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Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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 > 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 > --- > include/linux/pgalloc.h | 24 ++++++++++++++++++++++++ Could we put this in the correct place in MAINTAINERS please? I think MEMORY MANAGEMENT - CORE is correct, given the below file is there. > include/linux/pgtable.h | 4 ++-- Thanks!