linux-kernel - [PATCH 13/29] mm: add speculative_page_walk_begin() and speculative_page_walk

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Date:   Fri, 30 Apr 2021 12:52:14 -0700
From:   Michel Lespinasse <michel@...pinasse.org>
To:     Linux-MM <linux-mm@...ck.org>,
        Linux-Kernel <linux-kernel@...r.kernel.org>
Cc:     Laurent Dufour <ldufour@...ux.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Michal Hocko <mhocko@...e.com>,
        Matthew Wilcox <willy@...radead.org>,
        Rik van Riel <riel@...riel.com>,
        Paul McKenney <paulmck@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Michel Lespinasse <michel@...pinasse.org>
Subject: [PATCH 13/29] mm: add speculative_page_walk_begin() and speculative_page_walk_end()

Speculative page faults will use these to protect against races with
page table reclamation.

This could always be handled by disabling local IRQs as the fast GUP
code does; however speculative page faults do not need to protect
against races with THP page splitting, so a weaker rcu read lock is
sufficient in the MMU_GATHER_RCU_TABLE_FREE case.

Signed-off-by: Michel Lespinasse <michel@...pinasse.org>
---
 mm/memory.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/mm/memory.c b/mm/memory.c
index 8258ff93a055..b28047765de7 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2566,6 +2566,28 @@ int apply_to_existing_page_range(struct mm_struct *mm, unsigned long addr,
 }
 EXPORT_SYMBOL_GPL(apply_to_existing_page_range);
 
+#ifdef CONFIG_SPECULATIVE_PAGE_FAULT
+
+/*
+ * speculative_page_walk_begin() ... speculative_page_walk_end() protects
+ * against races with page table reclamation.
+ *
+ * This is similar to what fast GUP does, but fast GUP also needs to
+ * protect against races with THP page splitting, so it always needs
+ * to disable interrupts.
+ * Speculative page faults only need to protect against page table reclamation,
+ * so rcu_read_lock() is sufficient in the MMU_GATHER_RCU_TABLE_FREE case.
+ */
+#ifdef CONFIG_MMU_GATHER_RCU_TABLE_FREE
+#define speculative_page_walk_begin() rcu_read_lock()
+#define speculative_page_walk_end()   rcu_read_unlock()
+#else
+#define speculative_page_walk_begin() local_irq_disable()
+#define speculative_page_walk_end()   local_irq_enable()
+#endif
+
+#endif	/* CONFIG_SPECULATIVE_PAGE_FAULT */
+
 /*
  * handle_pte_fault chooses page fault handler according to an entry which was
  * read non-atomically.  Before making any commitment, on those architectures
-- 
2.20.1