Re: Delayed allocation and page_lock vs transaction start ordering

[Mingming Cao <cmm@us.ibm.com>]

On Wed, 2008-04-16 at 12:35 +0200, Jan Kara wrote:
> On Tue 15-04-08 16:33:17, Mingming Cao wrote:
> > On Tue, 2008-04-15 at 16:28 -0700, Mingming Cao wrote:
> > > On Tue, 2008-04-15 at 11:08 -0700, Mingming Cao wrote:
> > > > On Tue, 2008-04-15 at 18:14 +0200, Jan Kara wrote:
> > > > >   Hi,
> > > > > 
> > > > >   I've ported my patch inversing locking ordering of page_lock and
> > > > > transaction start to ext4 (on top of ext4 patch queue). Everything except
> > > > > delayed allocation is converted (the patch is below for interested
> > > > > readers). The question is how to proceed with delayed allocation. Its
> > > > > current implementation in VFS is designed to work well with the old
> > > > > ordering (page lock first, then start a transaction). We could bend it to
> > > > > work with the new locking ordering but I really see no point since ext4 is
> > > > > the only user. 
> > > > 
> > > > I think the plan is port the changes to ext2/3/JFS and support delayed
> > > > allocation on those filesystems. 
> > > > 
> > > > > Also XFS has AFAIK ordering first start transaction, then
> > > > > lock pages so if we should ever merge delayed alloc implementations the new
> > > > > ordering would make it easier.
> > > > >   So what do people think here? Do you agree with reimplementing current
> > > > > mpage_da_... functions?
> > > > 
> > > > It worth a try, but I could not see how to bend delayed allocation to
> > > > work the new ordering:( With delayed allocation Ext4 gets into
> > > > writepage() directly with page locked, but we need to start transaction
> > > > to do block allocation...:(
> > > 
> > > Looked again it seems possible to reservse the order with delayed
> > > allocation. with ext3_da_writepgaes() we could start the journal before
> > > calling mpage_da_writepages()(which will lock the pages), instead of
> > > start the journal inside ext4_da_get_block_write(). So that we could get
> > > the locking order right. Just need to taking care of the estimated
> > > credits right.
> > > 
> > > How about this? (untested, just throw out for comment)
> > 
> > Seems sent out an old version, this version compiles
>   Thanks for the patch. Some comments are below.
> 
> > ---
> >  fs/ext4/inode.c |   53 ++++++++++++++++++++++++++++++++++++++++-------------
> >  1 file changed, 40 insertions(+), 13 deletions(-)
> > 
> > Index: linux-2.6.25-rc9/fs/ext4/inode.c
> > ===================================================================
> > --- linux-2.6.25-rc9.orig/fs/ext4/inode.c	2008-04-15 15:40:33.000000000 -0700
> > +++ linux-2.6.25-rc9/fs/ext4/inode.c	2008-04-15 16:32:10.000000000 -0700
> > @@ -1437,18 +1437,12 @@ static int ext4_da_get_block_prep(struct
> >  static int ext4_da_get_block_write(struct inode *inode, sector_t iblock,
> >  				   struct buffer_head *bh_result, int create)
> >  {
> > -	int ret, needed_blocks = ext4_writepage_trans_blocks(inode);
> > +	int ret;
> >  	unsigned max_blocks = bh_result->b_size >> inode->i_blkbits;
> >  	loff_t disksize = EXT4_I(inode)->i_disksize;
> >  	handle_t *handle = NULL;
> >  
> > -	if (create) {
> > -		handle = ext4_journal_start(inode, needed_blocks);
> > -		if (IS_ERR(handle)) {
> > -			ret = PTR_ERR(handle);
> > -			goto out;
> > -		}
> > -	}
> > +	handle = ext4_journal_current_handle();
>   Maybe we could assert that handle != NULL? When using delayed allocation,
> a transaction should always be started.
> 
Agreed.

