linux-kernel - [RFC][PATCH 5/8] x86/mm: Fix up comments around PMD preallocation

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Message-Id: <20250123172436.A367153A@davehans-spike.ostc.intel.com>
Date: Thu, 23 Jan 2025 09:24:36 -0800
From: Dave Hansen <dave.hansen@...ux.intel.com>
To: linux-kernel@...r.kernel.org
Cc: x86@...nel.org,tglx@...utronix.de,bp@...en8.de,joro@...tes.org,luto@...nel.org,peterz@...radead.org,kirill.shutemov@...ux.intel.com,rick.p.edgecombe@...el.com,jgross@...e.com,Dave Hansen <dave.hansen@...ux.intel.com>
Subject: [RFC][PATCH 5/8] x86/mm: Fix up comments around PMD preallocation


From: Dave Hansen <dave.hansen@...ux.intel.com>

The "paravirt environment" is no longer in the tree. Axe that part of the
comment. Also add a blurb to remind readers that "USER_PMDS" refer to
the PTI user *copy* of the page tables, not the user *portion*.

Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
---

 b/arch/x86/mm/pgtable.c |    9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff -puN arch/x86/mm/pgtable.c~simplify-PREALLOCATED_PMDS-2 arch/x86/mm/pgtable.c
--- a/arch/x86/mm/pgtable.c~simplify-PREALLOCATED_PMDS-2	2025-01-23 09:20:52.098399043 -0800
+++ b/arch/x86/mm/pgtable.c	2025-01-23 09:20:52.102399187 -0800
@@ -160,16 +160,17 @@ static void pgd_dtor(pgd_t *pgd)
  * processor notices the update.  Since this is expensive, and
  * all 4 top-level entries are used almost immediately in a
  * new process's life, we just pre-populate them here.
- *
- * Also, if we're in a paravirt environment where the kernel pmd is
- * not shared between pagetables (!SHARED_KERNEL_PMDS), we allocate
- * and initialize the kernel pmds here.
  */
 #define PREALLOCATED_PMDS	(static_cpu_has(X86_FEATURE_PTI) ? \
 					PTRS_PER_PGD : KERNEL_PGD_BOUNDARY)
 #define MAX_PREALLOCATED_PMDS	PTRS_PER_PGD
 
 /*
+ * "USER_PMDS" are the PMDs for the user copy of the page tables when
+ * PTI is enabled. They do not exist when PTI is disabled.  Note that
+ * this is distinct from the user _portion_ of the kernel page tables
+ * which always exists.
+ *
  * We allocate separate PMDs for the kernel part of the user page-table
  * when PTI is enabled. We need them to map the per-process LDT into the
  * user-space page-table.
_