[PATCH v2 13/18] btrfs: extent_io: introduce read_extent_buffer_subpage()

[Qu Wenruo <wqu@suse.com>]

Introduce a new helper, read_extent_buffer_subpage(), to do the subpage
extent buffer read.

The difference between regular and subpage routines are:
- No page locking
  Here we completely rely on extent locking.
  Page locking can reduce the concurrency greatly, as if we lock one
  page to read one extent buffer, all the other extent buffers in the
  same page will have to wait.

- Extent uptodate condition
  Despite the existing PageUptodate() and EXTENT_BUFFER_UPTODATE check,
  We also need to check btrfs_subpage::uptodate_bitmap.

- No page loop
  Just one page, no need to loop, this greately simplified the subpage
  routine.

This patch only implemented the bio submit part, no endio support yet.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/disk-io.c   |  1 +
 fs/btrfs/extent_io.c | 70 ++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 71 insertions(+)

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 765deefda92b..b6c03a8b0c72 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -602,6 +602,7 @@ int btrfs_validate_metadata_buffer(struct btrfs_io_bio *io_bio,
 	ASSERT(page->private);
 	eb = (struct extent_buffer *)page->private;
 
+
 	/*
 	 * The pending IO might have been the only thing that kept this buffer
 	 * in memory.  Make sure we have a ref for all this other checks
diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index 4d55803302e9..1ec9de2aa910 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -5641,6 +5641,73 @@ void set_extent_buffer_uptodate(struct extent_buffer *eb)
 	}
 }
 
+static int read_extent_buffer_subpage(struct extent_buffer *eb, int wait,
+				      int mirror_num)
+{
+	struct btrfs_fs_info *fs_info = eb->fs_info;
+	struct extent_io_tree *io_tree;
+	struct page *page = eb->pages[0];
+	struct bio *bio = NULL;
+	int ret = 0;
+
+	ASSERT(!test_bit(EXTENT_BUFFER_UNMAPPED, &eb->bflags));
+	ASSERT(PagePrivate(page));
+	io_tree = &BTRFS_I(fs_info->btree_inode)->io_tree;
+
+	if (wait == WAIT_NONE) {
+		ret = try_lock_extent(io_tree, eb->start,
+				      eb->start + eb->len - 1);
+		if (ret <= 0)
+			return ret;
+	} else {
+		ret = lock_extent(io_tree, eb->start, eb->start + eb->len - 1);
+		if (ret < 0)
+			return ret;
+	}
+
+	ret = 0;
+	if (test_bit(EXTENT_BUFFER_UPTODATE, &eb->bflags) ||
+	    PageUptodate(page) ||
+	    btrfs_subpage_test_uptodate(fs_info, page, eb->start, eb->len)) {
+		set_bit(EXTENT_BUFFER_UPTODATE, &eb->bflags);
+		unlock_extent(io_tree, eb->start, eb->start + eb->len - 1);
+		return ret;
+	}
+
+	clear_bit(EXTENT_BUFFER_READ_ERR, &eb->bflags);
+	eb->read_mirror = 0;
+	atomic_set(&eb->io_pages, 1);
+	check_buffer_tree_ref(eb);
+
+	ret = submit_extent_page(REQ_OP_READ | REQ_META, NULL, page, eb->start,
+				 eb->len, eb->start - page_offset(page), &bio,
+				 end_bio_extent_readpage, mirror_num, 0, 0,
+				 true);
+	if (ret) {
+		/*
+		 * In the endio function, if we hit something wrong we will
+		 * increase the io_pages, so here we need to decrease it for error
+		 * path.
+		 */
+		atomic_dec(&eb->io_pages);
+	}
+	if (bio) {
+		int tmp;
+
+		tmp = submit_one_bio(bio, mirror_num, 0);
+		if (tmp < 0)
+			return tmp;
+	}
+	if (ret || wait != WAIT_COMPLETE)
+		return ret;
+
+	wait_extent_bit(io_tree, eb->start, eb->start + eb->len - 1,
+			EXTENT_LOCKED);
+	if (!test_bit(EXTENT_BUFFER_UPTODATE, &eb->bflags))
+		ret = -EIO;
+	return ret;
+}
+
 int read_extent_buffer_pages(struct extent_buffer *eb, int wait, int mirror_num)
 {
 	int i;
@@ -5657,6 +5724,9 @@ int read_extent_buffer_pages(struct extent_buffer *eb, int wait, int mirror_num)
 	if (test_bit(EXTENT_BUFFER_UPTODATE, &eb->bflags))
 		return 0;
 
+	if (eb->fs_info->sectorsize < PAGE_SIZE)
+		return read_extent_buffer_subpage(eb, wait, mirror_num);
+
 	num_pages = num_extent_pages(eb);
 	for (i = 0; i < num_pages; i++) {
 		page = eb->pages[i];
-- 
2.29.2