[RFC DRAFT PATCH] per-buffered-write stream IDs

[Ming Lin <ming.l@samsung.com>]

Hi Jens,

This RFC DRAFT patch is on top of your "[PATCH v2] Support for write stream IDs"
I throw it out early to get comments if it's the way to go.

Quote LWN(http://lwn.net/Articles/638722):

"There would be clear value in a closer association between stream IDs
and specific buffered-write operations. Getting there would require storing
the stream ID with each dirtied page, though; that, in turn, almost certainly 
implies shoehorning the stream ID into the associated page structure. 
That would not be an easy task; it is not surprising that it is not a part of 
this patch set. Should the lack of per-buffered-write stream IDs prove to be 
a serious constraint in the future, somebody will certainly be motivated to 
try to find a place to store another eight bits in struct page."

This draft patch stores stream_id in buffer head instead of page.

---
 fs/buffer.c                 |  4 ++--
 fs/ext4/page-io.c           |  2 +-
 fs/mpage.c                  |  5 ++++-
 include/linux/buffer_head.h |  1 +
 mm/filemap.c                | 17 +++++++++++++++++
 5 files changed, 25 insertions(+), 4 deletions(-)

diff --git a/fs/buffer.c b/fs/buffer.c
index 5191523..274fcc5 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -1774,7 +1774,7 @@ static int __block_write_full_page(struct inode *inode, struct page *page,
 	do {
 		struct buffer_head *next = bh->b_this_page;
 		if (buffer_async_write(bh)) {
-			_submit_bh(write_op, bh, streamid_to_flags(inode_streamid(inode)));
+			_submit_bh(write_op, bh, streamid_to_flags(bh->b_streamid));
 			nr_underway++;
 		}
 		bh = next;
@@ -1828,7 +1828,7 @@ recover:
 		struct buffer_head *next = bh->b_this_page;
 		if (buffer_async_write(bh)) {
 			clear_buffer_dirty(bh);
-			_submit_bh(write_op, bh, streamid_to_flags(inode_streamid(inode)));
+			_submit_bh(write_op, bh, streamid_to_flags(bh->b_streamid));
 			nr_underway++;
 		}
 		bh = next;
diff --git a/fs/ext4/page-io.c b/fs/ext4/page-io.c
index 980c5a9..c912129 100644
--- a/fs/ext4/page-io.c
+++ b/fs/ext4/page-io.c
@@ -406,7 +406,7 @@ submit_and_retry:
 		ret = io_submit_init_bio(io, bh);
 		if (ret)
 			return ret;
-		bio_set_streamid(io->io_bio, inode_streamid(inode));
+		bio_set_streamid(io->io_bio, bh->b_streamid);
 	}
 	ret = bio_add_page(io->io_bio, page, bh->b_size, bh_offset(bh));
 	if (ret != bh->b_size)
diff --git a/fs/mpage.c b/fs/mpage.c
index fba13f4..1c5ae1c 100644
--- a/fs/mpage.c
+++ b/fs/mpage.c
@@ -481,12 +481,15 @@ static int __mpage_writepage(struct page *page, struct writeback_control *wbc,
 	int length;
 	struct buffer_head map_bh;
 	loff_t i_size = i_size_read(inode);
+	int streamid = 0;
 	int ret = 0;
 
 	if (page_has_buffers(page)) {
 		struct buffer_head *head = page_buffers(page);
 		struct buffer_head *bh = head;
 
+		streamid = bh->b_streamid;
+
 		/* If they're all mapped and dirty, do it */
 		page_block = 0;
 		do {
@@ -605,7 +608,7 @@ alloc_new:
 				bio_get_nr_vecs(bdev), GFP_NOFS|__GFP_HIGH);
 		if (bio == NULL)
 			goto confused;
-		bio_set_streamid(bio, inode_streamid(inode));
+		bio_set_streamid(bio, streamid);
 	}
 
 	/*
diff --git a/include/linux/buffer_head.h b/include/linux/buffer_head.h
index 73b4522..5dbb460 100644
--- a/include/linux/buffer_head.h
+++ b/include/linux/buffer_head.h
@@ -75,6 +75,7 @@ struct buffer_head {
 	struct address_space *b_assoc_map;	/* mapping this buffer is
 						   associated with */
 	atomic_t b_count;		/* users using this buffer_head */
+	unsigned int b_streamid;
 };
 
 /*
diff --git a/mm/filemap.c b/mm/filemap.c
index 6bf5e42..3a31b95 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2419,6 +2419,21 @@ struct page *grab_cache_page_write_begin(struct address_space *mapping,
 }
 EXPORT_SYMBOL(grab_cache_page_write_begin);
 
+static void buffer_head_set_streamid(struct page *page, struct file *file)
+{
+	struct buffer_head *bh, *head;
+
+	if (!page_has_buffers(page))
+		return;
+
+	head = page_buffers(page);
+	bh = head;
+	do {
+		bh->b_streamid = file->f_streamid;
+		bh = bh->b_this_page;
+	} while (bh != head);
+}
+
 ssize_t generic_perform_write(struct file *file,
 				struct iov_iter *i, loff_t pos)
 {
@@ -2466,6 +2481,8 @@ again:
 		if (unlikely(status < 0))
 			break;
 
+		buffer_head_set_streamid(page, file);
+
 		if (mapping_writably_mapped(mapping))
 			flush_dcache_page(page);
 
-- 
1.9.1