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Subject: Re: [RFC PATCH v2 0/3] mm/gup: fix gup_fast with dynamic page table
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To:     Gerald Schaefer <gerald.schaefer@linux.ibm.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        John Hubbard <jhubbard@nvidia.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        linux-mm <linux-mm@kvack.org>, Paul Mackerras <paulus@samba.org>,
        linux-sparc <sparclinux@vger.kernel.org>,
        Alexander Gordeev <agordeev@linux.ibm.com>,
        Claudio Imbrenda <imbrenda@linux.ibm.com>,
        Will Deacon <will@kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        linux-s390 <linux-s390@vger.kernel.org>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Richard Weinberger <richard@nod.at>,
        linux-x86 <x86@kernel.org>, Russell King <linux@armlinux.org.uk>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Ingo Molnar <mingo@redhat.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Heiko Carstens <hca@linux.ibm.com>,
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        linux-um <linux-um@lists.infradead.org>,
        Borislav Petkov <bp@alien8.de>,
        Andy Lutomirski <luto@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-arm <linux-arm-kernel@lists.infradead.org>,
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        LKML <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Mike Rapoport <rppt@kernel.org>
References: <20200907180058.64880-1-gerald.schaefer@linux.ibm.com>
From:   Christophe Leroy <christophe.leroy@csgroup.eu>
Message-ID: <9f9acde7-c400-53b0-38ec-ad3e48de5371@csgroup.eu>
Date:   Tue, 8 Sep 2020 06:42:50 +0200
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Le 07/09/2020 à 20:00, Gerald Schaefer a écrit :
> This is v2 of an RFC previously discussed here:
> https://lore.kernel.org/lkml/20200828140314.8556-1-gerald.schaefer@linux.ibm.com/
> 
> Patch 1 is a fix for a regression in gup_fast on s390, after our conversion
> to common gup_fast code. It will introduce special helper functions
> pXd_addr_end_folded(), which have to be used in places where pagetable walk
> is done w/o lock and with READ_ONCE, so currently only in gup_fast.
> 
> Patch 2 is an attempt to make that more generic, i.e. change pXd_addr_end()
> themselves by adding an extra pXd value parameter. That was suggested by
> Jason during v1 discussion, because he is already thinking of some other
> places where he might want to switch to the READ_ONCE logic for pagetable
> walks. In general, that would be the cleanest / safest solution, but there
> is some impact on other architectures and common code, hence the new and
> greatly enlarged recipient list.
> 
> Patch 3 is a "nice to have" add-on, which makes pXd_addr_end() inline
> functions instead of #defines, so that we get some type checking for the
> new pXd value parameter.
> 
> Not sure about Fixes/stable tags for the generic solution. Only patch 1
> fixes a real bug on s390, and has Fixes/stable tags. Patches 2 + 3 might
> still be nice to have in stable, to ease future backports, but I guess
> "nice to have" does not really qualify for stable backports.

If one day you have to backport a fix that requires patch 2 and/or 3, 
just mark it "depends-on:" and the patches will go in stable at the 
relevant time.

Christophe