Re: Is _PAGE_PROTNONE set only for user mappings?

[Hyeonggon Yoo <42.hyeyoo@gmail.com>]

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