Re: [PATCH v2 0/8] arm64: Unmap FF-A lent memory from direct map

[Sumit Garg]

On Tue, 29 Sep 2026 at 12:33:07 +0100, Will Deacon wrote:
> On Sat, Sep 26, 2026 at 05:43:08PM +0530, Sumit Garg wrote:
> > Looks like you dropped me from your reply.
>
> Huh, that's weird. If I hit reply-all ('g') in mutt, it moves the entire
> CC list to To: and drops you. I suspect the same thing happened to Thierry
> when he replied here:
>
> https://lore.kernel.org/all/arJcN60nagFKXj2Q@orome/
>
> I can't figure out why that's happening :/
> I manually tried to fix the To: and Cc: lines in this message.

Thanks.

> > On Tue, 22 Sep 2026 at 15:55:59 +0100, Will Deacon wrote:
> > > I spoke to Brendan at LPC (?) last year but this doesn't really work
> > > for arm64 because it relies on being able to unmap arbitrary parts of
> > > the linear map, which isn't generally possible unless you force pte-level
> > > mappings for everything, which is prohibitive for perf/power.
> >
> > As per the cover letter it's about allocating pages that are not present
> > in the direct map. This essentially fits the protected DMABufs use-case
> > where we don't want any kernel mapping to exist at any time. Bufers
> > allocated from protected DMABufs are only meant to be accessed by the
> > TEE implementation or HW accelerators like in the secure media pipeline.
>
> That sounds like it should probably build on top of this series, then.

The main problem I see with this series is it tries to move from the generic
CMA allocator to a reserved FF-A CMA pool which again comes from the
DT. We already support reserved pool allocations via "no-map" which are
discovered dynamically from OP-TEE.

If we can rather support a generic CMA like allocator for arm64 which
can support unmapped allocations then I am all for it. These platform
specific reserved pools isn't a scalable solution.

> You could allocate the DMABufs from a CMA region that has no linear alias.

Allocation from generic CMA pool is already the case here:

tee_shm_alloc_dma_mem() ->
  dma_alloc_pages()

Do you mean it's just fine to unmap buffers allocated from generic CMA
region since it doesn't have any linear alias?

> > > Vincent's series tackles that by using a pool so that only that part of
> > > memory requires the pte-level mappings in the linear map. That's the
> > > whole point of it, so I don't think it makes sense to drop it in favour
> > > of Brendan's approach (which doesn't work).
> > >
> >
> > For protected DMABufs, we even don't require any pte-level mappings in
> > linear map. The whole idea of mapping and unmapping is just a bottleneck
> > for performance sensitive secure media pipeline use-cases. That's why if
> > we can support __GFP_UNMAPPED with the memory allocator on arm64 would
> > be the best fit for this use-case.
>
> The point is that you will require pte-level mappings for the linear map
> if you want to unmap from it at runtime on arm64. If you don't, then you
> can end up needing to split a block mapping (e.g. pmd-level) when you
> decide to unmap only part of it and there isn't a safe way to do that on
> arm64 without transiently unmapping the entire block, which can lead to
> fatal translation faults during that window.

I agree with you here, what I am rather trying to find if on arm64 there
is any possibilty to have a generic allocator for unmapped pages. That's
the use-case here as well as what Brendan't patch-set was trying to address.

> > Now the question is why arm64 can't support __GFP_UNMAPPED similar to
> > how Brendan is doing it for x86?
>
> Because arm64 != x86? Presumably x86 doesn't have the strict
> break-before-make requirements we have on arm64.

Sure, sounds like you are eluding to that the generic MM flag
__GFP_UNMAPPED being proposed can't be supported on arm64, right?

-Sumit