[PATCH v8 13/20] mm: pagewalk: Add test_p?d callbacks

[Steven Price <steven.price@xxxxxxx>]

It is useful to be able to skip parts of the page table tree even when
walking without VMAs. Add test_p?d callbacks similar to test_walk but
which are called just before a table at that level is walked. If the
callback returns non-zero then the entire table is skipped.

Signed-off-by: Steven Price <steven.price@xxxxxxx>
---
 include/linux/mm.h | 11 +++++++++++
 mm/pagewalk.c      | 24 ++++++++++++++++++++++++
 2 files changed, 35 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index f6de08c116e6..a4c1ed255455 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1382,6 +1382,11 @@ void unmap_vmas(struct mmu_gather *tlb, struct vm_area_struct *start_vma,
  *             value means "do page table walk over the current vma,"
  *             and a negative one means "abort current page table walk
  *             right now." 1 means "skip the current vma."
+ * @test_pmd:  similar to test_walk(), but called for every pmd.
+ * @test_pud:  similar to test_walk(), but called for every pud.
+ * @test_p4d:  similar to test_walk(), but called for every p4d.
+ *             Returning 0 means walk this part of the page tables,
+ *             returning 1 means to skip this range.
  * @mm:        mm_struct representing the target process of page table walk
  * @vma:       vma currently walked (NULL if walking outside vmas)
  * @private:   private data for callbacks' usage
@@ -1406,6 +1411,12 @@ struct mm_walk {
 			     struct mm_walk *walk);
 	int (*test_walk)(unsigned long addr, unsigned long next,
 			struct mm_walk *walk);
+	int (*test_pmd)(unsigned long addr, unsigned long next,
+			pmd_t *pmd_start, struct mm_walk *walk);
+	int (*test_pud)(unsigned long addr, unsigned long next,
+			pud_t *pud_start, struct mm_walk *walk);
+	int (*test_p4d)(unsigned long addr, unsigned long next,
+			p4d_t *p4d_start, struct mm_walk *walk);
 	struct mm_struct *mm;
 	struct vm_area_struct *vma;
 	void *private;
diff --git a/mm/pagewalk.c b/mm/pagewalk.c
index dac0c848b458..231655db1295 100644
--- a/mm/pagewalk.c
+++ b/mm/pagewalk.c
@@ -32,6 +32,14 @@ static int walk_pmd_range(pud_t *pud, unsigned long addr, unsigned long end,
 	unsigned long next;
 	int err = 0;
 
+	if (walk->test_pmd) {
+		err = walk->test_pmd(addr, end, pmd_offset(pud, 0), walk);
+		if (err < 0)
+			return err;
+		if (err > 0)
+			return 0;
+	}
+
 	pmd = pmd_offset(pud, addr);
 	do {
 again:
@@ -82,6 +90,14 @@ static int walk_pud_range(p4d_t *p4d, unsigned long addr, unsigned long end,
 	unsigned long next;
 	int err = 0;
 
+	if (walk->test_pud) {
+		err = walk->test_pud(addr, end, pud_offset(p4d, 0), walk);
+		if (err < 0)
+			return err;
+		if (err > 0)
+			return 0;
+	}
+
 	pud = pud_offset(p4d, addr);
 	do {
  again:
@@ -124,6 +140,14 @@ static int walk_p4d_range(pgd_t *pgd, unsigned long addr, unsigned long end,
 	unsigned long next;
 	int err = 0;
 
+	if (walk->test_p4d) {
+		err = walk->test_p4d(addr, end, p4d_offset(pgd, 0), walk);
+		if (err < 0)
+			return err;
+		if (err > 0)
+			return 0;
+	}
+
 	p4d = p4d_offset(pgd, addr);
 	do {
 		next = p4d_addr_end(addr, end);
-- 
2.20.1