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Aug 11, 2025 at 09:12:08PM +0900, Harry Yoo wrote: > > On Mon, Aug 11, 2025 at 12:38:37PM +0100, Lorenzo Stoakes wrote: > > > On Mon, Aug 11, 2025 at 02:34:19PM +0900, Harry Yoo wrote: > > > > Introduce and use {pgd,p4d}_populate_kernel() in core MM code when > > > > populating PGD and P4D entries for the kernel address space. > > > > These helpers ensure proper synchronization of page tables when > > > > updating the kernel portion of top-level page tables. > > > > > > > > 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 for following reasons: > > > > > > > > 1) It is easy to forget to perform the necessary page table > > > > synchronization when introducing new changes. > > > > For instance, commit 4917f55b4ef9 ("mm/sparse-vmemmap: improve memory > > > > savings for compound devmaps") overlooked the need to synchronize > > > > page tables for the vmemmap area. > > > > > > > > 2) It is also easy to overlook that the vmemmap and direct mapping areas > > > > must not be accessed before explicit page table synchronization. > > > > For example, commit 8d400913c231 ("x86/vmemmap: handle unpopulated > > > > sub-pmd ranges")) caused crashes by accessing the vmemmap area > > > > before calling sync_global_pgds(). > > > > > > > > To address this, as suggested by Dave Hansen, introduce _kernel() variants > > > > of the page table population helpers, which invoke architecture-specific > > > > hooks to properly synchronize page tables. These are introduced in a new > > > > header file, include/linux/pgalloc.h, so they can be called from common code. > > > > > > > > They reuse existing infrastructure for vmalloc and ioremap. > > > > Synchronization requirements are determined by ARCH_PAGE_TABLE_SYNC_MASK, > > > > and the actual synchronization is performed by arch_sync_kernel_mappings(). > > > > > > > > This change currently targets only x86_64, so only PGD and P4D level > > > > Hi Lorenzo, thanks for looking at this! > > > > > Well, arm defines ARCH_PAGE_TABLE_SYNC_MASK in arch/arm/include/asm/page.h. But > > > it aliases this to PGTBL_PMD_MODIFIED so will remain unaffected :) > > > > Oh, here I just intended to explain why I didn't implement > > {pud,pmd}_populate_kernel(). > > I'd add that arm handles PGTBL_PMD_MODIFIED and therefore remains unaffected > just to be super clear. Will do: This change currently targets only x86_64, so only PGD and P4D level helpers are introduced. Currently, these helpers are no-ops since no architecture sets PGTBL_{PGD,P4D}_MODIFIED in ARCH_PAGE_TABLE_SYNC_MASK. In theory, PUD and PMD level helpers can be added later if needed by other architectures. For now, 32-bit architectures (x86-32 and arm) only handle PGTBL_PMD_MODIFIED, so p*d_populate_kernel() will never affect them unless we introduce a PMD level helper. > > > > helpers are introduced. In theory, PUD and PMD level helpers can be added > > > > later if needed by other architectures. > > > > > > > > Currently this is a no-op, since no architecture sets > > > > PGTBL_{PGD,P4D}_MODIFIED in ARCH_PAGE_TABLE_SYNC_MASK. > > > > > > > > Cc: > > > > Fixes: 8d400913c231 ("x86/vmemmap: handle unpopulated sub-pmd ranges") > > > > Suggested-by: Dave Hansen > > > > Signed-off-by: Harry Yoo > > Given that I missed you fixed the vmalloc.h thing, this LGTM so: > > Reviewed-by: Lorenzo Stoakes Thanks! -- Cheers, Harry / Hyeonggon