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Subject[RFC 4/4] speculative pag fault

Lookup vma in lockless style, do page fault, and check mm's version
after takine page table lock. If racy, mm's version is invalid .
Then, retry page fault.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
---
arch/x86/mm/fault.c | 28 +++++++++++++++++++++++++---
mm/memory.c | 21 ++++++++++++++-------
2 files changed, 39 insertions(+), 10 deletions(-)
Index: mmotm-mm-accessor/arch/x86/mm/fault.c
===================================================================
--- mmotm-mm-accessor.orig/arch/x86/mm/fault.c
+++ mmotm-mm-accessor/arch/x86/mm/fault.c
@@ -11,6 +11,7 @@
#include <linux/kprobes.h> /* __kprobes, ... */
#include <linux/mmiotrace.h> /* kmmio_handler, ... */
#include <linux/perf_event.h> /* perf_sw_event */
+#include <linux/hugetlb.h> /* is_vm_hugetlb...*/

#include <asm/traps.h> /* dotraplinkage, ... */
#include <asm/pgalloc.h> /* pgd_*(), ... */
@@ -952,6 +953,7 @@ do_page_fault(struct pt_regs *regs, unsi
struct mm_struct *mm;
int write;
int fault;
+ int speculative;

tsk = current;
mm = tsk->mm;
@@ -1040,6 +1042,17 @@ do_page_fault(struct pt_regs *regs, unsi
return;
}

+ if ((error_code & PF_USER) && mm_version_check(mm)) {
+ vma = lookup_vma_cache(mm, address);
+ if (vma && mm_version_check(mm) &&
+ (vma->vm_start <= address) && (address < vma->vm_end)) {
+ speculative = 1;
+ goto found_vma;
+ }
+ if (vma)
+ vma_release(vma);
+ }
+
/*
* When running in the kernel we expect faults to occur only to
* addresses in user space. All other faults represent errors in
@@ -1056,6 +1069,8 @@ do_page_fault(struct pt_regs *regs, unsi
* validate the source. If this is invalid we can skip the address
* space check, thus avoiding the deadlock:
*/
+retry_with_lock:
+ speculative = 0;
if (unlikely(!mm_read_trylock(mm))) {
if ((error_code & PF_USER) == 0 &&
!search_exception_tables(regs->ip)) {
@@ -1073,6 +1088,7 @@ do_page_fault(struct pt_regs *regs, unsi
}

vma = find_vma(mm, address);
+found_vma:
if (unlikely(!vma)) {
bad_area(regs, error_code, address);
return;
@@ -1119,6 +1135,7 @@ good_area:
*/
fault = handle_mm_fault(mm, vma, address, write ? FAULT_FLAG_WRITE : 0);

+
if (unlikely(fault & VM_FAULT_ERROR)) {
mm_fault_error(regs, error_code, address, fault);
return;
@@ -1128,13 +1145,18 @@ good_area:
tsk->maj_flt++;
perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MAJ, 1, 0,
regs, address);
- } else {
+ } else if (!speculative || mm_version_check(mm)) {
tsk->min_flt++;
perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MIN, 1, 0,
regs, address);
+ } else {
+ vma_release(vma);
+ goto retry_with_lock;
}

check_v8086_mode(regs, address, tsk);
-
- mm_read_unlock(mm);
+ if (!speculative)
+ mm_read_unlock(mm);
+ else
+ vma_release(vma);
}
Index: mmotm-mm-accessor/mm/memory.c
===================================================================
--- mmotm-mm-accessor.orig/mm/memory.c
+++ mmotm-mm-accessor/mm/memory.c
@@ -121,6 +121,13 @@ static int __init init_zero_pfn(void)
}
core_initcall(init_zero_pfn);

+static bool test_valid_pte(struct mm_struct *mm, pte_t pte, pte_t orig)
+{
+ if (likely(mm_version_check(mm) && pte_same(pte, orig)))
+ return true;
+ return false;
+}
+
/*
* If a p?d_bad entry is found while walking page tables, report
* the error, before resetting entry to p?d_none. Usually (but
@@ -2044,7 +2051,7 @@ static int do_wp_page(struct mm_struct *
lock_page(old_page);
page_table = pte_offset_map_lock(mm, pmd, address,
&ptl);
- if (!pte_same(*page_table, orig_pte)) {
+ if (!test_valid_pte(mm, *page_table, orig_pte)) {
unlock_page(old_page);
page_cache_release(old_page);
goto unlock;
@@ -2105,7 +2112,7 @@ static int do_wp_page(struct mm_struct *
*/
page_table = pte_offset_map_lock(mm, pmd, address,
&ptl);
- if (!pte_same(*page_table, orig_pte)) {
+ if (!test_valid_pte(mm, *page_table, orig_pte)) {
unlock_page(old_page);
page_cache_release(old_page);
goto unlock;
@@ -2169,7 +2176,7 @@ gotten:
* Re-check the pte - we dropped the lock
*/
page_table = pte_offset_map_lock(mm, pmd, address, &ptl);
- if (likely(pte_same(*page_table, orig_pte))) {
+ if (test_valid_pte(mm, *page_table, orig_pte)) {
if (old_page) {
if (!PageAnon(old_page)) {
dec_mm_counter(mm, file_rss);
@@ -2555,7 +2562,7 @@ static int do_swap_page(struct mm_struct
* while we released the pte lock.
*/
page_table = pte_offset_map_lock(mm, pmd, address, &ptl);
- if (likely(pte_same(*page_table, orig_pte)))
+ if (pte_same(*page_table, orig_pte))
ret = VM_FAULT_OOM;
delayacct_clear_flag(DELAYACCT_PF_SWAPIN);
goto unlock;
@@ -2588,7 +2595,7 @@ static int do_swap_page(struct mm_struct
* Back out if somebody else already faulted in this pte.
*/
page_table = pte_offset_map_lock(mm, pmd, address, &ptl);
- if (unlikely(!pte_same(*page_table, orig_pte)))
+ if (unlikely(!test_valid_pte(mm, *page_table, orig_pte)))
goto out_nomap;

if (unlikely(!PageUptodate(page))) {
@@ -2844,7 +2851,7 @@ static int __do_fault(struct mm_struct *
* handle that later.
*/
/* Only go through if we didn't race with anybody else... */
- if (likely(pte_same(*page_table, orig_pte))) {
+ if (likely(test_valid_pte(mm, *page_table, orig_pte))) {
flush_icache_page(vma, page);
entry = mk_pte(page, vma->vm_page_prot);
if (flags & FAULT_FLAG_WRITE)
@@ -2991,7 +2998,7 @@ static inline int handle_pte_fault(struc

ptl = pte_lockptr(mm, pmd);
spin_lock(ptl);
- if (unlikely(!pte_same(*pte, entry)))
+ if (unlikely(!test_valid_pte(mm, *pte, entry)))
goto unlock;
if (flags & FAULT_FLAG_WRITE) {
if (!pte_write(entry))


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