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_RELEASED flag, which indicates that the call site holds mmap_lock
and wishes for it to be released if blocking on a pending disk transfer.
In that case, filemap_fault() returns the VM_FAULT_RELEASED 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.
Signed-off-by: Michel Lespinasse <walken@xxxxxxxxxx>
---
arch/x86/mm/fault.c | 35 ++++++++++++++++++++++++++---------
include/linux/mm.h | 2 ++
mm/filemap.c | 20 +++++++++++++++++++-
mm/memory.c | 3 ++-
4 files changed, 49 insertions(+), 11 deletions(-)
diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 4c4508e..58109ba 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -954,6 +954,7 @@ do_page_fault(struct pt_regs *regs, unsigned long error_code)
struct mm_struct *mm;
int write;
int fault;
+ unsigned int release_flag = FAULT_FLAG_RELEASE;
tsk = current;
mm = tsk->mm;
@@ -1064,6 +1065,7 @@ 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 {
/*
@@ -1119,21 +1121,36 @@ 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,
+ release_flag | (write ? FAULT_FLAG_WRITE : 0));
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);
+ if (release_flag) { /* Did not go through a 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_RELEASED) {
+ /*
+ * handle_mm_fault() found that the desired page was
+ * locked. We asked for it to release mmap_sem in that
+ * case, so as to avoid holding it for too long.
+ * Retry starting at the mmap_sem acquire, this time
+ * without FAULT_FLAG_RETRY so that we avoid any
+ * risk of starvation.
+ */
+ release_flag = 0;
+ goto retry;
+ }
}
check_v8086_mode(regs, address, tsk);
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 74949fb..7782c30 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_RELEASE 0x08 /* Release mmap_sem 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_RELEASED 0x0400 /* mmap_sem got released */
#define VM_FAULT_ERROR (VM_FAULT_OOM | VM_FAULT_SIGBUS | VM_FAULT_HWPOISON)
diff --git a/mm/filemap.c b/mm/filemap.c
index 8ed709a..74197e2 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1550,7 +1550,25 @@ retry_find:
goto no_cached_page;
}
- lock_page(page);
+ /* Lock the page. */
+ if (!trylock_page(page)) {
+ if (!(vmf->flags & FAULT_FLAG_RELEASE))
+ __lock_page(page);
+ else {
+ /*
+ * Caller passed FAULT_FLAG_RELEASE flag.
+ * This indicates it has read-acquired mmap_sem,
+ * and requests that it be released if we have to
+ * wait for the page to be transferred from disk.
+ * Caller will then retry starting with the
+ * mmap_sem read-acquire.
+ */
+ up_read(&vma->vm_mm->mmap_sem);
+ wait_on_page_locked(page);
+ page_cache_release(page);
+ return ret | VM_FAULT_RELEASED;
+ }
+ }
/* Did it get truncated? */
if (unlikely(page->mapping != mapping)) {
diff --git a/mm/memory.c b/mm/memory.c
index 0e18b4d..2efb59d 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2926,7 +2926,8 @@ 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_RELEASED)))
return ret;
if (unlikely(PageHWPoison(vmf.page))) {
--
1.7.1
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