On Sun, Jul 13, 2025 at 08:39:53PM +0900, Harry Yoo wrote:
On Fri, Jul 11, 2025 at 06:18:44PM +0200, David Hildenbrand wrote:
On 09.07.25 15:16, Harry Yoo wrote:
Intrdocue and use {pgd,p4d}_pouplate_kernel() in core MM code when populating PGD and P4D entries corresponding to the kernel address space. The main purpose of these helpers is to ensure synchronization of the kernel portion of the top-level page tables whenever such an entry is populated.
Until now, the kernel has relied on each architecture to handle synchronization of top-level page tables in an ad-hoc manner. For example, see commit 9b861528a801 ("x86-64, mem: Update all PGDs for direct mapping and vmemmap mapping changes").
However, this approach has proven fragile, as it's easy to forget to perform the necessary synchronization when introducing new changes.
To address this, introduce _kernel() varients of the page table
s/varients/variants/
Will fix. Thanks.
population helpers that invoke architecture-specific hooks to properly synchronize the page tables.
I was expecting to see the sync be done in common code -- such that it cannot be missed :)
You mean something like an arch-independent implementation of sync_global_pgds()?
That would be a "much more robust" approach ;)
To do that, the kernel would need to maintain a list of page tables that have kernel portion mapped and perform the sync in the common code.
But determining which page tables to add to the list would be highly architecture-specific. For example, I think some architectures use separate page tables for kernel space, unlike x86 (e.g., arm64 TTBR1, SPARC) and user page tables should not be affected.
sync_global_pgds() can be still implemented per architecture, but it can be called from the common code. We already have something like that for vmalloc that calls arch_sync_kernel_mappings(). It's implemented only by x86-32 and arm, other architectures do not define it.
While doing the sync in common code might be a more robust option in the long term, I'm afraid that making it work correctly across all architectures would be challenging, due to differences in how each architecture manages the kernel address space.
But it's really just rerouting to the arch code where the sync can be done, correct?
Yes, that's correct.
Thanks for taking a look!
-- Cheers, Harry / Hyeonggon