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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? Definitely yes! Since this series will be backported to about five -stable kernels (v5.13.x and later), I will add that as part of a follow-up series that is not intended for backporting. Does that sound okay? > I think MEMORY MANAGEMENT - CORE is correct, given the below file is there. Thanks for confirming that! -- Cheers, Harry / Hyeonggon