From 14fb1fd751fa09440634b909e6f5f53e2cba9ae0 Mon Sep 17 00:00:00 2001 From: David Hildenbrand Date: Thu, 3 Aug 2023 16:32:06 +0200 Subject: [PATCH] pgtable: improve pte_protnone() comment Especially the "For PROT_NONE VMAs, the PTEs are not marked _PAGE_PROTNONE" part is wrong: doing an mprotect(PROT_NONE) will end up marking all PTEs on x86_64 as _PAGE_PROTNONE, making pte_protnone() indicate "yes". So let's improve the comment, so it's easier to grasp which semantics pte_protnone() actually has. Link: https://lkml.kernel.org/r/20230803143208.383663-6-david@redhat.com Signed-off-by: David Hildenbrand Acked-by: Mel Gorman Cc: Hugh Dickins Cc: Jason Gunthorpe Cc: John Hubbard Cc: Linus Torvalds Cc: liubo Cc: Matthew Wilcox (Oracle) Cc: Mel Gorman Cc: Paolo Bonzini Cc: Peter Xu Cc: Shuah Khan Signed-off-by: Andrew Morton --- include/linux/pgtable.h | 16 ++++++++++------ 1 file changed, 10 insertions(+), 6 deletions(-) diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h index f34e0f2..6064f45 100644 --- a/include/linux/pgtable.h +++ b/include/linux/pgtable.h @@ -1333,12 +1333,16 @@ static inline int pud_trans_unstable(pud_t *pud) #ifndef CONFIG_NUMA_BALANCING /* - * Technically a PTE can be PROTNONE even when not doing NUMA balancing but - * the only case the kernel cares is for NUMA balancing and is only ever set - * when the VMA is accessible. For PROT_NONE VMAs, the PTEs are not marked - * _PAGE_PROTNONE so by default, implement the helper as "always no". It - * is the responsibility of the caller to distinguish between PROT_NONE - * protections and NUMA hinting fault protections. + * In an inaccessible (PROT_NONE) VMA, pte_protnone() may indicate "yes". It is + * perfectly valid to indicate "no" in that case, which is why our default + * implementation defaults to "always no". + * + * In an accessible VMA, however, pte_protnone() reliably indicates PROT_NONE + * page protection due to NUMA hinting. NUMA hinting faults only apply in + * accessible VMAs. + * + * So, to reliably identify PROT_NONE PTEs that require a NUMA hinting fault, + * looking at the VMA accessibility is sufficient. */ static inline int pte_protnone(pte_t pte) { -- 2.7.4