linux-kernel - [RFC][PATCH 1/9] mm: Update ptep_get

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Message-ID: <20201130113602.841001656@infradead.org>
Date:   Mon, 30 Nov 2020 12:27:06 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     x86@...nel.org, willy@...radead.org
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        aarcange@...hat.com, kirill.shutemov@...ux.intel.com,
        jroedel@...e.de, peterz@...radead.org
Subject: [RFC][PATCH 1/9] mm: Update ptep_get_lockless()s comment

Improve the comment and add a lockdep assertion because nobody reads
comments.

Suggested-by: Matthew Wilcox <willy@...radead.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 include/linux/pgtable.h |   17 ++++++++---------
 1 file changed, 8 insertions(+), 9 deletions(-)

--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -260,15 +260,12 @@ static inline pte_t ptep_get(pte_t *ptep
 
 #ifdef CONFIG_GUP_GET_PTE_LOW_HIGH
 /*
- * WARNING: only to be used in the get_user_pages_fast() implementation.
- *
- * With get_user_pages_fast(), we walk down the pagetables without taking any
- * locks.  For this we would like to load the pointers atomically, but sometimes
- * that is not possible (e.g. without expensive cmpxchg8b on x86_32 PAE).  What
- * we do have is the guarantee that a PTE will only either go from not present
- * to present, or present to not present or both -- it will not switch to a
- * completely different present page without a TLB flush in between; something
- * that we are blocking by holding interrupts off.
+ * For walking the pagetables without holding any locks.  Some architectures
+ * (eg x86-32 PAE) cannot load the entries atomically without using expensive
+ * instructions.  We are guaranteed that a PTE will only either go from not
+ * present to present, or present to not present -- it will not switch to a
+ * completely different present page without a TLB flush inbetween; which we
+ * are blocking by holding interrupts off.
  *
  * Setting ptes from not present to present goes:
  *
@@ -294,6 +291,8 @@ static inline pte_t ptep_get_lockless(pt
 {
 	pte_t pte;
 
+	lockdep_assert_irqs_disabled();
+
 	do {
 		pte.pte_low = ptep->pte_low;
 		smp_rmb();