Re: [BUG 5.14] arm64/mm: dma memory mapping fails (in some cases)

[David Hildenbrand <david@redhat.com>]

On 25.08.21 12:20, Catalin Marinas wrote:
> + hch
> 
> On Tue, Aug 24, 2021 at 08:59:22PM +0200, David Hildenbrand wrote:
>> On 24.08.21 20:46, Robin Murphy wrote:
>>> On 2021-08-24 19:28, Mike Rapoport wrote:
>>>> On Tue, Aug 24, 2021 at 06:37:41PM +0100, Catalin Marinas wrote:
>>>>> On Tue, Aug 24, 2021 at 03:40:47PM +0200, Alex Bee wrote:
>>>>>> it seems there is a regression in arm64 memory mapping in 5.14, since it
>>>>>> fails on Rockchip RK3328 when the pl330 dmac tries to map with:
>>>>>>
>>>>>>   ------------[ cut here ]------------
>>>>>>   WARNING: CPU: 2 PID: 373 at kernel/dma/mapping.c:235 dma_map_resource+0x68/0xc0
>>>>>>   Modules linked in: spi_rockchip(+) fuse
>>>>>>   CPU: 2 PID: 373 Comm: systemd-udevd Not tainted 5.14.0-rc7 #1
>>>>>>   Hardware name: Pine64 Rock64 (DT)
>>>>>>   pstate: 80000005 (Nzcv daif -PAN -UAO -TCO BTYPE=--)
>>>>>>   pc : dma_map_resource+0x68/0xc0
>>>>>>   lr : pl330_prep_slave_fifo+0x78/0xd0
>>>>>>   sp : ffff800012102ae0
>>>>>>   x29: ffff800012102ae0 x28: ffff000005c94800 x27: 0000000000000000
>>>>>>   x26: ffff000000566bd0 x25: 0000000000000001 x24: 0000000000000001
>>>>>>   x23: 0000000000000002 x22: ffff000000628c00 x21: 0000000000000001
>>>>>>   x20: ffff000000566bd0 x19: 0000000000000001 x18: 0000000000000000
>>>>>>   x17: 0000000000000000 x16: 0000000000000000 x15: 0000000000000000
>>>>>>   x14: 0000000000000277 x13: 0000000000000001 x12: 0000000000000000
>>>>>>   x11: 0000000000000001 x10: 00000000000008e0 x9 : ffff800012102a80
>>>>>>   x8 : ffff000000d14b80 x7 : ffff0000fe7b12f0 x6 : ffff0000fe7b1100
>>>>>>   x5 : fffffc000000000f x4 : 0000000000000000 x3 : 0000000000000001
>>>>>>   x2 : 0000000000000001 x1 : 00000000ff190800 x0 : ffff000000628c00
>>>>>>   Call trace:
>>>>>>     dma_map_resource+0x68/0xc0
>>>>>>     pl330_prep_slave_sg+0x58/0x220
>>>>>>     rockchip_spi_prepare_dma+0xd8/0x2c0 [spi_rockchip]
>>>>>>     rockchip_spi_transfer_one+0x294/0x3d8 [spi_rockchip]
>>>>> [...]
>>>>>> Note: This does not relate to the spi driver - when disabling this device in
>>>>>> the device tree it fails for any other (i2s, for instance) which uses dma.
>>>>>> Commenting out the failing check at [1], however, helps and the mapping
>>>>>> works again.
>>>>
>>>>> Do you know which address dma_map_resource() is trying to map (maybe
>>>>> add some printk())? It's not supposed to map RAM, hence the warning.
>>>>> Random guess, the address is 0xff190800 (based on the x1 above but the
>>>>> regs might as well be mangled).
>>>>
>>>> 0xff190800 will cause this warning for sure. It has a memory map, but it is
>>>> not RAM so old version of pfn_valid() would return 0 and the new one
>>>> returns 1.
>>>
>>> How does that happen, though? It's not a memory address, and it's not
>>> even within the bounds of anywhere there should or could be memory. This
>>> SoC has a simple memory map - everything from 0 to 0xfeffffff goes to
>>> the DRAM controller (which may not all be populated, and may have pieces
>>> carved out by secure firmware), while 0xff000000-0xffffffff is MMIO. Why
>>> do we have pages (or at least the assumption of pages) for somewhere
>>> which by all rights should not have them?
>>
>> Simple: we allocate the vmemmap for whole sections (e.g., 128 MiB) to avoid
>> any such hacks. If there is a memory hole, it gets a memmap as well.
>>
>> Tricking pfn_valid() into returning "false" where we actually have a memmap
>> only makes it look like there is no memmap; but there is one, and
>> it's PG_reserved.
> 
> I can see the documentation for pfn_valid() does not claim anything more
> than the presence of an memmap entry. But I wonder whether the confusion
> is wider-spread than just the DMA code. At a quick grep, try_ram_remap()
> assumes __va() can be used on pfn_valid(), though I suspect it relies on
> the calling function to check that the resource was RAM. The arm64
> kern_addr_valid() returns true based on pfn_valid() and kcore.c uses
> standard memcpy on it, which wouldn't work for I/O (should we change
> this check to pfn_is_map_memory() for arm64?).
> 

kern_addr_valid() checks that there is a direct map entry, and that the 
mapped address has a valid mmap. (copied from x86-64)

Would you expect to have a direct map for memory holes and similar (IOW, 
!System RAM)?


>>>>> Either pfn_valid() gets confused in 5.14 or something is wrong with the
>>>>> DT. I have a suspicion it's the former since reverting the above commit
>>>>> makes it disappear.
>>>>
>>>> I think pfn_valid() actually behaves as expected but the caller is wrong
>>>> because pfn_valid != RAM (this applies btw to !arm64 as well).
>>>>
>>>> 	/* Don't allow RAM to be mapped */
>>>> 	if (WARN_ON_ONCE(pfn_valid(PHYS_PFN(phys_addr))))
>>>> 		return DMA_MAPPING_ERROR;
>>>>
>>>> Alex, can you please try this patch:
>>>
>>> That will certainly paper over the issue, but it's avoiding the question
>>> of what went wrong with the memory map in the first place. The comment
>>> is indeed a bit inaccurate, but ultimately dma_map_resource() exists for
>>> addresses that would be wrong to pass to dma_map_page(), so I believe
>>> pfn_valid() is still the correct check.
>>
>> If we want to check for RAM, pfn_valid() would be wrong. If we want to check
>> for "is there a memmap, for whatever lives or does not live there",
>> pfn_valid() is the right check.
> 
> So what should the DMA code use instead? Last time we needed something
> similar, the recommendation was to use pfn_to_online_page(). Mike is
> suggesting memblock_is_memory().

We use pfn_to_online_page() when we want to know if it's system RAM and 
that the memmap actually contains something sane (-> memmap content has 
a well defined state).

You can have offline memory blocks where pfn_to_online_page() would 
return "false", memblock_is_memory() would return "true". IOW, there is 
a memmap, it's System RAM, but the memmap is stale and not trustworthy.

If you want to make sure no System RAM (online/offline/...) will get 
mapped, memblock_is_memory() should be the right thing to use. I recall 
that x86 traverse the resource tree instead to exclude system ram 
regions similarly.

> 
> Given how later we are in the -rc cycle, I suggest we revert Anshuman's
> commit 16c9afc77660 ("arm64/mm: drop HAVE_ARCH_PFN_VALID") and try to
> assess the implications in 5.15 (the patch doesn't seem to have the
> arm64 maintainers' ack anyway ;)).

-- 
Thanks,

David / dhildenb