Re: [PATCH v3 05/26] x86: move PAE PMD preallocation defines to header

[Yosry Ahmed]

On Sun, Jul 26, 2026 at 10:22:38PM +0000, Brendan Jackman wrote:
> In a subsequent patch these defines will need to be referenced from a new
> file, move them as a separate patch for easy review.
> 
> The comments have style violations (personal pronouns etc), do not fix
> them as this is just code movement.
> 
> No functional change intended.
> 
> Signed-off-by: Brendan Jackman <jackmanb@xxxxxxxxxx>

AFAICT, this is only needed to expose PREALLOCATED_PMDS for
mm_local_map_to_user() in patch #7. The only usage is a BUILD_BUG_ON().

I wonder if it would be more preferrable to keep the definitions in
pgtable.c and add a runtime check instead in mm_local_map_to_user(),
something like:

diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
index 09b8d8e6a56ea..15055cb80092c 100644
--- a/arch/x86/include/asm/mmu_context.h
+++ b/arch/x86/include/asm/mmu_context.h
@@ -268,7 +268,6 @@ static inline pmd_t *pgd_to_pmd_walk(pgd_t *pgd, unsigned long va)

 static inline int mm_local_map_to_user(struct mm_struct *mm)
 {
-       BUILD_BUG_ON(!PREALLOCATED_PMDS);
        pgd_t *k_pgd = pgd_offset(mm, MM_LOCAL_BASE_ADDR);
        pgd_t *u_pgd = kernel_to_user_pgdp(k_pgd);
        pmd_t *k_pmd, *u_pmd;
@@ -277,6 +276,9 @@ static inline int mm_local_map_to_user(struct mm_struct *mm)
        k_pmd = pgd_to_pmd_walk(k_pgd, MM_LOCAL_BASE_ADDR);
        u_pmd = pgd_to_pmd_walk(u_pgd, MM_LOCAL_BASE_ADDR);

+       if (WARN_ON_ONCE(!k_pmd || !u_pmd))
+               return -EINVAL;
+
        BUILD_BUG_ON(MM_LOCAL_END_ADDR - MM_LOCAL_BASE_ADDR > PMD_SIZE);

        /* Preallocate the PTE table so it can be shared. */