Re: [PATCH] drm/panthor: Check VMA boundaries for PMD mappings

[Boris Brezillon <boris.brezillon@collabora.com>]

Hi Matthew,

On Tue, 23 Jun 2026 19:33:42 +0100
Matthew Wilcox <willy@infradead.org> wrote:

> On Tue, Jun 23, 2026 at 08:19:42PM +0200, Christian A. Ehrhardt wrote:
> > The panthor version is only compile tested because I don't
> > have the hardware. However, the code is identical to that
> > fixed in [1] and I have a reproducer for that.
> > 
> > No need for for stable backports. The code is new in 7.1.  
> 
> What documentation did you need to see to persuade you to use
> map_pages instead of writing a huge_fault handler?
> 
> (yes, you're all Doing It Wrong, please stop, but help us help you)

We tried [1], but couldn't figure out how to make it work with the
current locking in the gem_shmem logic, so we decided to postpone the
map_page() addition. We plan to get back to it and implement map_page(),
but my understanding is that we still need the fault()+huge_fault() as a
fallback for when we can't acquire the locks we need to service the
fault. Oh, and BTW, we had just the fault() implementation in the
version that was merged, no huge_fault(), but this fault()
implementation was mapping PMDs which led to another issue [2] that
forced us to add this huge_fault() implem.

I'm all for improving that, but if I'm completely honest, there are
times where our MM-related questions are left unanswered ([3] is one
example), and it gets hard to figure out how to do things properly
without proper guidance from the MM experts. I thought [2] was one of
these, but apparently the MM list/maintainers were not Cc-ed on that
one, oops.

Just to be clear, I'm not blaming anyone here (I know how busy subsystem
maintainers are in general), I'm just trying to explain how we ended up
with these new huge_fault() implementations (one being a fork of the
original implementation) that you were not expecting, but also wanted to
point out that we've been pretty transparent about where we were heading
during the submission process, and we didn't get a clear "no, stop
there!" back then.

Anyway, I'll try to get back to this map_pages() implementation, and
see how we can handle the fallback when locks can't be acquired in that
path (I assume this would mean no PMD mapping in the fallback path if
we get rid of huge_fault()).

Regards,

Boris

[1]https://patchew.org/linux/20251015153018.43735-1-loic.molinari@collabora.com/20251015153018.43735-4-loic.molinari@collabora.com/
[2]https://lore.kernel.org/all/20260319015224.46896-1-pedrodemargomes@gmail.com/
[3]https://lore.kernel.org/all/20260319155028.291f3157@fedora/