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From:   Rob Clark <robdclark@chromium.org>
Date:   Thu, 8 Aug 2019 09:44:32 -0700
Message-ID: <CAJs_Fx5fJ31CsFODBgBbhcCvoxSX_D1NHDjQs4LtJ_0GwuxMVA@mail.gmail.com>
Subject: Re: [PATCH 1/2] drm: add cache support for arm64
To:     Christoph Hellwig <hch@lst.de>
Cc:     Mark Rutland <mark.rutland@arm.com>,
        Rob Clark <robdclark@gmail.com>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Maxime Ripard <maxime.ripard@bootlin.com>,
        Sean Paul <sean@poorly.run>, David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        Allison Randal <allison@lohutok.net>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-arm-kernel@lists.infradead.org,
        LKML <linux-kernel@vger.kernel.org>
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On Thu, Aug 8, 2019 at 12:59 AM Christoph Hellwig <hch@lst.de> wrote:
>
> On Wed, Aug 07, 2019 at 10:30:04AM -0700, Rob Clark wrote:
> > So, we do end up using GFP_HIGHUSER, which appears to get passed thru
> > when shmem gets to the point of actually allocating pages.. not sure
> > if that just ends up being a hint, or if it guarantees that we don't
> > get something in the linear map.
> >
> > (Bear with me while I "page" this all back in.. last time I dug thru
> > the shmem code was probably pre-armv8, or at least before I had any
> > armv8 hw)
>
> GFP_HIGHUSER basically just means that this is an allocation that could
> dip into highmem, in which case it would not have a kernel mapping.
> This can happen on arm + LPAE, but not on arm64.

Just a dumb question, but why is *all* memory in the linear map on
arm64?  It would seem useful to have a source of pages that is not in
the linear map.
I guess it is mapped as huge pages (or something larger than 4k pages)?

Any recommended reading to understand how/why the kernel address space
is setup the way it is (so I can ask fewer dumb questions)?

BR,
-R