Re: [PATCH v2 4/5] mm: Add new ptep_deref() helper to fully encapsulate pte_t

[Yu Zhao]

On Thu, May 18, 2023 at 5:07 AM Ryan Roberts <ryan.roberts@xxxxxxx> 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.