Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
From:   Laurent Dufour <ldufour@linux.ibm.com>
To:     akpm@linux-foundation.org, mhocko@kernel.org, peterz@infradead.org,
        kirill@shutemov.name, ak@linux.intel.com, dave@stgolabs.net,
        jack@suse.cz, Matthew Wilcox <willy@infradead.org>,
        aneesh.kumar@linux.ibm.com, benh@kernel.crashing.org,
        mpe@ellerman.id.au, paulus@samba.org,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, hpa@zytor.com,
        Will Deacon <will.deacon@arm.com>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        sergey.senozhatsky.work@gmail.com,
        Andrea Arcangeli <aarcange@redhat.com>,
        Alexei Starovoitov <alexei.starovoitov@gmail.com>,
        kemi.wang@intel.com, Daniel Jordan <daniel.m.jordan@oracle.com>,
        David Rientjes <rientjes@google.com>,
        Jerome Glisse <jglisse@redhat.com>,
        Ganesh Mahendran <opensource.ganesh@gmail.com>,
        Minchan Kim <minchan@kernel.org>,
        Punit Agrawal <punitagrawal@gmail.com>,
        vinayak menon <vinayakm.list@gmail.com>,
        Yang Shi <yang.shi@linux.alibaba.com>,
        zhong jiang <zhongjiang@huawei.com>,
        Haiyan Song <haiyanx.song@intel.com>,
        Balbir Singh <bsingharora@gmail.com>, sj38.park@gmail.com,
        Michel Lespinasse <walken@google.com>,
        Mike Rapoport <rppt@linux.ibm.com>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        haren@linux.vnet.ibm.com, npiggin@gmail.com,
        paulmck@linux.vnet.ibm.com, Tim Chen <tim.c.chen@linux.intel.com>,
        linuxppc-dev@lists.ozlabs.org, x86@kernel.org
Subject: [PATCH v12 07/31] mm: make pte_unmap_same compatible with SPF
Date:   Tue, 16 Apr 2019 15:44:58 +0200
In-Reply-To: <20190416134522.17540-1-ldufour@linux.ibm.com>
References: <20190416134522.17540-1-ldufour@linux.ibm.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Message-Id: <20190416134522.17540-8-ldufour@linux.ibm.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

pte_unmap_same() is making the assumption that the page table are still
around because the mmap_sem is held.
This is no more the case when running a speculative page fault and
additional check must be made to ensure that the final page table are still
there.

This is now done by calling pte_spinlock() to check for the VMA's
consistency while locking for the page tables.

This is requiring passing a vm_fault structure to pte_unmap_same() which is
containing all the needed parameters.

As pte_spinlock() may fail in the case of a speculative page fault, if the
VMA has been touched in our back, pte_unmap_same() should now return 3
cases :
	1. pte are the same (0)
	2. pte are different (VM_FAULT_PTNOTSAME)
	3. a VMA's changes has been detected (VM_FAULT_RETRY)

The case 2 is handled by the introduction of a new VM_FAULT flag named
VM_FAULT_PTNOTSAME which is then trapped in cow_user_page().
If VM_FAULT_RETRY is returned, it is passed up to the callers to retry the
page fault while holding the mmap_sem.

Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Laurent Dufour <ldufour@linux.ibm.com>
---
 include/linux/mm_types.h |  6 +++++-
 mm/memory.c              | 37 +++++++++++++++++++++++++++----------
 2 files changed, 32 insertions(+), 11 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 8ec38b11b361..fd7d38ee2e33 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -652,6 +652,8 @@ typedef __bitwise unsigned int vm_fault_t;
  * @VM_FAULT_NEEDDSYNC:		->fault did not modify page tables and needs
  *				fsync() to complete (for synchronous page faults
  *				in DAX)
+ * @VM_FAULT_PTNOTSAME		Page table entries have changed during a
+ *				speculative page fault handling.
  * @VM_FAULT_HINDEX_MASK:	mask HINDEX value
  *
  */
@@ -669,6 +671,7 @@ enum vm_fault_reason {
 	VM_FAULT_FALLBACK       = (__force vm_fault_t)0x000800,
 	VM_FAULT_DONE_COW       = (__force vm_fault_t)0x001000,
 	VM_FAULT_NEEDDSYNC      = (__force vm_fault_t)0x002000,
+	VM_FAULT_PTNOTSAME	= (__force vm_fault_t)0x004000,
 	VM_FAULT_HINDEX_MASK    = (__force vm_fault_t)0x0f0000,
 };
 
@@ -693,7 +696,8 @@ enum vm_fault_reason {
 	{ VM_FAULT_RETRY,               "RETRY" },	\
 	{ VM_FAULT_FALLBACK,            "FALLBACK" },	\
 	{ VM_FAULT_DONE_COW,            "DONE_COW" },	\
-	{ VM_FAULT_NEEDDSYNC,           "NEEDDSYNC" }
+	{ VM_FAULT_NEEDDSYNC,           "NEEDDSYNC" },	\
+	{ VM_FAULT_PTNOTSAME,		"PTNOTSAME" }
 
 struct vm_special_mapping {
 	const char *name;	/* The name, e.g. "[vdso]". */
diff --git a/mm/memory.c b/mm/memory.c
index 221ccdf34991..d5bebca47d98 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2094,21 +2094,29 @@ static inline bool pte_map_lock(struct vm_fault *vmf)
  * parts, do_swap_page must check under lock before unmapping the pte and
  * proceeding (but do_wp_page is only called after already making such a check;
  * and do_anonymous_page can safely check later on).
+ *
+ * pte_unmap_same() returns:
+ *	0			if the PTE are the same
+ *	VM_FAULT_PTNOTSAME	if the PTE are different
+ *	VM_FAULT_RETRY		if the VMA has changed in our back during
+ *				a speculative page fault handling.
  */
-static inline int pte_unmap_same(struct mm_struct *mm, pmd_t *pmd,
-				pte_t *page_table, pte_t orig_pte)
+static inline vm_fault_t pte_unmap_same(struct vm_fault *vmf)
 {
-	int same = 1;
+	int ret = 0;
+
 #if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT)
 	if (sizeof(pte_t) > sizeof(unsigned long)) {
-		spinlock_t *ptl = pte_lockptr(mm, pmd);
-		spin_lock(ptl);
-		same = pte_same(*page_table, orig_pte);
-		spin_unlock(ptl);
+		if (pte_spinlock(vmf)) {
+			if (!pte_same(*vmf->pte, vmf->orig_pte))
+				ret = VM_FAULT_PTNOTSAME;
+			spin_unlock(vmf->ptl);
+		} else
+			ret = VM_FAULT_RETRY;
 	}
 #endif
-	pte_unmap(page_table);
-	return same;
+	pte_unmap(vmf->pte);
+	return ret;
 }
 
 static inline void cow_user_page(struct page *dst, struct page *src, unsigned long va, struct vm_area_struct *vma)
@@ -2714,8 +2722,17 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
 	int exclusive = 0;
 	vm_fault_t ret = 0;
 
-	if (!pte_unmap_same(vma->vm_mm, vmf->pmd, vmf->pte, vmf->orig_pte))
+	ret = pte_unmap_same(vmf);
+	if (ret) {
+		/*
+		 * If pte != orig_pte, this means another thread did the
+		 * swap operation in our back.
+		 * So nothing else to do.
+		 */
+		if (ret == VM_FAULT_PTNOTSAME)
+			ret = 0;
 		goto out;
+	}
 
 	entry = pte_to_swp_entry(vmf->orig_pte);
 	if (unlikely(non_swap_entry(entry))) {
-- 
2.21.0