Re: issue with MEMBLOCK_NOMAP

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On 29 January 2016 at 16:53, Mark Salter <msalter@xxxxxxxxxx> wrote:
> On Fri, 2016-01-29 at 15:06 +0100, Ard Biesheuvel wrote:
>> On 29 January 2016 at 15:00, Mark Salter <msalter@xxxxxxxxxx> wrote:
>> > Hi Ard,
>> >
>> > I ran into an issue with your MEMBLOCK_NOMAP changes on a particular
>> > firmware. The symptom is the kernel panics at boot time when it hits
>> > an unmapped page while unpacking the initramfs. As it turns out, the
>> > start of the initramfs shares a 64k kernel page with the UEFI memmap.
>> > I can avoid the problem with:
>> >
>> > @@ -203,7 +203,7 @@ void __init efi_init(void)
>> >
>> >         reserve_regions();
>> >         early_memunmap(memmap.map, params.mmap_size);
>> > -       memblock_mark_nomap(params.mmap & PAGE_MASK,
>> > -                           PAGE_ALIGN(params.mmap_size +
>> > -                                      (params.mmap & ~PAGE_MASK)));
>> > +       memblock_reserve(params.mmap & PAGE_MASK,
>> > +                        PAGE_ALIGN(params.mmap_size +
>> > +                                   (params.mmap & ~PAGE_MASK)));
>> >  }
>> >
>> >
>> > But it makes me worry about the same potential problem with
>> > other reserved regions which we nomap. What do you think?
>> >
>>
>> So I take it this initramfs allocation is not made by the stub but by
>> GRUB? Since the stub rounds all allocations to 64 KB ...
>>
> Yes. GRUB.
>

We have already fixed EDK2 a while ago to round up all regions
returned by AllocatePages() to round up to 64 KB. Do you know if this
is a GRUB issue (i.e., it traverses the memory map and finds a free
range and explicitly allocates it) or a firmware issue?

>> In any case, regardless of the underlying cause, if any part of the
>> initramfs turns out not to be covered by the linear mapping, we should
>> invoke your code to move it. So I think it should be a matter of
>> refining the logic in relocate_initrd() to do the right thing in this
>> case
>
> That thought had crossed my mind. I think it would be easy enough to
> trigger the copy if first or last page of initrd is unmapped.

Indeed. If some page in the middle is missing, then you're really
doing something fishy, so I don't see why we should care about that as
well.

> Somewhat
> related to this is that I want to rework this old patch to deal with
> acpi tables outside mapped ram:
>
>   https://lkml.org/lkml/2015/5/14/357
>
> Basically, we should be able to just do:
>
> diff --git a/arch/arm64/include/asm/acpi.h b/arch/arm64/include/asm/acpi.h
> index 15e0aad..4ea638c 100644
> --- a/arch/arm64/include/asm/acpi.h
> +++ b/arch/arm64/include/asm/acpi.h
> @@ -32,7 +32,7 @@
>  static inline void __iomem *acpi_os_ioremap(acpi_physical_address phys,
>                                             acpi_size size)
>  {
> -       if (!page_is_ram(phys >> PAGE_SHIFT))
> +       if (!memblock_is_memory(phys))
>                 return ioremap(phys, size);
>
>         return ioremap_cache(phys, size);
>

I think we should fix acpi_os_ioremap(). IIRC it is used via two
different code paths that distinguish between memory and I/O, and end
up using the same function for no good reason.

> But this doesn't currently work wrt mem= which works by removing
> the end range of memblocks. If I have mem= use the nomap flag
> rather than removing the range, the above acpi_os_ioremap change
> works, but other issues crop up due to memblock_end_of_DRAM()
> returning end of all DRAM regardless of mem=. So we end up with
> PFNs and struct pages for memory which will never be in linear
> map. Fixing memblock_end_of_DRAM() to look at the flags and
> return end of mapped DRAM gets things working but I wonder about
> other potential trouble spots with this approach. Any thoughts?
>

Actually, I think mem= should be considered a development feature, not
a production feature, and if its use is suboptimal in this respect, so
be it.

But to address this particular issue, it would probably be better to
fix page_is_ram(). I have made some attempts in that direction in the
past, but that never landed anywhere. Since ACPI on arm64 is tightly
coupled to UEFI, implementing page_is_ram() as something that
interrogates the UEFI memory map if efi_enabled(EFI_MEMMAP) would be
reasonable imo. (Or perhaps putting that in acpi_os_ioremap()
directly?)

>
>>
>> Your suggested change will break 32-bit ARM, since we use
>> ioremap_nocache() to map the UEFI memory map, and ARM does not allow
>> that on ranges that are part of the linear mapping.
>
> okay. I'll put together a patch to the initrd relocating code.
>

Great!
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