Messages in this thread | | | From | Roman Skakun <> | Date | Thu, 29 Jul 2021 13:17:09 +0300 | Subject | Re: [GIT PULL] dma-mapping fix for Linux 5.14 |
| |
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.
| |