osdc/WritebackHandler: add may_copy_on_write()

Implement a new method to tell us whether a read to a particular region
could be affected by a write-triggered copy-on-write.

Signed-off-by: Sage Weil <sage@inktank.com>

src/client/ObjecterWriteback.h

@@ -19,6 +19,10 @@ class ObjecterWriteback : public WritebackHandler {
 				  trunc_size, trunc_seq, onfinish);
   }
 
+  virtual bool may_copy_on_write(const object_t& oid, uint64_t read_off, uint64_t read_len, snapid_t snapid) {
+    return false;
+  }
+
   virtual tid_t write(const object_t& oid, const object_locator_t& oloc,
 		      uint64_t off, uint64_t len, const SnapContext& snapc,
 		      const bufferlist &bl, utime_t mtime, uint64_t trunc_size,

src/librbd/LibrbdWriteback.cc

@@ -86,6 +86,29 @@ namespace librbd {
     return ++m_tid;
   }
 
+  bool LibrbdWriteback::may_copy_on_write(const object_t& oid, uint64_t read_off, uint64_t read_len, snapid_t snapid)
+  {
+    m_ictx->snap_lock.Lock();
+    librados::snap_t snap_id = m_ictx->snap_id;
+    m_ictx->parent_lock.Lock();
+    uint64_t overlap = 0;
+    m_ictx->get_parent_overlap(snap_id, &overlap);
+    m_ictx->parent_lock.Unlock();
+    m_ictx->snap_lock.Unlock();
+
+    uint64_t object_no = oid_to_object_no(oid.name, m_ictx->object_prefix);
+
+    // reverse map this object extent onto the parent
+    vector<pair<uint64_t,uint64_t> > objectx;
+    Striper::extent_to_file(m_ictx->cct, &m_ictx->layout,
+			  object_no, 0, m_ictx->layout.fl_object_size,
+			  objectx);
+    uint64_t object_overlap = m_ictx->prune_parent_extents(objectx, overlap);
+    bool may = object_overlap > 0;
+    ldout(m_ictx->cct, 10) << "may_copy_on_write " << oid << " " << read_off << "~" << read_len << " = " << may << dendl;
+    return may;
+  }
+
   tid_t LibrbdWriteback::write(const object_t& oid,
 			       const object_locator_t& oloc,
 			       uint64_t off, uint64_t len,

src/librbd/LibrbdWriteback.h

@@ -26,6 +26,9 @@ namespace librbd {
 		       bufferlist *pbl, uint64_t trunc_size,  __u32 trunc_seq,
 		       Context *onfinish);
 
+    // Determine whether a read to this extent could be affected by a write-triggered copy-on-write
+    virtual bool may_copy_on_write(const object_t& oid, uint64_t read_off, uint64_t read_len, snapid_t snapid);
+
     // Note that oloc, trunc_size, and trunc_seq are ignored
     virtual tid_t write(const object_t& oid, const object_locator_t& oloc,
 			uint64_t off, uint64_t len, const SnapContext& snapc,

src/osdc/WritebackHandler.h

@@ -16,6 +16,18 @@ class WritebackHandler {
 		     uint64_t off, uint64_t len, snapid_t snapid,
 		     bufferlist *pbl, uint64_t trunc_size,  __u32 trunc_seq,
 		     Context *onfinish) = 0;
+  /**
+   * check if a given extent read result may change due to a write
+   *
+   * Check if the content we see at the given read offset may change due to a write to
+   * this object.
+   *
+   * @param oid object
+   * @param read_off read offset
+   * @param read_len read length
+   * @param snapid read snapid
+   */
+  virtual bool may_copy_on_write(const object_t& oid, uint64_t read_off, uint64_t read_len, snapid_t snapid) = 0;
   virtual tid_t write(const object_t& oid, const object_locator_t& oloc,
 		      uint64_t off, uint64_t len, const SnapContext& snapc,
 		      const bufferlist &bl, utime_t mtime, uint64_t trunc_size,