> >  	ret = ext4_get_blocks_wrap(handle, inode, iblock, max_blocks,
> >  				   bh_result, create, 0);
> > @@ -1483,17 +1477,51 @@ static int ext4_da_get_block_write(struc
> >  		ret = 0;
> >  	}
> >  
> > -out:
> > -	if (handle && !IS_ERR(handle))
> > -		ext4_journal_stop(handle);
> > -
> >  	return ret;
> >  }
> >  
> > +/*
> > + * For now just follow the DIO way to estimate the max credits
> > + * needed to write out EXT4_MAX_BUF_BLOCKS pages.
> > + * todo: need to calculate the max credits need for
> > + * extent based files, currently the DIO credits is based on
> > + * indirect-blocks mapping way.
> > + *
> > + * Probably should have a generic way to calculate credits
> > + * for DIO, writepages, and truncate
> > + */
> > +#define EXT4_MAX_BUF_BLOCKS	DIO_MAX_BLOCKS
> > +#define EXT4_MAX_BUF_CREDITS	DIO_CREDITS
> > +
> >  static int ext4_da_writepages(struct address_space *mapping,
> >  				struct writeback_control *wbc)
> >  {
> > -	return mpage_da_writepages(mapping, wbc, ext4_da_get_block_write);
> > +	struct inode *inode = mapping->host;
> > +	handle_t *handle = NULL;
> > +	int needed_blocks;
> > +	int ret;
> > +
> > +	/*
> > +	 * Estimate the worse case needed credits to write out
> > +	 * EXT4_MAX_BUF_BLOCKS pages
> > +	 */
> > +	needed_blocks = EXT4_MAX_BUF_CREDITS;
> > +
> > +	/* start the transaction with credits*/
> > +	handle = ext4_journal_start(inode, needed_blocks);
> > +	if (IS_ERR(handle)) {
> > +		ret = PTR_ERR(handle);
> > +		return ret;
> > +	}
> > +
> > +	/* set the max pages could be write-out at a time */
> > +	wbc->range_end = wbc->range_start +
> > +			EXT4_MAX_BUF_BLOCKS << PAGE_CACHE_SHIFT - 1;
>   I think limiting mpage_da_writepages through nr_to_write is better than
> through range_end. That way you don't count clean pages...
> 

You are right. 

> > +
> > +	ret = mpage_da_writepages(mapping, wbc, ext4_da_get_block_write);
> > +	ext4_journal_stop(handle);
>   But here we can't just stop. We have to write everything original caller
> has asked about (at least in WB_SYNC_ALL mode). But the question is where
> to resume because scanning the whole range again is kind-of excessive and
> prone do livelock with other process dirtying the file via mmap. Maybe if
> we slightly modified write_cache_pages() to always store in writeback_index
> where they finished, we could use this value.

Thanks for pointing this out.
How about this? 
---
 fs/ext4/inode.c     |   70 ++++++++++++++++++++++++++++++++++++++++++----------
 mm/page-writeback.c |    2 -
 2 files changed, 58 insertions(+), 14 deletions(-)

Index: linux-2.6.25-rc9/fs/ext4/inode.c
===================================================================
--- linux-2.6.25-rc9.orig/fs/ext4/inode.c	2008-04-16 09:59:00.000000000 -0700
+++ linux-2.6.25-rc9/fs/ext4/inode.c	2008-04-16 11:23:12.000000000 -0700
@@ -1437,18 +1437,13 @@ static int ext4_da_get_block_prep(struct
 static int ext4_da_get_block_write(struct inode *inode, sector_t iblock,
 				   struct buffer_head *bh_result, int create)
 {
-	int ret, needed_blocks = ext4_writepage_trans_blocks(inode);
+	int ret;
 	unsigned max_blocks = bh_result->b_size >> inode->i_blkbits;
 	loff_t disksize = EXT4_I(inode)->i_disksize;
 	handle_t *handle = NULL;
 
