linux-kernel - Re: [PATCH v2 4/5] mm: Add new ptep_deref() helper to fully encapsulate pte

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Message-ID: <6dfa35b0-7e1d-5b0d-1ee8-d7f5d58b4ed0@arm.com>
Date:   Thu, 25 May 2023 10:08:57 +0100
From:   Ryan Roberts <ryan.roberts@....com>
To:     Yu Zhao <yuzhao@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        SeongJae Park <sj@...nel.org>,
        Christoph Hellwig <hch@...radead.org>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Lorenzo Stoakes <lstoakes@...il.com>,
        Uladzislau Rezki <urezki@...il.com>, Zi Yan <ziy@...dia.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        damon@...ts.linux.dev
Subject: Re: [PATCH v2 4/5] mm: Add new ptep_deref() helper to fully
 encapsulate pte_t

On 19/05/2023 10:12, Ryan Roberts wrote:
> On 18/05/2023 20:28, Yu Zhao wrote:
>> On Thu, May 18, 2023 at 5:07 AM Ryan Roberts <ryan.roberts@....com> wrote:
>>>
>>> There are many call sites that directly dereference a pte_t pointer.
>>> This makes it very difficult to properly encapsulate a page table in the
>>> arch code without having to allocate shadow page tables. ptep_deref()
>>> aims to solve this by replacing all direct dereferences with a call to
>>> this function.
>>>
>>> The default implementation continues to just dereference the pointer
>>> (*ptep), so generated code should be exactly the same. However, it is
>>> possible for the architecture to override the default with their own
>>> implementation, that can (e.g.) hide certain bits from the core code, or
>>> determine young/dirty status by mixing in state from another source.
>>>
>>> While ptep_get() and ptep_get_lockless() already exist, these are
>>> implemented as atomic accesses (e.g. READ_ONCE() in the default case).
>>> So rather than using ptep_get() and risking performance regressions,
>>> introduce an new variant.
>>
>> We should reuse ptep_get():
>> 1. I don't think READ_ONCE() can cause measurable regressions in this case.
>> 2. It's technically wrong without it.
> 
> Can you clarify what you mean by technically wrong? Are you saying that the
> current code that does direct dereferencing is buggy?
> 
> I previously convinced myself that the potential for the compiler generating
> multiple loads was safe because the code in question is under the PTL so there
> are no concurrent stores. And we shouldn't see any tearing for the same reason.
> 
> That said, if there is concensus that we can just use ptep_get() (==
> READ_ONCE()) everywhere, then I agree that would be cleaner. Does anyone object?

Hi all,

A politie bump: It would be great to hear opinions on this before I go ahead and
make the change.

Thanks,
Ryan