linux-kernel - [PATCH v11 25/26] powerpc/mm: add speculative page fault

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Date:   Thu, 17 May 2018 13:06:32 +0200
From:   Laurent Dufour <ldufour@...ux.vnet.ibm.com>
To:     akpm@...ux-foundation.org, mhocko@...nel.org, peterz@...radead.org,
        kirill@...temov.name, ak@...ux.intel.com, dave@...olabs.net,
        jack@...e.cz, Matthew Wilcox <willy@...radead.org>,
        khandual@...ux.vnet.ibm.com, aneesh.kumar@...ux.vnet.ibm.com,
        benh@...nel.crashing.org, mpe@...erman.id.au, paulus@...ba.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, hpa@...or.com,
        Will Deacon <will.deacon@....com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        sergey.senozhatsky.work@...il.com,
        Andrea Arcangeli <aarcange@...hat.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        kemi.wang@...el.com, Daniel Jordan <daniel.m.jordan@...cle.com>,
        David Rientjes <rientjes@...gle.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Ganesh Mahendran <opensource.ganesh@...il.com>,
        Minchan Kim <minchan@...nel.org>,
        Punit Agrawal <punitagrawal@...il.com>,
        vinayak menon <vinayakm.list@...il.com>,
        Yang Shi <yang.shi@...ux.alibaba.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        haren@...ux.vnet.ibm.com, npiggin@...il.com, bsingharora@...il.com,
        paulmck@...ux.vnet.ibm.com, Tim Chen <tim.c.chen@...ux.intel.com>,
        linuxppc-dev@...ts.ozlabs.org, x86@...nel.org
Subject: [PATCH v11 25/26] powerpc/mm: add speculative page fault

This patch enable the speculative page fault on the PowerPC
architecture.

This will try a speculative page fault without holding the mmap_sem,
if it returns with VM_FAULT_RETRY, the mmap_sem is acquired and the
traditional page fault processing is done.

The speculative path is only tried for multithreaded process as there is no
risk of contention on the mmap_sem otherwise.

Signed-off-by: Laurent Dufour <ldufour@...ux.vnet.ibm.com>
---
 arch/powerpc/mm/fault.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
index ef268d5d9db7..d7b5742ffb2b 100644
--- a/arch/powerpc/mm/fault.c
+++ b/arch/powerpc/mm/fault.c
@@ -465,6 +465,21 @@ static int __do_page_fault(struct pt_regs *regs, unsigned long address,
 	if (is_exec)
 		flags |= FAULT_FLAG_INSTRUCTION;
 
+	/*
+	 * Try speculative page fault before grabbing the mmap_sem.
+	 * The Page fault is done if VM_FAULT_RETRY is not returned.
+	 * But if the memory protection keys are active, we don't know if the
+	 * fault is due to key mistmatch or due to a classic protection check.
+	 * To differentiate that, we will need the VMA we no more have, so
+	 * let's retry with the mmap_sem held.
+	 */
+	fault = handle_speculative_fault(mm, address, flags);
+	if (fault != VM_FAULT_RETRY && (IS_ENABLED(CONFIG_PPC_MEM_KEYS) &&
+					fault != VM_FAULT_SIGSEGV)) {
+		perf_sw_event(PERF_COUNT_SW_SPF, 1, regs, address);
+		goto done;
+	}
+
 	/* When running in the kernel we expect faults to occur only to
 	 * addresses in user space.  All other faults represent errors in the
 	 * kernel and should generate an OOPS.  Unfortunately, in the case of an
@@ -565,6 +580,7 @@ static int __do_page_fault(struct pt_regs *regs, unsigned long address,
 
 	up_read(&current->mm->mmap_sem);
 
+done:
 	if (unlikely(fault & VM_FAULT_ERROR))
 		return mm_fault_error(regs, address, fault);
 
-- 
2.7.4