os/bluestore: enlarege aligned_size avoid too many vector(> IOV_MAX)

Fixes: http://tracker.ceph.com/issues/21932
Signed-off-by: Jianpeng Ma <jianpeng.ma@intel.com>

src/os/bluestore/KernelDevice.cc

@@ -592,9 +592,17 @@ int KernelDevice::write(
 	   << dendl;
   assert(is_valid_io(off, len));
 
-  if ((!buffered || bl.get_num_buffers() >= IOV_MAX) &&
-      bl.rebuild_aligned_size_and_memory(block_size, block_size)) {
-    dout(20) << __func__ << " rebuilding buffer to be aligned" << dendl;
+  if ((!buffered || bl.get_num_buffers() >= IOV_MAX)) {
+    uint64_t aligned_size = block_size;
+    // Avoid after rebuild_aligned_size_and_memory, get_num_buffers() > IOV_MAX
+    if (bl.get_num_buffers() >= IOV_MAX && (len > (IOV_MAX * block_size))) {
+      aligned_size = ROUND_UP_TO(ROUND_UP_TO(len, IOV_MAX) / IOV_MAX, block_size);
+      dout(5) << __func__ << " because too many buffers, using  " << aligned_size
+	      << " as aligned_size" << dendl;
+    }
+    if (bl.rebuild_aligned_size_and_memory(aligned_size, block_size)) {
+      dout(20) << __func__ << " rebuilding buffer to be aligned" << dendl;
+    }
   }
   dout(40) << "data: ";
   bl.hexdump(*_dout);
@@ -619,10 +627,19 @@ int KernelDevice::aio_write(
   assert(off < size);
   assert(off + len <= size);
 
-  if ((!buffered || bl.get_num_buffers() >= IOV_MAX) &&
-      bl.rebuild_aligned_size_and_memory(block_size, block_size)) {
-    dout(20) << __func__ << " rebuilding buffer to be aligned" << dendl;
+  if ((!buffered || bl.get_num_buffers() >= IOV_MAX)) {
+    uint64_t aligned_size = block_size;
+    // Avoid after rebuild_aligned_size_and_memory, get_num_buffers() > IOV_MAX
+    if (bl.get_num_buffers() >= IOV_MAX && (len > (IOV_MAX * block_size))) {
+      aligned_size = ROUND_UP_TO(ROUND_UP_TO(len, IOV_MAX) / IOV_MAX, block_size);
+      dout(5) << __func__ << " because too many buffers, using  " << aligned_size
+	      << " as aligned_size" << dendl;
+    }
+    if (bl.rebuild_aligned_size_and_memory(aligned_size, block_size)) {
+      dout(20) << __func__ << " rebuilding buffer to be aligned" << dendl;
+    }
   }
+
   dout(40) << "data: ";
   bl.hexdump(*_dout);
   *_dout << dendl;