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

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Message-ID: <CAHk-=wiiEUD_XvNnXysYsKiT4B3SajWbZ4VKY3jYk-17EEaaiA@mail.gmail.com>
Date:   Fri, 11 Sep 2020 09:45:33 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Alexander Gordeev <agordeev@...ux.ibm.com>
Cc:     Jason Gunthorpe <jgg@...pe.ca>, John Hubbard <jhubbard@...dia.com>,
        Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
        Dave Hansen <dave.hansen@...el.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>,
        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>,
        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 Fri, Sep 11, 2020 at 5:20 AM Alexander Gordeev
<agordeev@...ux.ibm.com> wrote:
>
> What if the entry is still pud_present, but got remapped after
> READ_ONCE(*pudp)? IOW, it is still valid, but points elsewhere?

That can't happen.

The GUP walk doesn't hold any locks, but it *is* done with interrupts
disabled, and anybody who is modifying the page tables needs to do the
TLB flush, and/or RCU-free them.

The interrupt disable means that on architectures where the TLB flush
involves an IPI, it will be delayed until afterwards, but it also acts
as a big RCU read lock hammer.

So the page tables can get modified under us, but the old pages won't
be released and re-used.

                Linus