linux-kernel - Re: [PATCH v5 05/15] mm/frame-vector: Use FOLL

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Date:   Wed, 4 Nov 2020 19:11:00 +0000
From:   Christoph Hellwig <hch@...radead.org>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     John Hubbard <jhubbard@...dia.com>,
        Christoph Hellwig <hch@...radead.org>,
        Daniel Vetter <daniel.vetter@...ll.ch>,
        J??r??me Glisse <jglisse@...hat.com>,
        linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
        Jan Kara <jack@...e.cz>, Pawel Osciak <pawel@...iak.com>,
        KVM list <kvm@...r.kernel.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        DRI Development <dri-devel@...ts.freedesktop.org>,
        Tomasz Figa <tfiga@...omium.org>,
        Linux MM <linux-mm@...ck.org>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@...r.kernel.org>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>
Subject: Re: [PATCH v5 05/15] mm/frame-vector: Use FOLL_LONGTERM

On Wed, Nov 04, 2020 at 03:02:14PM -0400, Jason Gunthorpe wrote:
> I saw that once and I really couldn't make sense of it..
> What use is having futex's that only work on THP pages? Confused
> 
> CH said there was no case of HAVE_FAST_GUP !ARCH_HAS_PTE_SPECIAL, is
> one hidden someplace then?

ARCH_HAS_PTE_SPECIAL is selected by:

arc
arm		(if LPAE)
arm64
mips		(if !(32BIT && CPU_HAS_RIXI))
powerpc
riscv
s390
sh
sparc		(if SPARC64)
x86

HAVE_FAST_GUP is supported by:

arm		(if LPAE)
arm64
mips
powerpc
s390
sh
x86

so mips would be a candidate.  But I think only in theory, as
the options selecting CPU_HAS_RIXI do not select
CPU_SUPPORTS_HUGEPAGES.  Adding the mips maintainer to double check
my logic.