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

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Date:   Wed, 9 Sep 2020 09:18:46 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Gerald Schaefer <gerald.schaefer@...ux.ibm.com>
Cc:     Jason Gunthorpe <jgg@...pe.ca>, John Hubbard <jhubbard@...dia.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Russell King <linux@...linux.org.uk>,
        Mike Rapoport <rppt@...nel.org>,
        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>,
        Jeff Dike <jdike@...toit.com>,
        Richard Weinberger <richard@....at>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Arnd Bergmann <arnd@...db.de>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        linux-x86 <x86@...nel.org>,
        linux-arm <linux-arm-kernel@...ts.infradead.org>,
        linux-power <linuxppc-dev@...ts.ozlabs.org>,
        linux-sparc <sparclinux@...r.kernel.org>,
        linux-um <linux-um@...ts.infradead.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>
Subject: Re: [RFC PATCH v2 1/3] mm/gup: fix gup_fast with dynamic page table
 folding

On 9/9/20 5:29 AM, Gerald Schaefer wrote:
> This only works well as long there are real pagetable pointers involved,
> that can also be used for iteration. For gup_fast, or any other future
> pagetable walkers using the READ_ONCE logic w/o lock, that is not true.
> There are pointers involved to local pXd values on the stack, because of
> the READ_ONCE logic, and our middle-level iteration will suddenly iterate
> over such stack pointers instead of pagetable pointers.

By "There are pointers involved to local pXd values on the stack", did
you mean "locate" instead of "local"?  That sentence confused me.

Which code is it, exactly that allocates these troublesome on-stack pXd
values, btw?

> This will be addressed by making the pXd_addr_end() dynamic, for which
> we need to see the pXd value in order to determine its level / type.

Thanks for the explanation!