[PATCH 4/4] ext4: fix suboptimal seek_{data,hole} extents traversial

[Dmitry Monakhov <dmonakhov@openvz.org>]

It is rediculus practice to scan inode block by block, this technique
applicable only for old indirect files. This takes signifficant amount
of time for really large files. Let's reuse ext4_fiemap which already
traverse inode-tree in most optimal meaner.

TESTCASE:
ftruncate64(fd, 0);
ftruncate64(fd, 1ULL << 40);
/* lseek will spin very long time */
lseek64(fd, 0, SEEK_DATA);
lseek64(fd, 0, SEEK_HOLE);


Original report: https://lkml.org/lkml/2014/10/16/620

##################################
BTW: Why do we need i_mutex here?

---
 fs/ext4/extents.c |    4 +-
 fs/ext4/file.c    |  220 +++++++++++++++++++++++++---------------------------
 2 files changed, 108 insertions(+), 116 deletions(-)

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index bed4308..e5d3ead 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -5166,8 +5166,8 @@ int ext4_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
 
 	/* fallback to generic here if not in extents fmt */
 	if (!(ext4_test_inode_flag(inode, EXT4_INODE_EXTENTS)))
-		return generic_block_fiemap(inode, fieinfo, start, len,
-			ext4_get_block);
+		return __generic_block_fiemap(inode, fieinfo, start, len,
+					      ext4_get_block);
 
 	if (fiemap_check_flags(fieinfo, EXT4_FIEMAP_FLAGS))
 		return -EBADR;
diff --git a/fs/ext4/file.c b/fs/ext4/file.c
index 8131be8..513c12c 100644
--- a/fs/ext4/file.c
+++ b/fs/ext4/file.c
@@ -273,24 +273,19 @@ static int ext4_file_open(struct inode * inode, struct file * filp)
  * we determine this extent as a data or a hole according to whether the
  * page cache has data or not.
  */
-static int ext4_find_unwritten_pgoff(struct inode *inode,
-				     int whence,
-				     struct ext4_map_blocks *map,
-				     loff_t *offset)
+static int ext4_find_unwritten_pgoff(struct inode *inode, int whence,
+				     loff_t endoff, loff_t *offset)
 {
 	struct pagevec pvec;
-	unsigned int blkbits;
 	pgoff_t index;
 	pgoff_t end;
-	loff_t endoff;
 	loff_t startoff;
 	loff_t lastoff;
 	int found = 0;
 
-	blkbits = inode->i_sb->s_blocksize_bits;
 	startoff = *offset;
 	lastoff = startoff;
-	endoff = (loff_t)(map->m_lblk + map->m_len) << blkbits;
+
 
 	index = startoff >> PAGE_CACHE_SHIFT;
 	end = endoff >> PAGE_CACHE_SHIFT;
@@ -408,147 +403,144 @@ out:
 static loff_t ext4_seek_data(struct file *file, loff_t offset, loff_t maxsize)
 {
 	struct inode *inode = file->f_mapping->host;
-	struct ext4_map_blocks map;
-	struct extent_status es;
-	ext4_lblk_t start, last, end;
-	loff_t dataoff, isize;
-	int blkbits;
-	int ret = 0;
+	struct fiemap_extent_info fie;
+	struct fiemap_extent ext[2];
+	loff_t next;
+	int i, ret = 0;
 
 	mutex_lock(&inode->i_mutex);
-
-	isize = i_size_read(inode);
-	if (offset >= isize) {
+	if (offset >= inode->i_size) {
 		mutex_unlock(&inode->i_mutex);
 		return -ENXIO;
 	}
-
-	blkbits = inode->i_sb->s_blocksize_bits;
-	start = offset >> blkbits;
-	last = start;
-	end = isize >> blkbits;
-	dataoff = offset;
-
-	do {
-		map.m_lblk = last;
-		map.m_len = end - last + 1;
-		ret = ext4_map_blocks(NULL, inode, &map, 0);
-		if (ret > 0 && !(map.m_flags & EXT4_MAP_UNWRITTEN)) {
-			if (last != start)
-				dataoff = (loff_t)last << blkbits;
+	fie.fi_flags = 0;
+	fie.fi_extents_max = 2;
+	fie.fi_extents_start = (struct fiemap_extent __user *) &ext;
+	while (1) {
+		mm_segment_t old_fs = get_fs();
+
+		fie.fi_extents_mapped = 0;
+		memset(ext, 0, sizeof(*ext) * fie.fi_extents_max);
+
+		set_fs(get_ds());
+		ret = ext4_fiemap(inode, &fie, offset, maxsize - offset);
+		set_fs(old_fs);
+		if (ret)
 			break;
-		}
 
-		/*
-		 * If there is a delay extent at this offset,
-		 * it will be as a data.
-		 */
-		ext4_es_find_delayed_extent_range(inode, last, last, &es);
-		if (es.es_len != 0 && in_range(last, es.es_lblk, es.es_len)) {
-			if (last != start)
-				dataoff = (loff_t)last << blkbits;
+		/* No extents found, EOF */
+		if (!fie.fi_extents_mapped) {
+			ret = -ENXIO;
 			break;
 		}
+		for (i = 0; i < fie.fi_extents_mapped; i++) {
+			next = (loff_t)(ext[i].fe_length + ext[i].fe_logical);
 
