Re: [PATCH] arm64: mm: Fix memmap to be initialized for the entire section

[Ard Biesheuvel]

On 17 November 2016 at 15:18, Robert Richter <robert.richter@xxxxxxxxxx> wrote:
> Thanks for your answer.
>
> On 17.11.16 14:25:29, Will Deacon wrote:
>> On Wed, Nov 09, 2016 at 08:51:32PM +0100, Robert Richter wrote:
>> > Thus, I don't see where my patch breaks code. Even acpi_os_ioremap()
>> > keeps the same behaviour as before since it still uses memblock_is_
>> > memory(). Could you more describe your concerns why do you think this
>> > patch breaks the kernel and moves the problem somewhere else? I
>> > believe it fixes the problem at all.
>>
>> acpi_os_ioremap always ends up in __ioremap_caller, regardless of
>> memblock_is_memory(). __ioremap_caller then fails if pfn_valid is true.
>
> But that's the reason my patch changed the code to use memblock_is_
> map_memory() instead. I was looking into the users of pfn_valid() esp.
> in arm64 code and changed it where required.
>
> This week I looked into the kernel again for code that might break by
> a pfn_valid() change. I found try_ram_remap() in memremap.c that has
> changed behaviour now, but this is explicit for MEMREMAP_WB, so it
> should be fine.
>
> Maybe it might be better to use page_is_ram() in addition to
> pfn_valid() where necessary. This should work now after commit:
>
>  e7cd190385d1 arm64: mark reserved memblock regions explicitly in iomem
>
> I still think pfn_valid() is not the correct use to determine the mem
> attributes for mappings, there are further checks required.
>
> The risk of breaking something with my patch is small and limited only
> to the mapping of efi reserved regions (which is the state of 4.4). If
> something breaks anyway it can easily be fixed by adding more checks
> to pfn_valid() as suggested above.
>

As I noted before, it looks to me like setting CONFIG_HOLES_IN_ZONE is
the correct way to address this. However, doing that does uncover a
bug in move_freepages() where the VM_BUG_ON_PAGE() dereferences struct
page fields before the pfn_valid_within() check, so it seems those
need to be switched around.

Robert, you mentioned that CONFIG_HOLES_IN_ZONE seems inappropriate
for sparsemem. Care to elaborate why?