Date:   Wed, 16 Mar 2022 12:06:16 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Gerald Schaefer <gerald.schaefer@...ux.ibm.com>
Cc:     linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        John Hubbard <jhubbard@...dia.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Yang Shi <shy828301@...il.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
        Michal Hocko <mhocko@...nel.org>,
        Nadav Amit <namit@...are.com>, Rik van Riel <riel@...riel.com>,
        Roman Gushchin <guro@...com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Peter Xu <peterx@...hat.com>,
        Donald Dutile <ddutile@...hat.com>,
        Christoph Hellwig <hch@....de>,
        Oleg Nesterov <oleg@...hat.com>, Jan Kara <jack@...e.cz>,
        Liang Zhang <zhangliang5@...wei.com>,
        Pedro Gomes <pedrodemargomes@...il.com>,
        Oded Gabbay <oded.gabbay@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, linux-mm@...ck.org,
        x86@...nel.org, linux-arm-kernel@...ts.infradead.org,
        linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
        Christian Borntraeger <borntraeger@...ux.ibm.com>
Subject: Re: [PATCH v1 5/7] s390/pgtable: support
 __HAVE_ARCH_PTE_SWP_EXCLUSIVE

On 16.03.22 11:56, Gerald Schaefer wrote:
> On Tue, 15 Mar 2022 18:12:16 +0100
> David Hildenbrand <david@...hat.com> wrote:
> 
>> On 15.03.22 17:58, David Hildenbrand wrote:
>>>
>>>>> This would mean that it is not OK to have bit 52 not zero for swap PTEs.
>>>>> But if I read the POP correctly, all bits except for the DAT-protection
>>>>> would be ignored for invalid PTEs, so maybe this comment needs some update
>>>>> (for both bits 52 and also 55).
>>>>>
>>>>> Heiko might also have some more insight.
>>>>
>>>> Indeed, I wonder why we should get a specification exception when the
>>>> PTE is invalid. I'll dig a bit into the PoP.
>>>
>>> SA22-7832-12 6-46 ("Translation-Specification Exception") is clearer
>>>
>>> "The page-table entry used for the translation is
>>> valid, and bit position 52 does not contain zero."
>>>
>>> "The page-table entry used for the translation is
>>> valid, EDAT-1 does not apply, the instruction-exe-
>>> cution-protection facility is not installed, and bit
>>> position 55 does not contain zero. It is model
>>> dependent whether this condition is recognized."
>>>
>>
>> I wonder if the following matches reality:
>>
>> diff --git a/arch/s390/include/asm/pgtable.h b/arch/s390/include/asm/pgtable.h
>> index 008a6c856fa4..6a227a8c3712 100644
>> --- a/arch/s390/include/asm/pgtable.h
>> +++ b/arch/s390/include/asm/pgtable.h
>> @@ -1669,18 +1669,16 @@ 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.
>>   * 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                        |XX1XX|type |S0|
>>   * |0000000000111111111122222222223333333333444444444455|55555|55566|66|
>>   * |0123456789012345678901234567890123456789012345678901|23456|78901|23|
>> + *
>> + * Bits 0-51 store the offset.
>> + * Bits 57-62 store the type.
>> + * Bit 62 (S) is used for softdirty tracking.
>> + * Bits 52, 53, 55 and 56 (X) are unused.
>>   */
>>  
>>  #define __SWP_OFFSET_MASK      ((1UL << 52) - 1)
>>
>>
>> I'm not sure why bit 53 was indicated as "1" and bit 55 was indicated as
>> "0". At least for 52 and 55 there was a clear description.
> 
> Bit 53 is the invalid bit, and that is always 1 for swap ptes, in addition

Ah, right, I missed the meaning of bot 53 because this documentation is just
sub-optimal.

> to protection bit 54. Bit 55, along with bit 52, has to be zero according
> to the (potentially deprecated) comment.

Yeah, that 52/55 comment is just wrong when dealing with invalid PTEs.

> 
> It is interesting that bit 56 seems to be unused, at least according
> to the comment, but that would also mention bit 62 as unused, so that
> clearly needs some update.

I currently have the following cleanup patch:

>From a4a8db2920e035e90a410b9170829326bb1fab92 Mon Sep 17 00:00:00 2001
From: David Hildenbrand <david@...hat.com>
Date: Tue, 15 Mar 2022 18:14:09 +0100
Subject: [PATCH] s390/pgtable: cleanup description of swp pte layout

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@...hat.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 008a6c856fa4..64fbe5fd3853 100644
--- a/arch/s390/include/asm/pgtable.h
+++ b/arch/s390/include/asm/pgtable.h
@@ -1669,18 +1669,17 @@ 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)
-- 
2.35.1


> 
> If bit 56 could be used for _PAGE_SWP_EXCLUSIVE, that would be better
> than stealing a bit from the offset, or using potentially dangerous
> bit 52. It is defined as _PAGE_UNUSED and only used for kvm, not sure
> if this is also relevant for swap ptes, similar to bit 62.

I don't think it is, and I also don't think there is anything wrong
with reusing bit 52.

> 
> Adding Christian on cc, maybe he has some insight on _PAGE_UNUSED
> bit 56 and swap ptes.


-- 
Thanks,

David / dhildenb

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