Received-SPF: softfail (google.com: domain of transitioning linux-kernel-owner@vger.kernel.org does not designate 23.128.96.19 as permitted sender) client-ip=23.128.96.19;
Date:   Wed, 30 Mar 2022 18:48:55 +0200
From:   Gerald Schaefer <gerald.schaefer@linux.ibm.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Hugh Dickins <hughd@google.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        David Rientjes <rientjes@google.com>,
        Shakeel Butt <shakeelb@google.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Jason Gunthorpe <jgg@nvidia.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Yang Shi <shy828301@gmail.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Matthew Wilcox <willy@infradead.org>,
        Vlastimil Babka <vbabka@suse.cz>, Jann Horn <jannh@google.com>,
        Michal Hocko <mhocko@kernel.org>,
        Nadav Amit <namit@vmware.com>, Rik van Riel <riel@surriel.com>,
        Roman Gushchin <guro@fb.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Peter Xu <peterx@redhat.com>,
        Donald Dutile <ddutile@redhat.com>,
        Christoph Hellwig <hch@lst.de>,
        Oleg Nesterov <oleg@redhat.com>, Jan Kara <jack@suse.cz>,
        Liang Zhang <zhangliang5@huawei.com>,
        Pedro Gomes <pedrodemargomes@gmail.com>,
        Oded Gabbay <oded.gabbay@gmail.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Heiko Carstens <hca@linux.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Alexander Gordeev <agordeev@linux.ibm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, linux-mm@kvack.org,
        x86@kernel.org, linux-arm-kernel@lists.infradead.org,
        linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org
Subject: Re: [PATCH v2 5/8] s390/pgtable: cleanup description of swp pte
 layout
Message-ID: <20220330184855.56916ba5@thinkpad>
In-Reply-To: <20220329164329.208407-6-david@redhat.com>
References: <20220329164329.208407-1-david@redhat.com>
        <20220329164329.208407-6-david@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Precedence: bulk

On Tue, 29 Mar 2022 18:43:26 +0200
David Hildenbrand <david@redhat.com> wrote:

> Bit 52 and bit 55 don't have to be zero: they only trigger a
> translation-specifiation exception if the PTE is marked as valid, which
> is not the case for swap ptes.
> 
> Document which bits are used for what, and which ones are unused.
> 
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>  arch/s390/include/asm/pgtable.h | 17 ++++++++---------
>  1 file changed, 8 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/s390/include/asm/pgtable.h b/arch/s390/include/asm/pgtable.h
> index 9df679152620..3982575bb586 100644
> --- a/arch/s390/include/asm/pgtable.h
> +++ b/arch/s390/include/asm/pgtable.h
> @@ -1712,18 +1712,17 @@ static inline int has_transparent_hugepage(void)
>  /*
>   * 64 bit swap entry format:
>   * A page-table entry has some bits we have to treat in a special way.
> - * Bits 52 and bit 55 have to be zero, otherwise a specification
> - * exception will occur instead of a page translation exception. The
> - * specification exception has the bad habit not to store necessary
> - * information in the lowcore.
> - * Bits 54 and 63 are used to indicate the page type.
> + * Bits 54 and 63 are used to indicate the page type. Bit 53 marks the pte
> + * as invalid.
>   * A swap pte is indicated by bit pattern (pte & 0x201) == 0x200
> - * This leaves the bits 0-51 and bits 56-62 to store type and offset.
> - * We use the 5 bits from 57-61 for the type and the 52 bits from 0-51
> - * for the offset.
> - * |			  offset			|01100|type |00|
> + * |			  offset			|X11XX|type |S0|
>   * |0000000000111111111122222222223333333333444444444455|55555|55566|66|
>   * |0123456789012345678901234567890123456789012345678901|23456|78901|23|
> + *
> + * Bits 0-51 store the offset.
> + * Bits 57-61 store the type.
> + * Bit 62 (S) is used for softdirty tracking.
> + * Bits 52, 55 and 56 (X) are unused.
>   */
>  
>  #define __SWP_OFFSET_MASK	((1UL << 52) - 1)

Thanks David!

Reviewed-by: Gerald Schaefer <gerald.schaefer@linux.ibm.com>