-	if (create) {
-		handle = ext4_journal_start(inode, needed_blocks);
-		if (IS_ERR(handle)) {
-			ret = PTR_ERR(handle);
-			goto out;
-		}
-	}
+	J_ASSERT(handle != NULL || create == 0);
+	handle = ext4_journal_current_handle();
 
 	ret = ext4_get_blocks_wrap(handle, inode, iblock, max_blocks,
 				   bh_result, create, 0);
@@ -1483,17 +1478,66 @@ static int ext4_da_get_block_write(struc
 		ret = 0;
 	}
 
-out:
-	if (handle && !IS_ERR(handle))
-		ext4_journal_stop(handle);
-
 	return ret;
 }
 
+/*
+ * For now just follow the DIO way to estimate the max credits
+ * needed to write out EXT4_MAX_WRITEBACK_PAGES.
+ * todo: need to calculate the max credits need for
+ * extent based files, currently the DIO credits is based on
+ * indirect-blocks mapping way.
+ *
+ * Probably should have a generic way to calculate credits
+ * for DIO, writepages, and truncate
+ */
+#define EXT4_MAX_WRITEBACK_PAGES	DIO_MAX_BLOCKS
+#define EXT4_MAX_WRITEBACK_CREDITS	DIO_CREDITS
+
 static int ext4_da_writepages(struct address_space *mapping,
 				struct writeback_control *wbc)
 {
-	return mpage_da_writepages(mapping, wbc, ext4_da_get_block_write);
+	struct inode *inode = mapping->host;
+	handle_t *handle = NULL;
+	int needed_blocks;
+	int ret = 0;
+	unsigned range_cyclic;
+	long to_write;
+
+	/*
+	 * Estimate the worse case needed credits to write out
+	 * EXT4_MAX_BUF_BLOCKS pages
+	 */
+	needed_blocks = EXT4_MAX_WRITEBACK_CREDITS;
+
+	to_write = wbc->nr_to_write;
+	range_cyclic = wbc->range_cyclic;
+	wbc->range_cyclic = 1;
+
+	while (!ret && to_write) {
+		/* start a new transaction*/
+		handle = ext4_journal_start(inode, needed_blocks);
+		if (IS_ERR(handle)) {
+			ret = PTR_ERR(handle);
+			goto out_writepages;
+		}
+		/*
+		 * set the max dirty pages could be write at a time
+		 * to fit into the reserved transaction credits
+		 */
+		if (wbc->nr_to_write > EXT4_MAX_WRITEBACK_PAGES)
+			wbc->nr_to_write = EXT4_MAX_WRITEBACK_PAGES;
+		to_write -= wbc->nr_to_write;
+
+		ret = mpage_da_writepages(mapping, wbc, ext4_da_get_block_write);
+		ext4_journal_stop(handle);
+		to_write +=wbc->nr_to_write;
+	}
+
+out_writepages:
+	wbc->nr_to_write = to_write;
+	wbc->range_cyclic = range_cyclic;
+	return ret;
 }
 
 static int ext4_da_write_begin(struct file *file, struct address_space *mapping,
Index: linux-2.6.25-rc9/mm/page-writeback.c
===================================================================
--- linux-2.6.25-rc9.orig/mm/page-writeback.c	2008-04-16 11:00:20.000000000 -0700
+++ linux-2.6.25-rc9/mm/page-writeback.c	2008-04-16 11:07:59.000000000 -0700
@@ -816,7 +816,7 @@ int write_cache_pages(struct address_spa
 	pagevec_init(&pvec, 0);
 	if (wbc->range_cyclic) {
 		index = mapping->writeback_index; /* Start from prev offset */
-		end = -1;
+		end = wbc->range_end >> PAGE_CACHE_SHIFT;
 	} else {
 		index = wbc->range_start >> PAGE_CACHE_SHIFT;
 		end = wbc->range_end >> PAGE_CACHE_SHIFT;