Re: [PATCH 2/3] Retry page fault when blocking on disk transfer.
From: Michel Lespinasse
Date: Fri Oct 08 2010 - 21:22:17 EST
Second try on adding the VM_FAULT_RETRY functionality to the swap in path.
This proposal would replace [patch 2/3] of this series (the initial
version of it, which was approved by linus / rik / hpa).
Changes since the approved version:
- split lock_page_or_retry() into an inline function in pagemap.h,
handling the trylock_page() fast path, and __lock_page_or_retry() in
filemap.c, handling the blocking path (with or without retry).
- make do_swap_page() call lock_page_or_retry() in place of lock_page(),
and handle the retry case.
---------------------------------- 8< -----------------------------------
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 or swap space.
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, handle_mm_fault() returns the
VM_FAULT_RETRY status flag 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.
Signed-off-by: Michel Lespinasse <walken@xxxxxxxxxx>
---
arch/x86/mm/fault.c | 38 ++++++++++++++++++++++++++------------
include/linux/mm.h | 2 ++
include/linux/pagemap.h | 13 +++++++++++++
mm/filemap.c | 16 +++++++++++++++-
mm/memory.c | 10 ++++++++--
5 files changed, 64 insertions(+), 15 deletions(-)
diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 4c4508e..b355b92 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -952,8 +952,10 @@ do_page_fault(struct pt_regs *regs, unsigned long error_code)
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;
@@ -1064,6 +1066,7 @@ do_page_fault(struct pt_regs *regs, unsigned long error_code)
bad_area_nosemaphore(regs, error_code, address);
return;
}
+retry:
down_read(&mm->mmap_sem);
} else {
/*
@@ -1107,8 +1110,6 @@ do_page_fault(struct pt_regs *regs, unsigned long error_code)
* 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;
@@ -1119,21 +1120,34 @@ good_area:
* 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 74949fb..0b4f9b2 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -144,6 +144,7 @@ extern pgprot_t protection_map[16];
#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
@@ -722,6 +723,7 @@ static inline int page_mapped(struct page *page)
#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_ERROR (VM_FAULT_OOM | VM_FAULT_SIGBUS | VM_FAULT_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 @@ static inline pgoff_t linear_page_index(struct vm_area_struct *vma,
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 @@ static inline void lock_page_nosync(struct page *page)
}
/*
+ * 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..2eeb6c5 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -612,6 +612,19 @@ void __lock_page_nosync(struct page *page)
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 @@ retry_find:
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 0e18b4d..362e803 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2626,6 +2626,7 @@ static int do_swap_page(struct mm_struct *mm, struct vm_area_struct *vma,
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;
@@ -2676,8 +2677,12 @@ static int do_swap_page(struct mm_struct *mm, struct vm_area_struct *vma,
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
@@ -2926,7 +2931,8 @@ static int __do_fault(struct mm_struct *mm, struct vm_area_struct *vma,
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))) {
--
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.
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