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Date:	Fri, 27 Jun 2008 14:44:59 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	Mingming <cmm@...ibm.com>
Cc:	Holger Kiehl <Holger.Kiehl@....de>, Theodore Tso <tytso@....edu>,
	Eric Sandeen <sandeen@...hat.com>, Jan Kara <jack@...e.cz>,
	Solofo.Ramangalahy@...l.net, Nick Dokos <nicholas.dokos@...com>,
	linux-ext4@...r.kernel.org,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: Performance of ext4

On Wed, Jun 25, 2008 at 05:46:39PM -0700, Mingming wrote:
> 
> On Wed, 2008-06-25 at 09:09 +0000, Holger Kiehl wrote:
> > On Tue, 24 Jun 2008, Mingming wrote:
> > 
> > >
> > > On Tue, 2008-06-24 at 21:12 +0000, Holger Kiehl wrote:
> > >> Yes, with this patch applied on top of latest patch queue I no longer
> > >> get truncated files, after running a short test. Tomorrow I will do some
> > >> more thorough testing and use the patch you have send to me in a separate
> > >> mail. The above patch did not apply but it was easy to apply by hand.
> > >
> > >
> > > Thanks for quick response and test. I have updated the patch queue with
> > > above patch merged. Please let me know if you still see apply issue and
> > > file size update issue with current patch queue.
> > >
> > Thanks, it applies without any problems. However I still hit an oops. What
> > I find strange is that I got the oops just as the benchmark is done and
> > all process where shutting down. The same behaviour I reported here:
> > http://www.ussg.iu.edu/hypermail/linux/kernel/0806.2/2113.html
> > Only this time I got just one oops. This is on x86_64 system (4 Opteron CPU's
> > and SW Raid 1+0). I have not seen this on my home system x86 (1 Dual Core
> > and HW Raid). Anyway, here the dmesg output:
> > 
> > kjournald2 starting.  Commit interval 15 seconds
> > EXT4 FS on md7, internal journal
> > EXT4-fs: mounted filesystem with ordered data mode.
> > EXT4-fs: file extents enabled
> > EXT4-fs: mballoc enabled
> > JBD: barrier-based sync failed on md7 - disabling barriers
> > ------------[ cut here ]------------
> > kernel BUG at fs/ext4/inode.c:1667!
> 
> Did not get a chance to look more closely today, but it's point to this
> code in ext4_da_writepage()
> 
>  page_bufs = page_buffers(page);
> 
> and appearently it's BUG_ON at 
>  BUG_ON(!PagePrivate(page)); in page_buffers().
> 
> 

Ok so we are doing the journal_commit and meanwhile shrink_page_list
dropped the buffer. I guess what is happening is


journal_submit_inode_data_buffers
generic_writepages
write_cache_pages
pagevec_lookup_tag(..PAGECACHE_TAG_DIRTY,..)
foreach(page)
							shrink_page_list
							lock_page
							ext4_releasepage
							try_to_free_buffers
							drop_buffers
							cancel_dirty_page
							unlock_page
lock_page()
BUG_ON(!PagePrivate(page));


How about the below ? I am not sure journalled mode writepage would need a
simillar change. We don't need to redirty the page because
that would mean the next journal_commit will hit the BUG_ON.
If we need to do a BUG_ON

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index fd67b34..94e5ab0 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -1662,8 +1662,17 @@ static int ext4_da_writepage(struct page *page,
 		 * writing the page in case we would have to do so.
 		 * We reach here also via journal_submit_inode_data_buffers
 		 */
+		if (!page_has_buffers(page)) {
+			/*
+			 * This can happen when we are called via
+			 * journal_submit_inode_data_buffers and
+			 * shrink_page_list parallely did drop_buffers
+			 * after write_cache_pages did a pagevec_lookup_tag
+			 */
+			unlock_page(page);
+			return 0;
+		}
 		size = i_size_read(inode);
-
 		page_bufs = page_buffers(page);
 		if (page->index == size >> PAGE_CACHE_SHIFT)
 			len = size & ~PAGE_CACHE_MASK;
@@ -2046,7 +2055,16 @@ static int ext4_normal_writepage(struct page *page,
 	loff_t len;
 
 	J_ASSERT(PageLocked(page));
-	J_ASSERT(page_has_buffers(page));
+	if (!page_has_buffers(page)) {
+		/*
+		 * This can happen when we are called via
+		 * journal_submit_inode_data_buffers and
+		 * shrink_page_list parallely did drop_buffers
+		 * after write_cache_pages did a pagevec_lookup_tag
+		 */
+		unlock_page(page);
+		return 0;
+	}
 	if (page->index == size >> PAGE_CACHE_SHIFT)
 		len = size & ~PAGE_CACHE_MASK;
 	else
--
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