linux-kernel - [RFC PATCH 33/37] mm: enable speculative fault handling only for multithreaded user space

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Message-Id: <20210407014502.24091-34-michel@lespinasse.org>
Date:   Tue,  6 Apr 2021 18:44:58 -0700
From:   Michel Lespinasse <michel@...pinasse.org>
To:     Linux-MM <linux-mm@...ck.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>,
        Rom Lemarchand <romlem@...gle.com>,
        Linux-Kernel <linux-kernel@...r.kernel.org>,
        Michel Lespinasse <michel@...pinasse.org>
Subject: [RFC PATCH 33/37] mm: enable speculative fault handling only for multithreaded user space

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@...pinasse.org>
---
 arch/x86/mm/fault.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 48b86911a6df..b1a07ca82d59 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -1318,6 +1318,10 @@ void do_user_addr_fault(struct pt_regs *regs,
 	}
 #endif
 
+	/* Only try spf for multithreaded user space faults. */
+	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)
@@ -1351,6 +1355,7 @@ void do_user_addr_fault(struct pt_regs *regs,
 
 spf_abort:
 	count_vm_event(SPF_ABORT);
+no_spf:
 
 	/*
 	 * Kernel-mode access to the user address space should only occur
-- 
2.20.1