[PATCH RFC v3 14/21] x86: mm: carve out the generic compile-time folded pgtable case in effective_prot()

[Yeoreum Yun]

The behaviour of pXdp_get() will change with generic compile-time folded
page tables. To prepare for that, rework effective_prot() to ignore any
folded page tables entries, as it will unconditionally get called by
ptdump core with pXd_val(pXdp_get()), and the value for folded entries
really should be ignored.

For this, introduce mm_first_pgtable_level() to identify the first real
page-table level, move mm_pXd_folded() in !__ASSEMBLER__ and
update effective_prot with higher_prot only when the current entry is not
folded nor the first level.

There should be no functional change.

Signed-off-by: Yeoreum Yun <yeoreum.yun@xxxxxxx>
---
 arch/x86/mm/dump_pagetables.c |  6 +++--
 include/linux/pgtable.h       | 61 ++++++++++++++++++++++++++++++++-----------
 2 files changed, 50 insertions(+), 17 deletions(-)

diff --git a/arch/x86/mm/dump_pagetables.c b/arch/x86/mm/dump_pagetables.c
index a18a61e732c3..2acbe41eb58f 100644
--- a/arch/x86/mm/dump_pagetables.c
+++ b/arch/x86/mm/dump_pagetables.c
@@ -38,6 +38,7 @@ struct pg_state {
 	bool check_wx;
 	unsigned long wx_pages;
 	struct seq_file *seq;
+	struct mm_struct *mm;
 };
 
 struct addr_marker {
@@ -254,7 +255,7 @@ static void effective_prot(struct ptdump_state *pt_st, enum pgtable_level level,
 	pgprotval_t prot = val & PTE_FLAGS_MASK;
 	pgprotval_t effective;
 
-	if (level > PGTABLE_LEVEL_PGD) {
+	if (level > mm_first_pgtable_level(st->mm)) {
 		pgprotval_t higher_prot = st->prot_levels[level - 1];
 
 		effective = (higher_prot & prot & (_PAGE_USER | _PAGE_RW)) |
@@ -452,7 +453,8 @@ bool ptdump_walk_pgd_level_core(struct seq_file *m,
 		.level = -1,
 		.to_dmesg	= dmesg,
 		.check_wx	= checkwx,
-		.seq		= m
+		.seq		= m,
+		.mm		= mm,
 	};
 
 	ptdump_walk_pgd(&st.ptdump, mm, pgd);
diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index 3186096a6e13..ed7101a13ce9 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -392,6 +392,22 @@ static inline void lazy_mmu_mode_pause(void) {}
 static inline void lazy_mmu_mode_resume(void) {}
 #endif
 
+/*
+ * On some architectures it depends on the mm if the p4d/pud or pmd
+ * layer of the page table hierarchy is folded or not.
+ */
+#ifndef mm_p4d_folded
+#define mm_p4d_folded(mm)	__is_defined(__PAGETABLE_P4D_FOLDED)
+#endif
+
+#ifndef mm_pud_folded
+#define mm_pud_folded(mm)	__is_defined(__PAGETABLE_PUD_FOLDED)
+#endif
+
+#ifndef mm_pmd_folded
+#define mm_pmd_folded(mm)	__is_defined(__PAGETABLE_PMD_FOLDED)
+#endif
+
 #ifndef pte_batch_hint
 /**
  * pte_batch_hint - Number of pages that can be added to batch without scanning.
@@ -2313,6 +2329,36 @@ static inline const char *pgtable_level_to_str(enum pgtable_level level)
 	}
 }
 
+#ifdef CONFIG_MMU
+/**
+ * mm_first_pgtable_level - return the first non-folded page-table level
+ * @mm: The mm structure.
+ *
+ * With runtime- or compile-time folded page tables, page-table walkers
+ * effectively skip the folded top-level page tables, treating them as
+ * always-present entries pointing to the next lower-level page table.
+ *
+ * Return the first page-table level in @mm that is not folded.
+ *
+ * Return: the first non-folded page-table level.
+ */
+static inline enum pgtable_level mm_first_pgtable_level(struct mm_struct *mm)
+{
+	/*
+	 * "PMD folded" actually means "PMD is folded into PUD", and
+	 * pud_present() etc. are hard-coded to look like present page table
+	 * levels, whereby pmd_present() etc. contain real logic.
+	 */
+	if (mm_pmd_folded(mm))
+		return PGTABLE_LEVEL_PMD;
+	if (mm_pud_folded(mm))
+		return PGTABLE_LEVEL_PUD;
+	if (mm_p4d_folded(mm))
+		return PGTABLE_LEVEL_P4D;
+	return PGTABLE_LEVEL_PGD;
+}
+#endif /* CONFIG_MMU */
+
 void ptval_bytes_to_hex_str(char *buf, size_t buf_size, const void *entry, size_t entry_size);
 
 #define ptval_to_str(buf, val)								\
@@ -2349,21 +2395,6 @@ void ptval_bytes_to_hex_str(char *buf, size_t buf_size, const void *entry, size_
 #ifndef has_transparent_pud_hugepage
 #define has_transparent_pud_hugepage() IS_BUILTIN(CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD)
 #endif
-/*
- * On some architectures it depends on the mm if the p4d/pud or pmd
- * layer of the page table hierarchy is folded or not.
- */
-#ifndef mm_p4d_folded
-#define mm_p4d_folded(mm)	__is_defined(__PAGETABLE_P4D_FOLDED)
-#endif
-
-#ifndef mm_pud_folded
-#define mm_pud_folded(mm)	__is_defined(__PAGETABLE_PUD_FOLDED)
-#endif
-
-#ifndef mm_pmd_folded
-#define mm_pmd_folded(mm)	__is_defined(__PAGETABLE_PMD_FOLDED)
-#endif
 
 #ifndef p4d_offset_lockless
 #define p4d_offset_lockless(pgdp, pgd, address) p4d_offset(&(pgd), address)

-- 
2.43.0