On Thu, May 12, 2022 at 11:37:48AM +0100, Mel Gorman wrote: > On Wed, May 11, 2022 at 02:20:45PM +0900, Hyeonggon Yoo wrote: > > > pgprot_t vm_get_page_prot(unsigned long vm_flags) > > > { > > > pgprot_t ret = __pgprot(pgprot_val(protection_map[vm_flags & > > > (VM_READ|VM_WRITE|VM_EXEC|VM_SHARED)]) | > > > pgprot_val(arch_vm_get_page_prot(vm_flags))); > > > > > > return arch_filter_pgprot(ret); > > > } > > > EXPORT_SYMBOL(vm_get_page_prot); > > > > I guess it's only set for processes' VMA if no caller is abusing > > vm_get_page_prot() for kernel mappings. > > > > But yeah, just quick guessing does not make us convinced. > > Let's Cc people working on mm. > > > > If kernel never uses _PAGE_PROTNONE for kernel mappings, it's just okay > > not to clear _PAGE_GLOBAL at first in __change_page_attr() if it's not user address, > > because no user will confuse _PAGE_GLOBAL as _PAGE_PROTNONE if it's kernel > > address. right? > > > > I'm not aware of a case where _PAGE_BIT_PROTNONE is used for a kernel > address expecting PROT_NONE semantics instead of the global bit. NUMA > Balancing is not going to accidentally treat a kernel address as if it's > a NUMA hinting fault. By the time a fault is determining if a PTE access > is a numa hinting fault or accesssing a PROT_NONE region, it has been > established that it is a userspace address backed by a valid VMA. > Thanks Mel, and IIUC nor does do_kern_addr_fault() in arch/x86/mm/fault.c expect _PAGE_PROTNONE instead of _PAGE_GLOBAL. I want to make it clear in the code that _PAGE_PROTNONE is only used for user mappings. How does below look? diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h index 40497a9020c6..f8d02b91a90c 100644 --- a/arch/x86/include/asm/pgtable_types.h +++ b/arch/x86/include/asm/pgtable_types.h @@ -35,7 +35,10 @@ #define _PAGE_BIT_DEVMAP _PAGE_BIT_SOFTW4 /* If _PAGE_BIT_PRESENT is clear, we use these: */ -/* - if the user mapped it with PROT_NONE; pte_present gives true */ +/* + * if the user mapped it with PROT_NONE; pte_present gives true + * this is only used for user mappings (with _PAGE_BIT_USER) + */ #define _PAGE_BIT_PROTNONE _PAGE_BIT_GLOBAL #define _PAGE_PRESENT (_AT(pteval_t, 1) << _PAGE_BIT_PRESENT) @@ -115,7 +118,8 @@ #define _PAGE_DEVMAP (_AT(pteval_t, 0)) #endif -#define _PAGE_PROTNONE (_AT(pteval_t, 1) << _PAGE_BIT_PROTNONE) +#define _PAGE_PROTNONE ((_AT(pteval_t, 1) << _PAGE_BIT_USER) | \ + (_AT(pteval_t, 1) << _PAGE_BIT_PROTNONE)) /* * Set of bits not changed in pte_modify. The pte's -- Thanks, Hyeonggon