-		/*
-		 * If there is a unwritten extent at this offset,
-		 * it will be as a data or a hole according to page
-		 * cache that has data or not.
-		 */
-		if (map.m_flags & EXT4_MAP_UNWRITTEN) {
-			int unwritten;
-			unwritten = ext4_find_unwritten_pgoff(inode, SEEK_DATA,
-							      &map, &dataoff);
-			if (unwritten)
-				break;
-		}
+			if (offset < (loff_t)ext[i].fe_logical)
+				offset = (loff_t)ext[i].fe_logical;
+			/*
+			 * If extent is not unwritten, then it contains valid
+			 * data, mapped or delayed.
+			 */
+			if (!(ext[i].fe_flags & FIEMAP_EXTENT_UNWRITTEN))
+				goto out;
 
-		last++;
-		dataoff = (loff_t)last << blkbits;
-	} while (last <= end);
+			/*
+			 * If there is a unwritten extent at this offset,
+			 * it will be as a data or a hole according to page
+			 * cache that has data or not.
+			 */
+			if (ext4_find_unwritten_pgoff(inode, SEEK_DATA,
+						      next, &offset))
+				goto out;
 
+			if (ext[i].fe_flags & FIEMAP_EXTENT_LAST) {
+				ret = -ENXIO;
+				goto out;
+			}
+			offset = next;
+		}
+	}
+	if (offset > inode->i_size)
+		offset = inode->i_size;
+out:
 	mutex_unlock(&inode->i_mutex);
+	if (ret)
+		return ret;
 
-	if (dataoff > isize)
-		return -ENXIO;
-
-	return vfs_setpos(file, dataoff, maxsize);
+	return vfs_setpos(file, offset, maxsize);
 }
 
 /*
- * ext4_seek_hole() retrieves the offset for SEEK_HOLE.
+ * ext4_seek_hole() retrieves the offset for SEEK_HOLE
  */
 static loff_t ext4_seek_hole(struct file *file, loff_t offset, loff_t maxsize)
 {
 	struct inode *inode = file->f_mapping->host;
-	struct ext4_map_blocks map;
-	struct extent_status es;
-	ext4_lblk_t start, last, end;
-	loff_t holeoff, isize;
-	int blkbits;
-	int ret = 0;
+	struct fiemap_extent_info fie;
+	struct fiemap_extent ext[2];
+	loff_t next;
+	int i, ret = 0;
 
 	mutex_lock(&inode->i_mutex);
-
-	isize = i_size_read(inode);
-	if (offset >= isize) {
+	if (offset >= inode->i_size) {
 		mutex_unlock(&inode->i_mutex);
 		return -ENXIO;
 	}
 
-	blkbits = inode->i_sb->s_blocksize_bits;
-	start = offset >> blkbits;
-	last = start;
-	end = isize >> blkbits;
-	holeoff = offset;
+	fie.fi_flags = 0;
+	fie.fi_extents_max = 2;
+	fie.fi_extents_start = (struct fiemap_extent __user *)&ext;
+	while (1) {
+		mm_segment_t old_fs = get_fs();
 
-	do {
-		map.m_lblk = last;
-		map.m_len = end - last + 1;
-		ret = ext4_map_blocks(NULL, inode, &map, 0);
-		if (ret > 0 && !(map.m_flags & EXT4_MAP_UNWRITTEN)) {
-			last += ret;
-			holeoff = (loff_t)last << blkbits;
-			continue;
-		}
+		fie.fi_extents_mapped = 0;
+		memset(ext, 0, sizeof(*ext));
 
-		/*
-		 * If there is a delay extent at this offset,
-		 * we will skip this extent.
-		 */
-		ext4_es_find_delayed_extent_range(inode, last, last, &es);
-		if (es.es_len != 0 && in_range(last, es.es_lblk, es.es_len)) {
-			last = es.es_lblk + es.es_len;
-			holeoff = (loff_t)last << blkbits;
-			continue;
-		}
+		set_fs(get_ds());
+		ret = ext4_fiemap(inode, &fie, offset, maxsize - offset);
+		set_fs(old_fs);
+		if (ret)
+			break;
 
-		/*
-		 * If there is a unwritten extent at this offset,
-		 * it will be as a data or a hole according to page
-		 * cache that has data or not.
-		 */
-		if (map.m_flags & EXT4_MAP_UNWRITTEN) {
-			int unwritten;
-			unwritten = ext4_find_unwritten_pgoff(inode, SEEK_HOLE,
-							      &map, &holeoff);
-			if (!unwritten) {
-				last += ret;
-				holeoff = (loff_t)last << blkbits;
+		/* No extents found */
+		if (!fie.fi_extents_mapped)
+			break;
+
+		for (i = 0; i < fie.fi_extents_mapped; i++) {
+			next = (loff_t)(ext[i].fe_logical + ext[i].fe_length);
+			/*
+			 * If extent is not unwritten, then it contains valid
+			 * data, mapped or delayed.
+			 */
+			if (!(ext[i].fe_flags & FIEMAP_EXTENT_UNWRITTEN)) {
+				if (offset < (loff_t)ext[i].fe_logical)
+					goto out;
+				offset = next;
 				continue;
 			}
-		}
-
-		/* find a hole */
-		break;
-	} while (last <= end);
+			/*
+			 * If there is a unwritten extent at this offset,
+			 * it will be as a data or a hole according to page
+			 * cache that has data or not.
+			 */
+			if (ext4_find_unwritten_pgoff(inode, SEEK_HOLE,
+						      next, &offset))
+				goto out;
 
+			offset = next;
+			if (ext[i].fe_flags & FIEMAP_EXTENT_LAST)
+				goto out;
+		}
+	}
+	if (offset > inode->i_size)
+		offset = inode->i_size;
+out:
 	mutex_unlock(&inode->i_mutex);
+	if (ret)
+		return ret;
 
-	if (holeoff > isize)
-		holeoff = isize;