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

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Message-ID: <20200910215921.GP87483@ziepe.ca>
Date:   Thu, 10 Sep 2020 18:59:21 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...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>,
        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 12:32:05PM -0700, Linus Torvalds wrote:
> Yeah, I get hung up on naming sometimes. I don't tend to care much
> about private local variables ("i" is a perfectly fine variable name),
> but these kinds of somewhat subtle cross-architecture definitions I
> feel matter.

One of the first replys to this patch was to ask "when would I use
_orig vs normal", so you are not alone. The name should convey it..

So, I suggest pXX_offset_unlocked()

Since it is safe to call without the page table lock, while pXX_offset()
requires the page table lock to be held as the internal *pXX is a data
race otherwise.

Patch 1 might be OK for a stable backport, but to get to a clear
pXX_offset_unlocked() all the arches would want to be changed to
implement that API and the generic code would provide the wrapper:

#define pXX_offset(pXXp, address) pXX_offset_unlocked(pXXp, *(pXXp), address)

Arches would not have a *pXX inside their code.

Then we can talk about auditing call sites of pXX_offset and think
about using the _unlocked version in places where the page table lock
is not held.

For instance mm/pagewalk.c should be changed. So should
huge_pte_offset() and probably other places. These places might
already be exsting data-race bugs.

It is code-as-documentation indicating an unlocked page table walk.

Now it is not just a S390 story but a change that makes the data
concurrency much clearer, so I think I prefer this version to the
addr_end one too.

Regards,
Jason