[RFC PATCH 2/2] mm: Gate stable page writes on the bdi flag

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

Create a helper function to decide if a particular address space is backed by a
device that requires stable page writes.  For each wait_on_page_writeback call
that was inserted in the original stable page write patchset, change it to wait
only if the helper says it's necessary.  This should eliminate unnecessary
waiting for devices that don't require page contents to be immutable during
writeout.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
---

 fs/buffer.c         |    3 ++-
 fs/ext4/inode.c     |    4 ++--
 include/linux/fs.h  |    2 ++
 mm/filemap.c        |    3 ++-
 mm/page-writeback.c |    7 +++++++
 5 files changed, 15 insertions(+), 4 deletions(-)

diff --git a/fs/buffer.c b/fs/buffer.c
index b5f0442..f508ece 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -2334,7 +2334,8 @@ 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);
+	if (page->mapping && mapping_needs_stable_writes(page->mapping))
+		wait_on_page_writeback(page);
 	return 0;
 out_unlock:
 	unlock_page(page);
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index b3c243b..82908b8 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -4813,8 +4813,8 @@ int ext4_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
 	if (page_has_buffers(page)) {
 		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);
+			if (mapping_needs_stable_writes(mapping))
+				wait_on_page_writeback(page);
 			ret = VM_FAULT_LOCKED;
 			goto out;
 		}
diff --git a/include/linux/fs.h b/include/linux/fs.h
index b33cfc9..d99db0e 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -466,6 +466,8 @@ struct block_device {
 	struct percpu_rw_semaphore	bd_block_size_semaphore;
 };
 
+int mapping_needs_stable_writes(struct address_space *mapping);
+
 /*
  * Radix-tree tags, for tagging dirty and writeback pages within the pagecache
  * radix trees
diff --git a/mm/filemap.c b/mm/filemap.c
index 83efee7..aedc6bd 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2274,7 +2274,8 @@ repeat:
 		return NULL;
 	}
 found:
-	wait_on_page_writeback(page);
+	if (mapping_needs_stable_writes(mapping))
+		wait_on_page_writeback(page);
 	return page;
 }
 EXPORT_SYMBOL(grab_cache_page_write_begin);
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 830893b..d0f042c 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -2275,3 +2275,10 @@ int mapping_tagged(struct address_space *mapping, int tag)
 	return radix_tree_tagged(&mapping->page_tree, tag);
 }
 EXPORT_SYMBOL(mapping_tagged);
+
+int mapping_needs_stable_writes(struct address_space *mapping)
+{
+	return mapping->backing_dev_info &&
+	       bdi_cap_stable_writes(mapping->backing_dev_info);
+}
+EXPORT_SYMBOL_GPL(mapping_needs_stable_writes);