Date:   Tue, 8 Sep 2020 14:09:01 +0200
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Christophe Leroy <christophe.leroy@...roup.eu>,
        Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        John Hubbard <jhubbard@...dia.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        linux-mm <linux-mm@...ck.org>, Paul Mackerras <paulus@...ba.org>,
        linux-sparc <sparclinux@...r.kernel.org>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Will Deacon <will@...nel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Richard Weinberger <richard@....at>,
        linux-x86 <x86@...nel.org>, Russell King <linux@...linux.org.uk>,
        Ingo Molnar <mingo@...hat.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Arnd Bergmann <arnd@...db.de>, Jeff Dike <jdike@...toit.com>,
        linux-um <linux-um@...ts.infradead.org>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-arm <linux-arm-kernel@...ts.infradead.org>,
        linux-power <linuxppc-dev@...ts.ozlabs.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Mike Rapoport <rppt@...nel.org>
Subject: Re: [RFC PATCH v2 1/3] mm/gup: fix gup_fast with dynamic page table
 folding



On 08.09.20 07:06, Christophe Leroy wrote:
> 
> 
> Le 07/09/2020 à 20:00, Gerald Schaefer a écrit :
>> From: Alexander Gordeev <agordeev@...ux.ibm.com>
>>
>> Commit 1a42010cdc26 ("s390/mm: convert to the generic get_user_pages_fast
>> code") introduced a subtle but severe bug on s390 with gup_fast, due to
>> dynamic page table folding.
>>
>> The question "What would it require for the generic code to work for s390"
>> has already been discussed here
>> https://lkml.kernel.org/r/20190418100218.0a4afd51@mschwideX1
>> and ended with a promising approach here
>> https://lkml.kernel.org/r/20190419153307.4f2911b5@mschwideX1
>> which in the end unfortunately didn't quite work completely.
>>
>> We tried to mimic static level folding by changing pgd_offset to always
>> calculate top level page table offset, and do nothing in folded pXd_offset.
>> What has been overlooked is that PxD_SIZE/MASK and thus pXd_addr_end do
>> not reflect this dynamic behaviour, and still act like static 5-level
>> page tables.
>>
> 
> [...]
> 
>>
>> Fix this by introducing new pXd_addr_end_folded helpers, which take an
>> additional pXd entry value parameter, that can be used on s390
>> to determine the correct page table level and return corresponding
>> end / boundary. With that, the pointer iteration will always
>> happen in gup_pgd_range for s390. No change for other architectures
>> introduced.
> 
> Not sure pXd_addr_end_folded() is the best understandable name, allthough I don't have any alternative suggestion at the moment.
> Maybe could be something like pXd_addr_end_fixup() as it will disappear in the next patch, or pXd_addr_end_gup() ?
> 
> Also, if it happens to be acceptable to get patch 2 in stable, I think you should switch patch 1 and patch 2 to avoid the step through pXd_addr_end_folded()

given that this fixes a data corruption issue, wouldnt it be the best to go forward
with this patch ASAP and then handle the other patches on top with all the time that
we need?
> 
> 
>>
>> Fixes: 1a42010cdc26 ("s390/mm: convert to the generic get_user_pages_fast code")
>> Cc: <stable@...r.kernel.org> # 5.2+
>> Reviewed-by: Gerald Schaefer <gerald.schaefer@...ux.ibm.com>
>> Signed-off-by: Alexander Gordeev <agordeev@...ux.ibm.com>
>> Signed-off-by: Gerald Schaefer <gerald.schaefer@...ux.ibm.com>
>> ---
>>   arch/s390/include/asm/pgtable.h | 42 +++++++++++++++++++++++++++++++++
>>   include/linux/pgtable.h         | 16 +++++++++++++
>>   mm/gup.c                        |  8 +++----
>>   3 files changed, 62 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/s390/include/asm/pgtable.h b/arch/s390/include/asm/pgtable.h
>> index 7eb01a5459cd..027206e4959d 100644
>> --- a/arch/s390/include/asm/pgtable.h
>> +++ b/arch/s390/include/asm/pgtable.h
>> @@ -512,6 +512,48 @@ static inline bool mm_pmd_folded(struct mm_struct *mm)
>>   }
>>   #define mm_pmd_folded(mm) mm_pmd_folded(mm)
>>   +/*
>> + * With dynamic page table levels on s390, the static pXd_addr_end() functions
>> + * will not return corresponding dynamic boundaries. This is no problem as long
>> + * as only pXd pointers are passed down during page table walk, because
>> + * pXd_offset() will simply return the given pointer for folded levels, and the
>> + * pointer iteration over a range simply happens at the correct page table
>> + * level.
>> + * It is however a problem with gup_fast, or other places walking the page
>> + * tables w/o locks using READ_ONCE(), and passing down the pXd values instead
>> + * of pointers. In this case, the pointer given to pXd_offset() is a pointer to
>> + * a stack variable, which cannot be used for pointer iteration at the correct
>> + * level. Instead, the iteration then has to happen by going up to pgd level
>> + * again. To allow this, provide pXd_addr_end_folded() functions with an
>> + * additional pXd value parameter, which can be used on s390 to determine the
>> + * folding level and return the corresponding boundary.
>> + */
>> +static inline unsigned long rste_addr_end_folded(unsigned long rste, unsigned long addr, unsigned long end)
> 
> What does 'rste' stands for ?
> 
> Isn't this line a bit long ?

this is region/segment table entry according to the architecture. 
On our platform we do have the pagetables with a different format that
next levels (segment table -> 1MB granularity, region 3rd table -> 2 GB
granularity, region 2nd table -> 4TB granularity, region 1st table -> 8 PB
granularity. ST,R3,R2,R1 have the same format and are thus often called
crste (combined region and segment table entry).

Powered by blists - more mailing lists