From:   Michael Ellerman <mpe@ellerman.id.au>
To:     David Hildenbrand <david@redhat.com>, linux-kernel@vger.kernel.org
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Hugh Dickins <hughd@google.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Jason Gunthorpe <jgg@nvidia.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Yang Shi <shy828301@gmail.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Nadav Amit <namit@vmware.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Peter Xu <peterx@redhat.com>, linux-mm@kvack.org,
        x86@kernel.org, linux-alpha@vger.kernel.org,
        linux-snps-arc@lists.infradead.org,
        linux-arm-kernel@lists.infradead.org, linux-csky@vger.kernel.org,
        linux-hexagon@vger.kernel.org, linux-ia64@vger.kernel.org,
        loongarch@lists.linux.dev, linux-m68k@lists.linux-m68k.org,
        linux-mips@vger.kernel.org, openrisc@lists.librecores.org,
        linux-parisc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        linux-riscv@lists.infradead.org, linux-s390@vger.kernel.org,
        linux-sh@vger.kernel.org, sparclinux@vger.kernel.org,
        linux-um@lists.infradead.org, linux-xtensa@linux-xtensa.org,
        David Hildenbrand <david@redhat.com>,
        Nicholas Piggin <npiggin@gmail.com>,
        Christophe Leroy <christophe.leroy@csgroup.eu>
Subject: Re: [PATCH mm-unstable v1 17/26] powerpc/mm: support
 __HAVE_ARCH_PTE_SWP_EXCLUSIVE on 32bit book3s
In-Reply-To: <20230113171026.582290-18-david@redhat.com>
References: <20230113171026.582290-1-david@redhat.com>
 <20230113171026.582290-18-david@redhat.com>
Date:   Fri, 10 Feb 2023 16:08:07 +1100
Message-ID: <87cz6iw1jc.fsf@mpe.ellerman.id.au>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

David Hildenbrand <david@redhat.com> writes:
> We already implemented support for 64bit book3s in commit bff9beaa2e80
> ("powerpc/pgtable: support __HAVE_ARCH_PTE_SWP_EXCLUSIVE for book3s")
>
> Let's support __HAVE_ARCH_PTE_SWP_EXCLUSIVE also in 32bit by reusing yet
> unused LSB 2 / MSB 29. There seems to be no real reason why that bit cannot
> be used, and reusing it avoids having to steal one bit from the swap
> offset.
>
> While at it, mask the type in __swp_entry().
>
> Cc: Michael Ellerman <mpe@ellerman.id.au>
> Cc: Nicholas Piggin <npiggin@gmail.com>
> Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>  arch/powerpc/include/asm/book3s/32/pgtable.h | 38 +++++++++++++++++---
>  1 file changed, 33 insertions(+), 5 deletions(-)

I gave this a quick test on a ppc32 machine, everything seems fine.

Your test_swp_exclusive.c passes, and an LTP run looks normal.

Acked-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc)

cheers

> diff --git a/arch/powerpc/include/asm/book3s/32/pgtable.h b/arch/powerpc/include/asm/book3s/32/pgtable.h
> index 75823f39e042..0ecb3a58f23f 100644
> --- a/arch/powerpc/include/asm/book3s/32/pgtable.h
> +++ b/arch/powerpc/include/asm/book3s/32/pgtable.h
> @@ -42,6 +42,9 @@
>  #define _PMD_PRESENT_MASK (PAGE_MASK)
>  #define _PMD_BAD	(~PAGE_MASK)
>  
> +/* We borrow the _PAGE_USER bit to store the exclusive marker in swap PTEs. */
> +#define _PAGE_SWP_EXCLUSIVE	_PAGE_USER
> +
>  /* And here we include common definitions */
>  
>  #define _PAGE_KERNEL_RO		0
> @@ -363,17 +366,42 @@ static inline void __ptep_set_access_flags(struct vm_area_struct *vma,
>  #define pmd_page(pmd)		pfn_to_page(pmd_pfn(pmd))
>  
>  /*
> - * Encode and decode a swap entry.
> - * Note that the bits we use in a PTE for representing a swap entry
> - * must not include the _PAGE_PRESENT bit or the _PAGE_HASHPTE bit (if used).
> - *   -- paulus
> + * Encode/decode swap entries and swap PTEs. Swap PTEs are all PTEs that
> + * are !pte_none() && !pte_present().
> + *
> + * Format of swap PTEs (32bit PTEs):
> + *
> + *                         1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
> + *   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
> + *   <----------------- offset --------------------> < type -> E H P
> + *
> + *   E is the exclusive marker that is not stored in swap entries.
> + *   _PAGE_PRESENT (P) and __PAGE_HASHPTE (H) must be 0.
> + *
> + * For 64bit PTEs, the offset is extended by 32bit.
>   */
>  #define __swp_type(entry)		((entry).val & 0x1f)
>  #define __swp_offset(entry)		((entry).val >> 5)
> -#define __swp_entry(type, offset)	((swp_entry_t) { (type) | ((offset) << 5) })
> +#define __swp_entry(type, offset)	((swp_entry_t) { ((type) & 0x1f) | ((offset) << 5) })
>  #define __pte_to_swp_entry(pte)		((swp_entry_t) { pte_val(pte) >> 3 })
>  #define __swp_entry_to_pte(x)		((pte_t) { (x).val << 3 })
>  
> +#define __HAVE_ARCH_PTE_SWP_EXCLUSIVE
> +static inline int pte_swp_exclusive(pte_t pte)
> +{
> +	return pte_val(pte) & _PAGE_SWP_EXCLUSIVE;
> +}
> +
> +static inline pte_t pte_swp_mkexclusive(pte_t pte)
> +{
> +	return __pte(pte_val(pte) | _PAGE_SWP_EXCLUSIVE);
> +}
> +
> +static inline pte_t pte_swp_clear_exclusive(pte_t pte)
> +{
> +	return __pte(pte_val(pte) & ~_PAGE_SWP_EXCLUSIVE);
> +}
> +
>  /* Generic accessors to PTE bits */
>  static inline int pte_write(pte_t pte)		{ return !!(pte_val(pte) & _PAGE_RW);}
>  static inline int pte_read(pte_t pte)		{ return 1; }
> -- 
> 2.39.0
>
>
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