Re: [PATCH V4 mm-hotfixes 0/3] mm, x86: fix crash due to missing page table sync and make it harder to miss

[Kiryl Shutsemau]

On Mon, Aug 11, 2025 at 02:34:17PM +0900, Harry Yoo wrote:
> # The solution: Make page table sync more code robust and harder to miss
> 
> To address this, Dave Hansen suggested [3] [4] introducing
> {pgd,p4d}_populate_kernel() for updating kernel portion
> of the page tables and allow each architecture to explicitly perform
> synchronization when installing top-level entries. With this approach,
> we no longer need to worry about missing the sync step, reducing the risk
> of future regressions.

Looks sane:

Acked-by: Kiryl Shutsemau <kas@xxxxxxxxxx>

> The new interface reuses existing ARCH_PAGE_TABLE_SYNC_MASK,
> PGTBL_P*D_MODIFIED and arch_sync_kernel_mappings() facility used by
> vmalloc and ioremap to synchronize page tables.
> 
> pgd_populate_kernel() looks like this:
> static inline void pgd_populate_kernel(unsigned long addr, pgd_t *pgd,
>                                        p4d_t *p4d)
> {
>         pgd_populate(&init_mm, pgd, p4d);
>         if (ARCH_PAGE_TABLE_SYNC_MASK & PGTBL_PGD_MODIFIED)
>                 arch_sync_kernel_mappings(addr, addr);
> }
> 
> It is worth noting that vmalloc() and apply_to_range() carefully
> synchronizes page tables by calling p*d_alloc_track() and
> arch_sync_kernel_mappings(), and thus they are not affected by
> this patch series.

Well, except ARCH_PAGE_TABLE_SYNC_MASK is not defined on x86-64 until
now. So I think it is affected.

-- 
Kiryl Shutsemau / Kirill A. Shutemov