evict truncated buffers more easily.

Performance tweak: when we truncate a file but encounter pages which are still
being journaled from a previous transaction, we can't evict the page from
memory immediately.

This patch just makes it a little easier for the VM to evict the page later on:
the page is marked unreferenced while we're committing the transaction which
pins it, and the commit logic tries to free the page completely once the
transaction has committed.

--- linux-2.4.19-ext3/fs/jbd/commit.c.=K0012=.orig	Fri Oct 11 15:52:00 2002
+++ linux-2.4.19-ext3/fs/jbd/commit.c	Fri Oct 11 15:52:01 2002
@@ -683,13 +683,25 @@
 			JBUFFER_TRACE(jh, "refile for checkpoint writeback");
 			__journal_refile_buffer(jh);
 		} else {
+			struct page *page = bh->b_page;
+			
 			J_ASSERT_BH(bh, !buffer_dirty(bh));
 			J_ASSERT_JH(jh, jh->b_next_transaction == NULL);
 			__journal_unfile_buffer(jh);
 			jh->b_transaction = 0;
 			__journal_remove_journal_head(bh);
-			__brelse(bh);
+
+			if (TryLockPage(page)) {	
+				__brelse(bh);
+			} else {
+				__brelse(bh);
+				page_cache_get(page);
+				try_to_free_buffers(page, 0);
+				unlock_page(page);
+				page_cache_release(page);
+			}
 		}
+		
 		spin_unlock(&journal_datalist_lock);
 	}
 
--- linux-2.4.19-ext3/fs/jbd/transaction.c.=K0012=.orig	Fri Oct 11 15:52:00 2002
+++ linux-2.4.19-ext3/fs/jbd/transaction.c	Fri Oct 11 15:52:01 2002
@@ -1904,8 +1904,29 @@
 	unlock_journal(journal);
 
 	if (!offset) {
-		if (!may_free || !try_to_free_buffers(page, 0))
+		if (!may_free || !try_to_free_buffers(page, 0)) {
+			if (!offset) {
+				/* We are still using the page, but only
+                                   because a transaction is pinning the
+                                   page.  Once it commits, we want to
+                                   encourage the page to be reaped as
+                                   quickly as possible. */
+				ClearPageReferenced(page);
+
+#if 0
+				/* Ugh, this is not exactly portable
+				   between VMs: we need a modular
+				   solution for this some day.. */
+				if (PageActive(page)) {
+					spin_lock(&pagemap_lru_lock);
+					del_page_from_active_list(page);
+					add_page_to_inactive_list(page);
+					spin_unlock(&pagemap_lru_lock);
+				}
+#endif
+			}
 			return 0;
+		}
 		J_ASSERT(page->buffers == NULL);
 	}
 	return 1;