[LogFS] Split large truncated into smaller chunks

Truncate would do an almost limitless amount of work without invoking
the garbage collector in between.  Split it up into more manageable,
though still large, chunks.
Signed-off-by: Joern Engel <joern@logfs.org>

fs/logfs/readwrite.c

@@ -1837,19 +1837,37 @@ static int __logfs_truncate(struct inode *inode, u64 size)
 	return logfs_truncate_direct(inode, size);
 }
 
-int logfs_truncate(struct inode *inode, u64 size)
+/*
+ * Truncate, by changing the segment file, can consume a fair amount
+ * of resources.  So back off from time to time and do some GC.
+ * 8 or 2048 blocks should be well within safety limits even if
+ * every single block resided in a different segment.
+ */
+#define TRUNCATE_STEP	(8 * 1024 * 1024)
+int logfs_truncate(struct inode *inode, u64 target)
 {
 	struct super_block *sb = inode->i_sb;
-	int err;
+	u64 size = i_size_read(inode);
+	int err = 0;
+
+	size = ALIGN(size, TRUNCATE_STEP);
+	while (size > target) {
+		if (size > TRUNCATE_STEP)
+			size -= TRUNCATE_STEP;
+		else
+			size = 0;
+		if (size < target)
+			size = target;
 
 		logfs_get_wblocks(sb, NULL, 1);
-	err = __logfs_truncate(inode, size);
+		err = __logfs_truncate(inode, target);
 		if (!err)
 			err = __logfs_write_inode(inode, 0);
 		logfs_put_wblocks(sb, NULL, 1);
+	}
 
 	if (!err)
-		err = vmtruncate(inode, size);
+		err = vmtruncate(inode, target);
 
 	/* I don't trust error recovery yet. */
 	WARN_ON(err);