Move mdwriteback() to better place

The previous order in the file didn't make sense and matched neither
the header file nor the smgr API.

Discussion: https://www.postgresql.org/message-id/flat/22fed8ba-01c3-2008-a256-4ea912d68fab%40enterprisedb.com

Author: petere

src/backend/storage/smgr/md.c

@@ -733,63 +733,6 @@ mdprefetch(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum)
 	return true;
 }
 
-/*
- * mdwriteback() -- Tell the kernel to write pages back to storage.
- *
- * This accepts a range of blocks because flushing several pages at once is
- * considerably more efficient than doing so individually.
- */
-void
-mdwriteback(SMgrRelation reln, ForkNumber forknum,
-			BlockNumber blocknum, BlockNumber nblocks)
-{
-	Assert((io_direct_flags & IO_DIRECT_DATA) == 0);
-
-	/*
-	 * Issue flush requests in as few requests as possible; have to split at
-	 * segment boundaries though, since those are actually separate files.
-	 */
-	while (nblocks > 0)
-	{
-		BlockNumber nflush = nblocks;
-		off_t		seekpos;
-		MdfdVec    *v;
-		int			segnum_start,
-					segnum_end;
-
-		v = _mdfd_getseg(reln, forknum, blocknum, true /* not used */ ,
-						 EXTENSION_DONT_OPEN);
-
-		/*
-		 * We might be flushing buffers of already removed relations, that's
-		 * ok, just ignore that case.  If the segment file wasn't open already
-		 * (ie from a recent mdwrite()), then we don't want to re-open it, to
-		 * avoid a race with PROCSIGNAL_BARRIER_SMGRRELEASE that might leave
-		 * us with a descriptor to a file that is about to be unlinked.
-		 */
-		if (!v)
-			return;
-
-		/* compute offset inside the current segment */
-		segnum_start = blocknum / RELSEG_SIZE;
-
-		/* compute number of desired writes within the current segment */
-		segnum_end = (blocknum + nblocks - 1) / RELSEG_SIZE;
-		if (segnum_start != segnum_end)
-			nflush = RELSEG_SIZE - (blocknum % ((BlockNumber) RELSEG_SIZE));
-
-		Assert(nflush >= 1);
-		Assert(nflush <= nblocks);
-
-		seekpos = (off_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
-
-		FileWriteback(v->mdfd_vfd, seekpos, (off_t) BLCKSZ * nflush, WAIT_EVENT_DATA_FILE_FLUSH);
-
-		nblocks -= nflush;
-		blocknum += nflush;
-	}
-}
-
 /*
  * mdread() -- Read the specified block from a relation.
  */
@@ -923,6 +866,63 @@ mdwrite(SMgrRelation reln, ForkNumber forknum, BlockNumber blocknum,
 		register_dirty_segment(reln, forknum, v);
 }
 
+/*
+ * mdwriteback() -- Tell the kernel to write pages back to storage.
+ *
+ * This accepts a range of blocks because flushing several pages at once is
+ * considerably more efficient than doing so individually.
+ */
+void
+mdwriteback(SMgrRelation reln, ForkNumber forknum,
+			BlockNumber blocknum, BlockNumber nblocks)
+{
+	Assert((io_direct_flags & IO_DIRECT_DATA) == 0);
+
+	/*
+	 * Issue flush requests in as few requests as possible; have to split at
+	 * segment boundaries though, since those are actually separate files.
+	 */
+	while (nblocks > 0)
+	{
+		BlockNumber nflush = nblocks;
+		off_t		seekpos;
+		MdfdVec    *v;
+		int			segnum_start,
+					segnum_end;
+
+		v = _mdfd_getseg(reln, forknum, blocknum, true /* not used */ ,
+						 EXTENSION_DONT_OPEN);
+
+		/*
+		 * We might be flushing buffers of already removed relations, that's
+		 * ok, just ignore that case.  If the segment file wasn't open already
+		 * (ie from a recent mdwrite()), then we don't want to re-open it, to
+		 * avoid a race with PROCSIGNAL_BARRIER_SMGRRELEASE that might leave
+		 * us with a descriptor to a file that is about to be unlinked.
+		 */
+		if (!v)
+			return;
+
+		/* compute offset inside the current segment */
+		segnum_start = blocknum / RELSEG_SIZE;
+
+		/* compute number of desired writes within the current segment */
+		segnum_end = (blocknum + nblocks - 1) / RELSEG_SIZE;
+		if (segnum_start != segnum_end)
+			nflush = RELSEG_SIZE - (blocknum % ((BlockNumber) RELSEG_SIZE));
+
+		Assert(nflush >= 1);
+		Assert(nflush <= nblocks);
+
+		seekpos = (off_t) BLCKSZ * (blocknum % ((BlockNumber) RELSEG_SIZE));
+
+		FileWriteback(v->mdfd_vfd, seekpos, (off_t) BLCKSZ * nflush, WAIT_EVENT_DATA_FILE_FLUSH);
+
+		nblocks -= nflush;
+		blocknum += nflush;
+	}
+}
+
 /*
  * mdnblocks() -- Get the number of blocks stored in a relation.
  *