[PATCH] Revert "mm/pgtable: add stubs for {pmd/pub}_{set/clear}_huge"

[Will Deacon]

On Sat, Jul 17, 2021 at 06:31:08PM +0200, Christophe Leroy wrote:
> Jonathan Marek <jonathan at marek.ca> a écrit :
> 
> > c742199a breaks arm64 for some configs because it stubs out functions which
> > should not have been stubbed out.
> > 
> > With 4K pages and ARM64_VA_BITS_39=y, the kernel crashes early on unmapped
> > 1G pages in the linear map caused by pud_set_huge() in alloc_init_pud()
> > being stubbed out. Reverting c742199a fixes the crash.
> 
> This patch is there for a reason. Reverting it will break some other config.

Which config? Not reverting it breaks arm64.

> The fix needs to allow it work with all platforms.

Hmm, there was already a report that selftests were failing with this
change:

https://lore.kernel.org/linux-arm-kernel/CAMuHMdXShORDox-xxaeUfDW3wx2PeggFSqhVSHVZNKCGK-y_vQ@mail.gmail.com/

That was a week ago and doesn't seem to have progressed, so I'm inclined to
revert this if it's not resolved this week as we usually use -rc3 as a base
for queuing patches for the next release.

> I don't understand, why does arm64 needs these PUD helpers when it defines
> __PAGETABLE_PUD_FOLDED ?

Probably for the huge vmap code; see arch_vmap_pXd_supported(). That also
lines up with the failing selftests afaict.

For 4k pages with 3 levels of walk, we want to be able to use 1GB mappings
at level 1 (pgd), 2MB mappings at level 2 (pmd) as well as the usual 4k
page mappings at level 3 (pte).

Will