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

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Message-ID: <20230524190618.GR4967@kernel.org>
Date:   Wed, 24 May 2023 22:06:18 +0300
From:   Mike Rapoport <rppt@...nel.org>
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 12:07:26PM +0100, Ryan Roberts 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.
> 
> Call sites will be converted to use the accessor in future commits.
> 
> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
> ---
>  include/linux/pgtable.h | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index c5a51481bbb9..1161beab2492 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -204,6 +204,13 @@ static inline int pudp_set_access_flags(struct vm_area_struct *vma,
>  #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
>  #endif
>  
> +#ifndef ptep_deref
> +static inline pte_t ptep_deref(pte_t *ptep)
> +{
> +	return *(pte_t *)ptep;

Why do you need the casting here?

> +}
> +#endif
> +
>  #ifndef __HAVE_ARCH_PTEP_TEST_AND_CLEAR_YOUNG
>  static inline int ptep_test_and_clear_young(struct vm_area_struct *vma,
>  					    unsigned long address,
> -- 
> 2.25.1
> 
> 

-- 
Sincerely yours,
Mike.