linux-kernel - Re: [PATCH 2/4] drm/i915/gem: Sync the vmap PTEs upon construction

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Date:   Fri, 21 Aug 2020 05:41:03 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Chris Wilson <chris@...is-wilson.co.uk>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        intel-gfx <intel-gfx@...ts.freedesktop.org>,
        Linux-MM <linux-mm@...ck.org>, Pavel Machek <pavel@....cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Joerg Roedel <jroedel@...e.de>,
        Dave Airlie <airlied@...hat.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        stable <stable@...r.kernel.org>
Subject: Re: [PATCH 2/4] drm/i915/gem: Sync the vmap PTEs upon construction

On Fri, Aug 21, 2020 at 1:50 AM Chris Wilson <chris@...is-wilson.co.uk> wrote:
>
> Since synchronising the PTE after assignment is a manual step, use the
> newly exported interface to flush the PTE after assigning via
> alloc_vm_area().

This commit message doesn't make much sense to me.

Are you talking about synchronizing the page directory structure
across processes after possibly creating new kernel page tables?

Because that has nothing to do with the PTE. It's all about making
sure the _upper_ layers of the page directories are populated
everywhere..

The name seems off to me too - what are you "flushing"? (And yes, I
know about the flush_cache_vmap(), but that looks just bogus, since
any non-mapped area shouldn't have any virtual caches to begin with,
so I suspect that is just the crazy architectures being confused -
flush_cache_vmap() is a no-op on any sane architecture - and powerpc
that mis-uses it for other things).

                   Linus