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From:   Michel Lespinasse <michel@lespinasse.org>
To:     Linux-MM <linux-mm@kvack.org>, linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     kernel-team@fb.com, Laurent Dufour <ldufour@linux.ibm.com>,
        Jerome Glisse <jglisse@google.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Michal Hocko <mhocko@suse.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Matthew Wilcox <willy@infradead.org>,
        Liam Howlett <liam.howlett@oracle.com>,
        Rik van Riel <riel@surriel.com>,
        Paul McKenney <paulmck@kernel.org>,
        Song Liu <songliubraving@fb.com>,
        Suren Baghdasaryan <surenb@google.com>,
        Minchan Kim <minchan@google.com>,
        Joel Fernandes <joelaf@google.com>,
        David Rientjes <rientjes@google.com>,
        Axel Rasmussen <axelrasmussen@google.com>,
        Andy Lutomirski <luto@kernel.org>,
        Michel Lespinasse <michel@lespinasse.org>
Subject: [PATCH v2 28/35] mm: disable speculative faults for single threaded user space
Date:   Fri, 28 Jan 2022 05:09:59 -0800
Message-Id: <20220128131006.67712-29-michel@lespinasse.org>
In-Reply-To: <20220128131006.67712-1-michel@lespinasse.org>
References: <20220128131006.67712-1-michel@lespinasse.org>
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Performance tuning: single threaded userspace does not benefit from
speculative page faults, so we turn them off to avoid any related
(small) extra overheads.

Signed-off-by: Michel Lespinasse <michel@lespinasse.org>
---
 arch/x86/mm/fault.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 6ba109413396..d6f8d4967c49 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -1328,6 +1328,13 @@ void do_user_addr_fault(struct pt_regs *regs,
 #endif
 
 #ifdef CONFIG_SPECULATIVE_PAGE_FAULT
+	/*
+	 * No need to try speculative faults for kernel or
+	 * single threaded user space.
+	 */
+	if (!(flags & FAULT_FLAG_USER) || atomic_read(&mm->mm_users) == 1)
+		goto no_spf;
+
 	count_vm_event(SPF_ATTEMPT);
 	seq = mmap_seq_read_start(mm);
 	if (seq & 1)
@@ -1362,7 +1369,9 @@ void do_user_addr_fault(struct pt_regs *regs,
 
 spf_abort:
 	count_vm_event(SPF_ABORT);
-#endif
+no_spf:
+
+#endif	/* CONFIG_SPECULATIVE_PAGE_FAULT */
 
 	/*
 	 * Kernel-mode access to the user address space should only occur
-- 
2.20.1