linux-kernel - Re: [RFC PATCH v2 1/3] mm/gup: fix gup_fast with dynamic page table folding

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Date:   Tue, 8 Sep 2020 15:38:57 +0200
From:   Gerald Schaefer <gerald.schaefer@...ux.ibm.com>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Christian Borntraeger <borntraeger@...ibm.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        John Hubbard <jhubbard@...dia.com>,
        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 Tue, 8 Sep 2020 14:40:10 +0200
Christophe Leroy <christophe.leroy@...roup.eu> wrote:

> 
> 
> Le 08/09/2020 à 14:09, Christian Borntraeger a écrit :
> > 
> > 
> > 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?
> 
> I have no strong opinion on this, but I feel rather tricky to have to 
> change generic part of GUP to use a new fonction then revert that change 
> in the following patch, just because you want the first patch in stable 
> and not the second one.
> 
> Regardless, I was wondering, why do we need a reference to the pXd at 
> all when calling pXd_addr_end() ?
> 
> Couldn't S390 retrieve the pXd by using the pXd_offset() dance with the 
> passed addr ?

Apart from performance impact when re-doing that what has already been
done by the caller, I think we would also break the READ_ONCE semantics.
After all, the pXd_offset() would also require some pXd pointer input,
which we don't have. So we would need to start over again from mm->pgd.

Also, it seems to be more in line with other primitives that take
a pXd value or pointer.