[PATCH] Btrfs: add skeleton code for compression heuristic

[Timofey Titovets <nefelim4ag@gmail.com>]

For now that code just return true
Later more complex heuristic code will be added

Signed-off-by: Timofey Titovets <nefelim4ag@gmail.com>
---
 fs/btrfs/compression.c | 22 ++++++++++++++++++++++
 fs/btrfs/compression.h |  2 ++
 fs/btrfs/inode.c       | 18 ++++++++++++------
 3 files changed, 36 insertions(+), 6 deletions(-)

diff --git a/fs/btrfs/compression.c b/fs/btrfs/compression.c
index 2c0b7b57fcd5..d0cf2024def7 100644
--- a/fs/btrfs/compression.c
+++ b/fs/btrfs/compression.c
@@ -1046,3 +1046,25 @@ int btrfs_decompress_buf2page(const char *buf, unsigned long buf_start,
 
 	return 1;
 }
+
+/*
+ * Heuristic skeleton
+ * For now just would be a naive and very optimistic 'return true'.
+ */
+int btrfs_compress_heuristic(struct inode *inode, u64 start, u64 end)
+{
+	u64 index = start >> PAGE_SHIFT;
+	u64 end_index = end >> PAGE_SHIFT;
+	struct page *page;
+	int ret = 1;
+
+	while (index <= end_index) {
+		page = find_get_page(inode->i_mapping, index);
+		kmap(page);
+		kunmap(page);
+		put_page(page);
+		index++;
+	}
+
+	return ret;
+}
diff --git a/fs/btrfs/compression.h b/fs/btrfs/compression.h
index 87f6d3332163..8508ba6b9aef 100644
--- a/fs/btrfs/compression.h
+++ b/fs/btrfs/compression.h
@@ -129,4 +129,6 @@ struct btrfs_compress_op {
 extern const struct btrfs_compress_op btrfs_zlib_compress;
 extern const struct btrfs_compress_op btrfs_lzo_compress;
 
+int btrfs_compress_heuristic(struct inode *inode, u64 start, u64 end);
+
 #endif
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 06dea7c89bbd..27922b29fa4e 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -392,20 +392,26 @@ static noinline int add_async_extent(struct async_cow *cow,
 	return 0;
 }
 
-static inline int inode_need_compress(struct inode *inode)
+static inline int inode_need_compress(struct inode *inode, u64 start, u64 end)
 {
+	int ret = 0;
 	struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb);
 
 	/* force compress */
 	if (btrfs_test_opt(fs_info, FORCE_COMPRESS))
-		return 1;
+		goto try_compress;
 	/* bad compression ratios */
 	if (BTRFS_I(inode)->flags & BTRFS_INODE_NOCOMPRESS)
-		return 0;
+		goto skip_compress;
 	if (btrfs_test_opt(fs_info, COMPRESS) ||
 	    BTRFS_I(inode)->flags & BTRFS_INODE_COMPRESS ||
 	    BTRFS_I(inode)->force_compress)
-		return 1;
+		goto try_compress;
+	goto skip_compress;
+try_compress:
+	ret = btrfs_compress_heuristic(inode, start, end);
+	return ret;
+skip_compress:
 	return 0;
 }
 
@@ -503,7 +509,7 @@ static noinline void compress_file_range(struct inode *inode,
 	 * inode has not been flagged as nocompress.  This flag can
 	 * change at any time if we discover bad compression ratios.
 	 */
-	if (inode_need_compress(inode)) {
+	if (inode_need_compress(inode, start, end)) {
 		WARN_ON(pages);
 		pages = kcalloc(nr_pages, sizeof(struct page *), GFP_NOFS);
 		if (!pages) {
@@ -1576,7 +1582,7 @@ static int run_delalloc_range(void *private_data, struct page *locked_page,
 	} else if (BTRFS_I(inode)->flags & BTRFS_INODE_PREALLOC && !force_cow) {
 		ret = run_delalloc_nocow(inode, locked_page, start, end,
 					 page_started, 0, nr_written);
-	} else if (!inode_need_compress(inode)) {
+	} else if (!inode_need_compress(inode, start, end)) {
 		ret = cow_file_range(inode, locked_page, start, end, end,
 				      page_started, nr_written, 1, NULL);
 	} else {
-- 
2.13.2