librbd: propagate diff-iterate range to parent in fast-diff mode

When getting parent diff, pass the overlap-reduced image extent instead
of the entire 0..overlap range to avoid a similar quadratic slowdown on
cloned images.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

1 files changed, 7 insertions, 5 deletions

diff --git a/src/librbd/api/DiffIterate.cc b/src/librbd/api/DiffIterate.cc
index a464b942ae0..b114c32faba 100644
--- a/src/librbd/api/DiffIterate.cc
+++ b/src/librbd/api/DiffIterate.cc
@@ -293,12 +293,14 @@ int DiffIterate<I>::execute() {
         std::shared_lock image_locker{m_image_ctx.image_lock};
         uint64_t raw_overlap = 0;
         m_image_ctx.get_parent_overlap(m_image_ctx.snap_id, &raw_overlap);
-        auto overlap = m_image_ctx.reduce_parent_overlap(raw_overlap, false);
-        if (overlap.first > 0 && overlap.second == io::ImageArea::DATA) {
+        io::Extents parent_extents = {{m_offset, m_length}};
+        if (m_image_ctx.prune_parent_extents(parent_extents, io::ImageArea::DATA,
+                                             raw_overlap, false) > 0) {
           ldout(cct, 10) << " first getting parent diff" << dendl;
-          DiffIterate diff_parent(*m_image_ctx.parent, {}, nullptr, 0,
-                                  overlap.first, true, true, &simple_diff_cb,
-                                  &parent_diff);
+          DiffIterate diff_parent(*m_image_ctx.parent, {}, nullptr,
+                                  parent_extents[0].first,
+                                  parent_extents[0].second, true, true,
+                                  &simple_diff_cb, &parent_diff);
           r = diff_parent.execute();
           if (r < 0) {
             return r;