Re: [GIT PULL] dma-mapping fix for Linux 5.14

[Roman Skakun <rm.skakun@gmail.com>]

Hi, Stefano!

> I don't know on which platform Roman Skakun (CC'ed) found the problem.
> But if we look at arch/arm/mm/dma-mapping.c:__dma_alloc, one of the
> possible options is the "remap_allocator", which calls
> __alloc_remap_buffer, which calls dma_common_contiguous_remap, which
> calls vmap.

Using  Renesas R-car H3 platform.
I have tested this case on 1:1 dom0 with < 4GB memory, but this case
still exists.
I'm still wondering why xen-swiotlb mapped vmalloc'ed addresses for
low memory DMA addresses.

пн, 26 июл. 2021 г. в 23:03, Stefano Stabellini <sstabellini@kernel.org>:
>
> On Mon, 26 Jul 2021, Boris Ostrovsky wrote:
> > On 7/25/21 12:50 PM, Linus Torvalds wrote:
> > > On Sat, Jul 24, 2021 at 11:03 PM Christoph Hellwig <hch@infradead.org> wrote:
> > >
> > >>   - handle vmalloc addresses in dma_common_{mmap,get_sgtable}
> > >>     (Roman Skakun)
> > > I've pulled this, but my reaction is that we've tried to avoid this in
> > > the past. Why is Xen using vmalloc'ed addresses and passing those in
> > > to the dma mapping routines?
> > >
> > > It *smells* to me like a Xen-swiotlb bug, and it would have been
> > > better to try to fix it there. Was that just too painful?
> >
> >
> > Stefano will probably know better but this appears to have something to do with how Pi (and possibly more ARM systems?) manage DMA memory: https://lore.kernel.org/xen-devel/CADz_WD5Ln7Pe1WAFp73d2Mz9wxspzTE3WgAJusp5S8LX4=83Bw@mail.gmail.com/.
>
> The original issue was found on the Raspberry Pi 4, and the fix was in
> swiotlb-xen.c, commit 8b1e868f6. More recently, Roman realized that
> dma_common_mmap might also end up calling virt_to_page on a vmalloc
> address. This is the fix for that.
>
>
> Why is Xen using vmalloc'ed addresses with dma routines at all?
>
> Xen is actually just calling the regular dma_direct_alloc to allocate
> pages (xen_swiotlb_alloc_coherent -> xen_alloc_coherent_pages ->
> dma_direct_alloc). dma_direct_alloc is the generic implementation. Back
> when the original issue was found, dma_direct_alloc returned a vmalloc
> address on RPi4.
>
> The original analysis was "xen_alloc_coherent_pages() eventually calls
> arch_dma_alloc() in remap.c which successfully allocates pages from
> atomic pool." See https://marc.info/?l=xen-devel&m=158878173207775.
>
>
> I don't know on which platform Roman Skakun (CC'ed) found the problem.
> But if we look at arch/arm/mm/dma-mapping.c:__dma_alloc, one of the
> possible options is the "remap_allocator", which calls
> __alloc_remap_buffer, which calls dma_common_contiguous_remap, which
> calls vmap.
>
> So unfortunately it seems that on certain arch/platforms
> dma_alloc_coherent can return a vmap'ed address. So I would imagine this
> issue could also happen on native (without Xen), at least in theory.



-- 
Best Regards, Roman.
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