mm: retry page fault when blocking on disk transfer
This change reduces mmap_sem hold times that are caused by waiting for
disk transfers when accessing file mapped VMAs.
It introduces the VM_FAULT_ALLOW_RETRY flag, which indicates that the call
site wants mmap_sem to be released if blocking on a pending disk transfer.
In that case, filemap_fault() returns the VM_FAULT_RETRY status bit and
do_page_fault() will then re-acquire mmap_sem and retry the page fault.
It is expected that the retry will hit the same page which will now be
cached, and thus it will complete with a low mmap_sem hold time.
Tests:
- microbenchmark: thread A mmaps a large file and does random read accesses
to the mmaped area - achieves about 55 iterations/s. Thread B does
mmap/munmap in a loop at a separate location - achieves 55 iterations/s
before, 15000 iterations/s after.
- We are seeing related effects in some applications in house, which show
significant performance regressions when running without this change.
[akpm@linux-foundation.org: fix warning & crash]
Signed-off-by: Michel Lespinasse <walken@google.com>
Acked-by: Rik van Riel <riel@redhat.com>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Reviewed-by: Wu Fengguang <fengguang.wu@intel.com>
Cc: Ying Han <yinghan@google.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Acked-by: "H. Peter Anvin" <hpa@zytor.com>
Cc: <linux-arch@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 852b319..9b2345c 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -956,8 +956,10 @@
struct task_struct *tsk;
unsigned long address;
struct mm_struct *mm;
- int write;
int fault;
+ int write = error_code & PF_WRITE;
+ unsigned int flags = FAULT_FLAG_ALLOW_RETRY |
+ (write ? FAULT_FLAG_WRITE : 0);
tsk = current;
mm = tsk->mm;
@@ -1068,6 +1070,7 @@
bad_area_nosemaphore(regs, error_code, address);
return;
}
+retry:
down_read(&mm->mmap_sem);
} else {
/*
@@ -1111,8 +1114,6 @@
* we can handle it..
*/
good_area:
- write = error_code & PF_WRITE;
-
if (unlikely(access_error(error_code, write, vma))) {
bad_area_access_error(regs, error_code, address);
return;
@@ -1123,21 +1124,34 @@
* make sure we exit gracefully rather than endlessly redo
* the fault:
*/
- fault = handle_mm_fault(mm, vma, address, write ? FAULT_FLAG_WRITE : 0);
+ fault = handle_mm_fault(mm, vma, address, flags);
if (unlikely(fault & VM_FAULT_ERROR)) {
mm_fault_error(regs, error_code, address, fault);
return;
}
- if (fault & VM_FAULT_MAJOR) {
- tsk->maj_flt++;
- perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MAJ, 1, 0,
- regs, address);
- } else {
- tsk->min_flt++;
- perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MIN, 1, 0,
- regs, address);
+ /*
+ * 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.
+ */
+ if (flags & FAULT_FLAG_ALLOW_RETRY) {
+ if (fault & VM_FAULT_MAJOR) {
+ tsk->maj_flt++;
+ perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MAJ, 1, 0,
+ regs, address);
+ } else {
+ tsk->min_flt++;
+ perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS_MIN, 1, 0,
+ regs, address);
+ }
+ if (fault & VM_FAULT_RETRY) {
+ /* Clear FAULT_FLAG_ALLOW_RETRY to avoid any risk
+ * of starvation. */
+ flags &= ~FAULT_FLAG_ALLOW_RETRY;
+ goto retry;
+ }
}
check_v8086_mode(regs, address, tsk);
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 2862009..3bf4665 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -144,6 +144,7 @@
#define FAULT_FLAG_WRITE 0x01 /* Fault was a write access */
#define FAULT_FLAG_NONLINEAR 0x02 /* Fault was via a nonlinear mapping */
#define FAULT_FLAG_MKWRITE 0x04 /* Fault was mkwrite of existing pte */
+#define FAULT_FLAG_ALLOW_RETRY 0x08 /* Retry fault if blocking */
/*
* This interface is used by x86 PAT code to identify a pfn mapping that is
@@ -723,6 +724,7 @@
#define VM_FAULT_NOPAGE 0x0100 /* ->fault installed the pte, not return page */
#define VM_FAULT_LOCKED 0x0200 /* ->fault locked the returned page */
+#define VM_FAULT_RETRY 0x0400 /* ->fault blocked, must retry */
#define VM_FAULT_HWPOISON_LARGE_MASK 0xf000 /* encodes hpage index for large hwpoison */
diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index e12cdc6..2d1ffe3 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -299,6 +299,8 @@
extern void __lock_page(struct page *page);
extern int __lock_page_killable(struct page *page);
extern void __lock_page_nosync(struct page *page);
+extern int __lock_page_or_retry(struct page *page, struct mm_struct *mm,
+ unsigned int flags);
extern void unlock_page(struct page *page);
static inline void __set_page_locked(struct page *page)
@@ -351,6 +353,17 @@
}
/*
+ * lock_page_or_retry - Lock the page, unless this would block and the
+ * caller indicated that it can handle a retry.
+ */
+static inline int lock_page_or_retry(struct page *page, struct mm_struct *mm,
+ unsigned int flags)
+{
+ might_sleep();
+ return trylock_page(page) || __lock_page_or_retry(page, mm, flags);
+}
+
+/*
* This is exported only for wait_on_page_locked/wait_on_page_writeback.
* Never use this directly!
*/
diff --git a/mm/filemap.c b/mm/filemap.c
index 8ed709a..33f8125 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -612,6 +612,19 @@
TASK_UNINTERRUPTIBLE);
}
+int __lock_page_or_retry(struct page *page, struct mm_struct *mm,
+ unsigned int flags)
+{
+ if (!(flags & FAULT_FLAG_ALLOW_RETRY)) {
+ __lock_page(page);
+ return 1;
+ } else {
+ up_read(&mm->mmap_sem);
+ wait_on_page_locked(page);
+ return 0;
+ }
+}
+
/**
* find_get_page - find and get a page reference
* @mapping: the address_space to search
@@ -1550,7 +1563,8 @@
goto no_cached_page;
}
- lock_page(page);
+ if (!lock_page_or_retry(page, vma->vm_mm, vmf->flags))
+ return ret | VM_FAULT_RETRY;
/* Did it get truncated? */
if (unlikely(page->mapping != mapping)) {
diff --git a/mm/memory.c b/mm/memory.c
index 92cc54e..714c443 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2627,6 +2627,7 @@
struct page *page, *swapcache = NULL;
swp_entry_t entry;
pte_t pte;
+ int locked;
struct mem_cgroup *ptr = NULL;
int exclusive = 0;
int ret = 0;
@@ -2677,8 +2678,12 @@
goto out_release;
}
- lock_page(page);
+ locked = lock_page_or_retry(page, mm, flags);
delayacct_clear_flag(DELAYACCT_PF_SWAPIN);
+ if (!locked) {
+ ret |= VM_FAULT_RETRY;
+ goto out_release;
+ }
/*
* Make sure try_to_free_swap or reuse_swap_page or swapoff did not
@@ -2927,7 +2932,8 @@
vmf.page = NULL;
ret = vma->vm_ops->fault(vma, &vmf);
- if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE)))
+ if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE |
+ VM_FAULT_RETRY)))
return ret;
if (unlikely(PageHWPoison(vmf.page))) {
