helper I originally
>> added, and am now using the existing ptep_get() and ptep_get_lockless() helpers.
> 
> When I first saw the name of ptep_get()/ptep_get_lockless(), I thought
> the pte seems like to be protected by the refcount mechanism (Why I have
> this though? Because Qi Zheng has proposed a approach to free pte page tables
> by using the refcount mechanism [1]). And your proposed name of ptep_deref()
> is intuitive for me, so I have another thought, should we rename ptep_get()
> to ptep_deref()? Just a thought from me, I'd like to hear if others object.

I see your point, but I think any renaming exercise should be discussed and
applied in the context of a separate patch series, given that ptep_get() and
ptep_get_lockless() already exist in the code base. This would be a much bigger
job, since it would need to cover all the arch code too.

> 
> Thanks.
> 
> [1] https://lore.kernel.org/lkml/20211110105428.32458-7-zhengqi.arch@bytedance.com/