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(p200300cbc706f900aa79cd2500e032d1.dip0.t-ipconnect.de. [2003:cb:c706:f900:aa79:cd25:e0:32d1]) by smtp.gmail.com with ESMTPSA id u8-20020a5d4348000000b00203dbfa4ff2sm1758365wrr.34.2022.03.16.07.00.58 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 16 Mar 2022 07:00:59 -0700 (PDT) Message-ID: <75d6cccd-ce22-bdf9-68d5-0792cec39ab7@redhat.com> Date: Wed, 16 Mar 2022 15:00:56 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.6.2 Subject: Re: [PATCH v1 5/7] s390/pgtable: support __HAVE_ARCH_PTE_SWP_EXCLUSIVE To: Gerald Schaefer , Christian Borntraeger Cc: linux-kernel@vger.kernel.org, Andrew Morton , Hugh Dickins , Linus Torvalds , David Rientjes , Shakeel Butt , John Hubbard , Jason Gunthorpe , Mike Kravetz , Mike Rapoport , Yang Shi , "Kirill A . Shutemov" , Matthew Wilcox , Vlastimil Babka , Jann Horn , Michal Hocko , Nadav Amit , Rik van Riel , Roman Gushchin , Andrea Arcangeli , Peter Xu , Donald Dutile , Christoph Hellwig , Oleg Nesterov , Jan Kara , Liang Zhang , Pedro Gomes , Oded Gabbay , Catalin Marinas , Will Deacon , Michael Ellerman , Benjamin Herrenschmidt , Paul Mackerras , Heiko Carstens , Vasily Gorbik , Alexander Gordeev , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , linux-mm@kvack.org, x86@kernel.org, linux-arm-kernel@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org References: <20220315141837.137118-1-david@redhat.com> <20220315141837.137118-6-david@redhat.com> <20220315172102.771bd2cf@thinkpad> <8b13b6c0-78d4-48e3-06f0-ec0680d013a9@redhat.com> <55b6b582-51ca-b869-2055-674fe4c563e6@redhat.com> <20220316115654.12823b78@thinkpad> <6f7b208b-ec38-571d-cd24-b9bfa79d1f40@linux.ibm.com> <20220316142722.76c691d2@thinkpad> From: David Hildenbrand Organization: Red Hat In-Reply-To: <20220316142722.76c691d2@thinkpad> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 37F45A0016 X-Stat-Signature: 7mzqkee9c5qw689jm77m3esz5z6m45px Authentication-Results: imf25.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=EsRw+0xo; spf=none (imf25.hostedemail.com: domain of david@redhat.com has no SPF policy when checking 170.10.129.124) smtp.mailfrom=david@redhat.com; dmarc=pass (policy=none) header.from=redhat.com X-HE-Tag: 1647439348-636404 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 16.03.22 14:27, Gerald Schaefer wrote: > On Wed, 16 Mar 2022 14:01:07 +0100 > Christian Borntraeger wrote: > >> >> >> Am 16.03.22 um 11:56 schrieb Gerald Schaefer: >>> On Tue, 15 Mar 2022 18:12:16 +0100 >>> David Hildenbrand 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 >>> to protection bit 54. Bit 55, along with bit 52, has to be zero according >>> to the (potentially deprecated) comment. >>> >>> 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. >>> >>> 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. >>> >>> Adding Christian on cc, maybe he has some insight on _PAGE_UNUSED >>> bit 56 and swap ptes. >> >> I think _PAGE_UNUSED is not used for swap ptes. It is used _before_ swapping >> to decide whether we swap or discard the page. >> >> Regarding bit 52, the POP says in chapter 3 for the page table entry >> >> [..] >> Page-Invalid Bit (I): Bit 53 controls whether the >> page associated with the page-table entry is avail- >> able. When the bit is zero, address translation pro- >> ceeds by using the page-table entry. When the bit is >> one, the page-table entry cannot be used for transla- >> tion. >> >> >> -->When the page-invalid bit is one, all other bits in the >> -->page-table entry are available for use by program- >> -->ming. >> >> this was added with the z14 POP, but I guess it was just a clarification >> and should be valid for older machines as well. >> So 52 and 56 should be ok, with 52 probably the better choice. > > Ok, bit 55 would then also be an option IIUC, since execution protection > should not be relevant for swap ptes. And Davids clean-up removing the > restriction for bit 52 and 55 in the comment would make sense. > > I would also favor bit 52 though (PAGE_LARGE), as in Davids initial patch > version, since this is never used for any real ptes. The PAGE_LARGE flag > is only set in the "virtual" large ptes that the hugetlb code is seeing > from huge_ptep_get(). But it will (and must) never be written as a valid > pte, or else it will generate an exception. IIRC, we only set it to detect > such possible bugs, e.g. hugetlb code writing a pte (which really is a > pmd/pud) directly, instead of using set_huge_pte_at(). > Agreed. I'll include the doc cleanup patch and a fixed-up version of this patch (still using bit 52, not messing with the offset bits) in the next version. Thanks all! -- Thanks, David / dhildenb