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

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Message-ID: <CAOUHufZaiTCxk4M4w5GaA-+5FoMnHZt+fzoU=cYCA1Ldk7CVEQ@mail.gmail.com>
Date:   Thu, 18 May 2023 13:28:35 -0600
From:   Yu Zhao <yuzhao@...gle.com>
To:     Ryan Roberts <ryan.roberts@....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 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.