[PATCH 1/1] arch/arm/mm/fault.c: Porting OOM changes into __do_page_fault

[Kautuk Consul]

Commits d065bd810b6deb67d4897a14bfe21f8eb526ba99 and
37b23e0525d393d48a7d59f870b3bc061a30ccdb introduced changes into
the x86 pagefault handler for makeing the page fault handler
retryable as well as killable.

These changes reduce the mmap_sem hold time(for x86), which is crucial
during OOM killer invocation.

Porting these changes to ARM.

Without these changes, my ARM board encounters many hang and livelock
scenarios.
After applying this patch, OOM feature performance improves according to
my testing.

Motivation for porting these changes:
------------------------------------
Embedded devices such as SMART TVs and SMART phones in the near future
will have the capability to download and run apps from the internet.
Due to this, the device user might run some malignant app that
allocates too much memory.
In that case, OOM killer performance is very important so that the
device can free up memory for other apps as well as the kernel.

Signed-off-by: Kautuk Consul <consul.kautuk at gmail.com>
---
 arch/arm/mm/fault.c |   60 ++++++++++++++++++++++++++++++++++++++------------
 1 files changed, 45 insertions(+), 15 deletions(-)

diff --git a/arch/arm/mm/fault.c b/arch/arm/mm/fault.c
index aa33949..2f89dba 100644
--- a/arch/arm/mm/fault.c
+++ b/arch/arm/mm/fault.c
@@ -231,11 +231,15 @@ static inline bool access_error(unsigned int fsr, struct vm_area_struct *vma)
 
 static int __kprobes
 __do_page_fault(struct mm_struct *mm, unsigned long addr, unsigned int fsr,
-		struct task_struct *tsk)
+		struct pt_regs *regs, struct task_struct *tsk)
 {
 	struct vm_area_struct *vma;
 	int fault;
+	int write = fsr & FSR_WRITE;
+	unsigned int flags = FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE |
+					(write ? FAULT_FLAG_WRITE : 0);
 
+retry:
 	vma = find_vma(mm, addr);
 	fault = VM_FAULT_BADMAP;
 	if (unlikely(!vma))
@@ -257,13 +261,44 @@ good_area:
 	 * If for any reason at all we couldn't handle the fault, make
 	 * sure we exit gracefully rather than endlessly redo the fault.
 	 */
-	fault = handle_mm_fault(mm, vma, addr & PAGE_MASK, (fsr & FSR_WRITE) ? FAULT_FLAG_WRITE : 0);
-	if (unlikely(fault & VM_FAULT_ERROR))
+	fault = handle_mm_fault(mm, vma, addr & PAGE_MASK, flags);
+
+	if (unlikely((fault & VM_FAULT_ERROR)))
 		return fault;
-	if (fault & VM_FAULT_MAJOR)
-		tsk->maj_flt++;
-	else
-		tsk->min_flt++;
+
+	if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current))
+		return fault;
+
+	/*
+	 * Major/minor page fault accounting is only done on the
+	 * initial attempt. If we go through a retry, it is extremely
+	 * likely that the page will be found in page cache at that point.
+	 */
+	perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, addr);
+	if (flags & FAULT_FLAG_ALLOW_RETRY) {
+		if (fault & VM_FAULT_MAJOR) {
+			tsk->maj_flt++;
+			perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MAJ, 1,
+				      regs, addr);
+		} else {
+			tsk->min_flt++;
+			perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MIN, 1,
+				      regs, addr);
+		}
+		if (fault & VM_FAULT_RETRY) {
+			/* Clear FAULT_FLAG_ALLOW_RETRY to avoid any risk
+			 * of starvation. */
+			flags &= ~FAULT_FLAG_ALLOW_RETRY;
+
+			/* Acquire the mmap_sem again before retrying this
+			 * pagefault. This would have been released by
+			 * __lock_page_or_retry() in mm/filemap.c. */
+			down_read(&mm->mmap_sem);
+
+			goto retry;
+		}
+	}
+
 	return fault;
 
 check_stack:
@@ -320,14 +355,9 @@ do_page_fault(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
 #endif
 	}
 
-	fault = __do_page_fault(mm, addr, fsr, tsk);
-	up_read(&mm->mmap_sem);
-
-	perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, addr);
-	if (fault & VM_FAULT_MAJOR)
-		perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MAJ, 1, regs, addr);
-	else if (fault & VM_FAULT_MINOR)
-		perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MIN, 1, regs, addr);
+	fault = __do_page_fault(mm, addr, fsr, regs, tsk);
+	if (unlikely(!(fault & VM_FAULT_RETRY)))
+		up_read(&mm->mmap_sem);
 
 	/*
 	 * Handle the "normal" case first - VM_FAULT_MAJOR / VM_FAULT_MINOR
-- 
1.7.5.4