[PATCH][1/2]page_fault retry with NOPAGE_RETRY

[Ying Han <yinghan@google.com>]

support for FAULT_FLAG_RETRY with no user change:

Signed-off-by: Ying Han <yinghan@google.com>
              Mike Waychison <mikew@google.com>

 include/linux/fs.h  |    2 +-
 include/linux/mm.h  |    2 +
 mm/filemap.c        |   72 ++++++++++++++++++++++++++++++++++++++++++++++++--
 mm/memory.c         |   33 +++++++++++++++++------


diff --git a/include/linux/fs.h b/include/linux/fs.h
index 4a853ef..29c2c39 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -793,7 +793,7 @@ struct file_ra_state {
 					   there are only # of pages ahead */

 	unsigned int ra_pages;		/* Maximum readahead window */
-	int mmap_miss;			/* Cache miss stat for mmap accesses */
+	unsigned int mmap_miss;		/* Cache miss stat for mmap accesses */
 	loff_t prev_pos;		/* Cache last read() position */
 };

diff --git a/include/linux/mm.h b/include/linux/mm.h
index ffee2f7..5a134a9 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_RETRY	0x04	/* Retry major fault */


 /*
@@ -690,6 +691,7 @@ static inline int page_mapped(struct page *page)

 #define VM_FAULT_MINOR	0 /* For backwards compat. Remove me quickly. */

+#define VM_FAULT_RETRY	0x0010
 #define VM_FAULT_OOM	0x0001
 #define VM_FAULT_SIGBUS	0x0002
 #define VM_FAULT_MAJOR	0x0004
diff --git a/mm/filemap.c b/mm/filemap.c
index f3e5f89..6eb7c36 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -714,6 +714,58 @@ repeat:
 EXPORT_SYMBOL(find_lock_page);

 /**
+ * find_lock_page_retry - locate, pin and lock a pagecache page
+ * @mapping: the address_space to search
+ * @offset: the page index
+ * @vma: vma in which the fault was taken
+ * @ppage: zero if page not present, otherwise point to the page in pagecache
+ * @retry: 1 indicate caller tolerate a retry.
+ *
+ * If retry flag is on, and page is already locked by someone else, return
+ * a hint of retry and leave *ppage untouched.
+ *
+ * Return *ppage==NULL if page is not in pagecache. Otherwise return *ppage
+ * points to the page in the pagecache with ret=VM_FAULT_RETRY indicate a
+ * hint to caller for retry, or ret=0 which means page is succefully
+ * locked.
+ */
+unsigned find_lock_page_retry(struct address_space *mapping, pgoff_t offset,
+				struct vm_area_struct *vma, struct page **ppage,
+				int retry)
+{
+	unsigned int ret = 0;
+	struct page *page;
+
+repeat:
+	page = find_get_page(mapping, offset);
+	if (page) {
+		if (!retry)
+			lock_page(page);
+		else {
+			if (!trylock_page(page)) {
+				struct mm_struct *mm = vma->vm_mm;
+
+				up_read(&mm->mmap_sem);
+				wait_on_page_locked(page);
+				down_read(&mm->mmap_sem);
+
+				page_cache_release(page);
+				return VM_FAULT_RETRY;
+			}
+		}
+		if (unlikely(page->mapping != mapping)) {
+			unlock_page(page);
+			page_cache_release(page);
+			goto repeat;
+		}
+		VM_BUG_ON(page->index != offset);
+	}
+	*ppage = page;
+	return ret;
+}
+EXPORT_SYMBOL(find_lock_page_retry);
+
+/**
  * find_or_create_page - locate or add a pagecache page
  * @mapping: the page's address_space
  * @index: the page's index into the mapping
@@ -1444,6 +1496,8 @@ int filemap_fault(struct vm_area_struct *vma, struct vm_
 	pgoff_t size;
 	int did_readaround = 0;
 	int ret = 0;
+	int retry_flag = vmf->flags & FAULT_FLAG_RETRY;
+	int retry_ret;

