From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Thu, 29 Jul 2010 19:23:58 +0000 (+0000)
Subject: Fix another longstanding problem in copy_relation_data: it was blithely
X-Git-Tag: REL8_1_22~24
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=9e468f94aa30cef7c188a2b8b0d26f26130d44c0;p=postgresql

Fix another longstanding problem in copy_relation_data: it was blithely
assuming that a local char[] array would be aligned on at least a word
boundary.  There are architectures on which that is pretty much guaranteed to
NOT be the case ... and those arches also don't like non-aligned memory
accesses, meaning that log_newpage() would crash if it ever got invoked.
Even on Intel-ish machines there's a potential for a large performance penalty
from doing I/O to an inadequately aligned buffer.  So palloc it instead.

Backpatch to 8.0 --- 7.4 doesn't have this code.
---

diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index 8f8d28e825..3d2e385b08 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/tablecmds.c,v 1.174.2.11 2010/07/29 16:15:33 rhaas Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/tablecmds.c,v 1.174.2.12 2010/07/29 19:23:58 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -5837,11 +5837,11 @@ static void
 copy_relation_data(Relation rel, SMgrRelation dst)
 {
 	SMgrRelation src;
+	char	   *buf;
+	Page		page;
 	bool		use_wal;
 	BlockNumber nblocks;
 	BlockNumber blkno;
-	char		buf[BLCKSZ];
-	Page		page = (Page) buf;
 
 	/*
 	 * Since we copy the file directly without looking at the shared buffers,
@@ -5851,6 +5851,15 @@ copy_relation_data(Relation rel, SMgrRelation dst)
 	 */
 	FlushRelationBuffers(rel);
 
+	/*
+	 * palloc the buffer so that it's MAXALIGN'd.  If it were just a local
+	 * char[] array, the compiler might align it on any byte boundary, which
+	 * can seriously hurt transfer speed to and from the kernel; not to
+	 * mention possibly making PageSetLSN fail.
+	 */
+	buf = (char *) palloc(BLCKSZ);
+	page = (Page) buf;
+
 	/*
 	 * We need to log the copied data in WAL iff WAL archiving is enabled AND
 	 * it's not a temp rel.
@@ -5914,6 +5923,8 @@ copy_relation_data(Relation rel, SMgrRelation dst)
 		smgrwrite(dst, blkno, buf, true);
 	}
 
+	pfree(buf);
+
 	/*
 	 * If the rel isn't temp, we must fsync it down to disk before it's safe
 	 * to commit the transaction.  (For a temp rel we don't care since the rel