[PATCH] ARM: mm: fix no-MMU ZERO_PAGE() implementation

[Arnd Bergmann]

On Tue, Oct 18, 2022, at 19:44, Giulio Benetti wrote:
> On 18/10/22 09:03, Arnd Bergmann wrote:
>> On Tue, Oct 18, 2022, at 1:37 AM, Giulio Benetti wrote:
>>> Actually in no-MMU SoCs(i.e. i.MXRT) ZERO_PAGE(vaddr) expands to
>
>> It looks like we dropped the ball on this when it came up last.
>> I'm also not sure when we started requiring this, any idea?
>
> No to be honest. But in my case I've met ZERO_PAGE() calling in sdhci
> driver. And as stated on the ML link above:
> ```
> But I wonder if it's safe for noMMU architectures to go on without a
> working ZERO_PAGE(0).  It has uses scattered throughout the tree, in
> drivers, fs, crypto and more, and it's not at all obvious (to me) that
> they all depend on CONFIG_MMU.
> ```
> And I've found this driver that requires it and probably is not the last
> since imxrt support is not complete.
>
>> I can see that microblaze-nommu used BUG() in ZERO_PAGE(), so at
>> whenever microblaze last worked, we clearly did not call it.
>
> This probably means that microblaze-nommu doesn't use drivers or other
> subsystems that require ZERO_PAGE().

To clarify: microblaze-nommu support was removed two years ago,
and probably was already broken for a while before that.

>> In addition to your fix, I see that arm is the only architecture
>> that defines 'empty_zero_page' as a pointer to the page, when
>> everything else just makes it a pointer to the data itself,
>> or an 'extern char empty_zero_page[]' array, which we may want
>> to change for consistency.
>
> I was about doing it, but then I tought to move one piece at a time.

Right, it would definitely be a separate patch, but it
can be a series of two patches. We probably wouldn't need to
backport the second patch that turns it into a static allocation.

> But yes, I can modify accordingly. That way we also avoid the early
> allocation in pagint_init() since it would be a .bss array.

>> There are three references to empty_zero_page in architecture
>> independent code, and while we don't seem to use any of them
>> on Arm, they would clearly be wrong if we did:
>> 
>> drivers/acpi/scan.c:#define INVALID_ACPI_HANDLE ((acpi_handle)empty_zero_page)
>> drivers/spi/spi-fsl-cpm.c:      mspi->dma_dummy_tx = dma_map_single(dev, empty_zero_page, PAGE_SIZE,
>> include/linux/raid/pq.h:# define raid6_empty_zero_page empty_zero_page
>
> For them I can send patches to substitute with PAGE_ZERO(0) correctly 
> adapted.
>
> What do you think?

That sounds like a good idea as well.

     Arnd