 	size = (i_size_read(inode) + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT;
 	if (vmf->pgoff >= size)
@@ -1458,6 +1512,7 @@ int filemap_fault(struct vm_area_struct *vma, struct vm_
 	 */
 retry_find:
 	page = find_lock_page(mapping, vmf->pgoff);
+
 	/*
 	 * For sequential accesses, we use the generic readahead logic.
 	 */
@@ -1465,7 +1520,13 @@ retry_find:
 		if (!page) {
 			page_cache_sync_readahead(mapping, ra, file,
 							   vmf->pgoff, 1);
-			page = find_lock_page(mapping, vmf->pgoff);
+			retry_ret = find_lock_page_retry(mapping, vmf->pgoff,
+						vma, &page, retry_flag);
+			if (retry_ret == VM_FAULT_RETRY) {
+				/* counteract the followed retry hit */
+				ra->mmap_miss++;
+				return retry_ret;
+			}
 			if (!page)
 				goto no_cached_page;
 		}
@@ -1504,7 +1565,14 @@ retry_find:
 				start = vmf->pgoff - ra_pages / 2;
 			do_page_cache_readahead(mapping, file, start, ra_pages);
 		}
-		page = find_lock_page(mapping, vmf->pgoff);
+retry_find_retry:
+		retry_ret = find_lock_page_retry(mapping, vmf->pgoff,
+				vma, &page, retry_flag);
+		if (retry_ret == VM_FAULT_RETRY) {
+			/* counteract the followed retry hit */
+			ra->mmap_miss++;
+			return retry_ret;
+		}
 		if (!page)
 			goto no_cached_page;
 	}
@@ -1548,7 +1616,7 @@ no_cached_page:
 	 * meantime, we'll just come back here and read it again.
 	 */
 	if (error >= 0)
-		goto retry_find;
+		goto retry_find_retry;

 	/*
 	 * An error return from page_cache_read can result if the
diff --git a/mm/memory.c b/mm/memory.c
index 164951c..5e215c9 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2467,6 +2467,13 @@ static int __do_fault(struct mm_struct *mm, struct vm_a
 	vmf.page = NULL;

 	ret = vma->vm_ops->fault(vma, &vmf);
+
+	/* page may be available, but we have to restart the process
+	 * because mmap_sem was dropped during the ->fault
+	 */
+	if (ret & VM_FAULT_RETRY)
+		return ret;
+
 	if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE)))
 		return ret;

@@ -2611,8 +2618,10 @@ static int do_linear_fault(struct mm_struct *mm, struct
 {
 	pgoff_t pgoff = (((address & PAGE_MASK)
 			- vma->vm_start) >> PAGE_SHIFT) + vma->vm_pgoff;
-	unsigned int flags = (write_access ? FAULT_FLAG_WRITE : 0);
+	int write = write_access & ~FAULT_FLAG_RETRY;
+	unsigned int flags = (write ? FAULT_FLAG_WRITE : 0);

+	flags |= (write_access & FAULT_FLAG_RETRY);
 	pte_unmap(page_table);
 	return __do_fault(mm, vma, address, pmd, pgoff, flags, orig_pte);
 }
@@ -2726,26 +2735,32 @@ int handle_mm_fault(struct mm_struct *mm, struct vm_ar
 	pud_t *pud;
 	pmd_t *pmd;
 	pte_t *pte;
+	int ret;

 	__set_current_state(TASK_RUNNING);

-	count_vm_event(PGFAULT);
-
-	if (unlikely(is_vm_hugetlb_page(vma)))
-		return hugetlb_fault(mm, vma, address, write_access);
+	if (unlikely(is_vm_hugetlb_page(vma))) {
+		ret = hugetlb_fault(mm, vma, address, write_access);
+		goto out;
+	}

+	ret = VM_FAULT_OOM;
 	pgd = pgd_offset(mm, address);
 	pud = pud_alloc(mm, pgd, address);
 	if (!pud)
-		return VM_FAULT_OOM;
+		goto out;
 	pmd = pmd_alloc(mm, pud, address);
 	if (!pmd)
-		return VM_FAULT_OOM;
+		goto out;
 	pte = pte_alloc_map(mm, pmd, address);
 	if (!pte)
-		return VM_FAULT_OOM;
+		goto out;

-	return handle_pte_fault(mm, vma, address, pte, pmd, write_access);
+	ret = handle_pte_fault(mm, vma, address, pte, pmd, write_access);
+out:
+	if (!(ret & VM_FAULT_RETRY))
+		count_vm_event(PGFAULT);
+	return ret;
 }

 #ifndef __PAGETABLE_PUD_FOLDED

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>