[PATCH 2/4] mm: Only enforce stable page writes if the backing device requires it

["Darrick J. Wong" <darrick.wong@oracle.com>]

Create a helper function to check if a backing device requires stable page
writes and, if so, performs the necessary wait.  Then, make it so that all
points in the memory manager that handle making pages writable use the helper
function.  This should provide stable page write support to most filesystems,
while eliminating unnecessary waiting for devices that don't require the
feature.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---
 fs/buffer.c             |    2 +-
 fs/ext4/inode.c         |    2 +-
 include/linux/pagemap.h |    1 +
 mm/filemap.c            |    3 ++-
 mm/page-writeback.c     |   11 +++++++++++
 5 files changed, 16 insertions(+), 3 deletions(-)


diff --git a/fs/buffer.c b/fs/buffer.c
index b5f0442..4cf0b78 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -2334,7 +2334,7 @@ int __block_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf,
 	if (unlikely(ret < 0))
 		goto out_unlock;
 	set_page_dirty(page);
-	wait_on_page_writeback(page);
+	wait_if_stable_page_write(page);
 	return 0;
 out_unlock:
 	unlock_page(page);
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index b3c243b..24c6193 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -4814,7 +4814,7 @@ int ext4_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
 		if (!walk_page_buffers(NULL, page_buffers(page), 0, len, NULL,
 					ext4_bh_unmapped)) {
 			/* Wait so that we don't change page under IO */
-			wait_on_page_writeback(page);
+			wait_if_stable_page_write(page);
 			ret = VM_FAULT_LOCKED;
 			goto out;
 		}
diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index e42c762..1d67f81 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -398,6 +398,7 @@ static inline void wait_on_page_writeback(struct page *page)
 }
 
 extern void end_page_writeback(struct page *page);
+void wait_if_stable_page_write(struct page *page);
 
 /*
  * Add an arbitrary waiter to a page's wait queue
diff --git a/mm/filemap.c b/mm/filemap.c
index 83efee7..34df185 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1728,6 +1728,7 @@ int filemap_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
 	 * see the dirty page and writeprotect it again.
 	 */
 	set_page_dirty(page);
+	wait_if_stable_page_write(page);
 out:
 	sb_end_pagefault(inode->i_sb);
 	return ret;
@@ -2274,7 +2275,7 @@ repeat:
 		return NULL;
 	}
 found:
-	wait_on_page_writeback(page);
+	wait_if_stable_page_write(page);
 	return page;
 }
 EXPORT_SYMBOL(grab_cache_page_write_begin);
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 830893b..cbbaeca 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -2275,3 +2275,14 @@ int mapping_tagged(struct address_space *mapping, int tag)
 	return radix_tree_tagged(&mapping->page_tree, tag);
 }
 EXPORT_SYMBOL(mapping_tagged);
+
+void wait_if_stable_page_write(struct page *page)
+{
+	struct backing_dev_info *bdi = page->mapping->backing_dev_info;
+
+	if (!bdi_cap_stable_pages_required(bdi))
+		return;
+
+	wait_on_page_writeback(page);
+}
+EXPORT_SYMBOL_GPL(wait_if_stable_page_write);