linux-kernel - [PATCH v2 14/35] mm: add speculative_page_walk_begin() and speculative_page_walk

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Date:   Fri, 28 Jan 2022 05:09:45 -0800
From:   Michel Lespinasse <michel@...pinasse.org>
To:     Linux-MM <linux-mm@...ck.org>, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     kernel-team@...com, Laurent Dufour <ldufour@...ux.ibm.com>,
        Jerome Glisse <jglisse@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Michal Hocko <mhocko@...e.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Davidlohr Bueso <dave@...olabs.net>,
        Matthew Wilcox <willy@...radead.org>,
        Liam Howlett <liam.howlett@...cle.com>,
        Rik van Riel <riel@...riel.com>,
        Paul McKenney <paulmck@...nel.org>,
        Song Liu <songliubraving@...com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Minchan Kim <minchan@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Michel Lespinasse <michel@...pinasse.org>
Subject: [PATCH v2 14/35] 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 aa24cd8c06e9..663952d14bad 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2725,6 +2725,28 @@ 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