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

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Message-ID: <20200910130233.GK87483@ziepe.ca>
Date:   Thu, 10 Sep 2020 10:02:33 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Alexander Gordeev <agordeev@...ux.ibm.com>
Cc:     Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
        Dave Hansen <dave.hansen@...el.com>,
        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>,
        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 Thu, Sep 10, 2020 at 11:39:25AM +0200, Alexander Gordeev wrote:

> As Gerald mentioned, it is very difficult to explain in a clear way.
> Hopefully, one could make sense ot of it.

I would say the page table API requires this invariant:

        pud = pud_offset(p4d, addr);
        do {
		WARN_ON(pud != pud_offset(p4d, addr);
                next = pud_addr_end(addr, end);
        } while (pud++, addr = next, addr != end);

ie pud++ is supposed to be a shortcut for 
  pud_offset(p4d, next)

While S390 does not follow this. Fixing addr_end brings it into
alignment by preventing pud++ from happening.

The only currently known side effect is that gup_fast crashes, but it
sure is an unexpected thing.

This suggests another fix, which is to say that pud++ is undefined and
pud_offset() must always be called, but I think that would cause worse
codegen on all other archs.

Jason