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

[Giulio Benetti]

On 18/10/22 20:35, Arnd Bergmann wrote:
> 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.

I've sent the patchset of 2:
https://lore.kernel.org/all/20221018222503.90118-1-giulio.benetti@xxxxxxxxxxxxxxxxxxxxxx/T/#t

I'm wondering if it makes sense to send a patchset for all those
architectures that have only one zero page. I've seen that for example
loongarch has more than one. But for the others I find the array
approach more linear, with less code all around and a bit faster in term
of code execution(of course really few, but better than nothing) since
that array is in .bss, so it will be zeroed earlier during a long
"memset" where assembly instructions for zeroing 8 bytes at a time are
used. What about this?

> > 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.

I've just sent a patchset for this:
https://lore.kernel.org/all/20221018215755.33566-1-giulio.benetti@xxxxxxxxxxxxxxxxxxxxxx/T/#t

Best regards
--
Giulio Benetti
CEO/CTO@Benetti Engineering sas