linux-kernel - [RFC PATCH 3/4] mm: pagewalk: Disallow user positive callback return values and use them for walk control

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Date:   Thu, 10 Oct 2019 15:40:57 +0200
From:   Thomas Hellström (VMware) 
        <thomas_os@...pmail.org>
To:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        torvalds@...ux-foundation.org, kirill@...temov.name
Cc:     Thomas Hellstrom <thellstrom@...are.com>
Subject: [RFC PATCH 3/4] mm: pagewalk: Disallow user positive callback return values and use them for walk control

From: Linus Torvalds <torvalds@...ux-foundation.org>

When we have both a pmd_entry() and a pte_entry() callback, in some
siutaions it is desirable not to traverse the pte level.
Reserve positive callback return values for walk control and define a
return value PAGE_WALK_CONTINUE that means skip lower level traversal
and continue the walk.

Co-developed-by: Thomas Hellstrom <thellstrom@...are.com>
Signed-off-by: Thomas Hellstrom <thellstrom@...are.com>
---
 include/linux/pagewalk.h | 6 ++++++
 mm/pagewalk.c            | 9 ++++++---
 2 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/include/linux/pagewalk.h b/include/linux/pagewalk.h
index 6ec82e92c87f..d9e5d1927315 100644
--- a/include/linux/pagewalk.h
+++ b/include/linux/pagewalk.h
@@ -4,6 +4,12 @@
 
 #include <linux/mm.h>
 
+/*
+ * pmd_entry() Return code meaning skip to next entry.
+ * Don't look for lower levels
+ */
+#define PAGE_WALK_CONTINUE 1
+
 struct mm_walk;
 
 /**
diff --git a/mm/pagewalk.c b/mm/pagewalk.c
index ea0b9e606ad1..d2483d432fda 100644
--- a/mm/pagewalk.c
+++ b/mm/pagewalk.c
@@ -52,8 +52,12 @@ static int walk_pmd_range(pud_t *pud, unsigned long addr, unsigned long end,
 		 */
 		if (ops->pmd_entry)
 			err = ops->pmd_entry(pmd, addr, next, walk);
-		if (err)
+		if (err < 0)
 			break;
+		if (err == PAGE_WALK_CONTINUE) {
+			err = 0;
+			continue;
+		}
 
 		/*
 		 * Check this here so we only break down trans_huge
@@ -291,8 +295,7 @@ static int __walk_page_range(unsigned long start, unsigned long end,
  *
  *  - 0  : succeeded to handle the current entry, and if you don't reach the
  *         end address yet, continue to walk.
- *  - >0 : succeeded to handle the current entry, and return to the caller
- *         with caller specific value.
+ *  - >0 : Reserved for walk control. Use only PAGE_WALK_XX values.
  *  - <0 : failed to handle the current entry, and return to the caller
  *         with error code.
  *
-- 
2.21.0