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Subject[RFC PATCH] ext3 writepage() journal avoidance
From
Date
Hi,

I am trying to speed up ext3 writepage() by avoiding
journaling in non-block allocation cases. Does this
look reasonable ? So far, my testing is fine. What am
I missing here ?

ext3_ordered_writepage() and ext3_writeback_writepage() starts
a transcation all the time. We need to start a transaction only
if we need to allocate a disk block. For normal writes, disk
block allocation happens in prepare_write(). We can find out
if the disk-block is already mapped, looking at buffers attached
to the page. So, for non-block allocation cases (for non-mapped
writes), we don't need to do all the journal stuff in writepage().

Thanks,
Badari

ext3_ordered_writepage() and ext3_writeback_writepage() starts
a transcation all the time. We need to start a transaction only
if we need to allocate a disk block. For normal writes, disk
block allocation happens in prepare_write(). We can find out
if the disk-block is already mapped, looking at buffers attached
to the page. So, for non-block allocation cases (for non-mapped
writes), we don't need to do all the journal stuff in writepage().

Signed-off-by: Badari Pulavarty <pbadari@us.ibm.com>

Index: linux-2.6.16-rc5/fs/ext3/inode.c
===================================================================
--- linux-2.6.16-rc5.orig/fs/ext3/inode.c 2006-03-09 10:19:41.000000000
-0800
+++ linux-2.6.16-rc5/fs/ext3/inode.c 2006-03-09 10:31:12.000000000 -0800
@@ -1201,6 +1201,11 @@ static int bput_one(handle_t *handle, st
return 0;
}

+static int check_bmap(handle_t *handle, struct buffer_head *bh)
+{
+ return !buffer_mapped(bh);
+}
+
static int journal_dirty_data_fn(handle_t *handle, struct buffer_head
*bh)
{
if (buffer_mapped(bh))
@@ -1268,6 +1273,7 @@ static int ext3_ordered_writepage(struct
handle_t *handle = NULL;
int ret = 0;
int err;
+ int need_trans = 1;

J_ASSERT(PageLocked(page));

@@ -1278,6 +1284,27 @@ static int ext3_ordered_writepage(struct
if (ext3_journal_current_handle())
goto out_fail;

+ if (!page_has_buffers(page)) {
+ create_empty_buffers(page, inode->i_sb->s_blocksize,
+ (1 << BH_Dirty)|(1 << BH_Uptodate));
+ } else {
+ /*
+ * Check to see if buffers are mapped to disk blocks.
+ * If disk blocks are already there, no reason for
+ * starting a transaction.
+ */
+ page_bufs = page_buffers(page);
+ need_trans = walk_page_buffers(handle, page_bufs, 0,
+ PAGE_CACHE_SIZE, NULL, check_bmap);
+ }
+
+ if (need_trans == 0) {
+ /* No need to allocate disk blocks - just do IO */
+ ret = block_write_full_page(page, ext3_get_block, wbc);
+ goto out;
+ }
+
+ /* We may need to allocate blocks - start a transaction etc. */
handle = ext3_journal_start(inode, ext3_writepage_trans_blocks
(inode));

if (IS_ERR(handle)) {
@@ -1285,10 +1312,6 @@ static int ext3_ordered_writepage(struct
goto out_fail;
}

- if (!page_has_buffers(page)) {
- create_empty_buffers(page, inode->i_sb->s_blocksize,
- (1 << BH_Dirty)|(1 << BH_Uptodate));
- }
page_bufs = page_buffers(page);
walk_page_buffers(handle, page_bufs, 0,
PAGE_CACHE_SIZE, NULL, bget_one);
@@ -1318,6 +1341,7 @@ static int ext3_ordered_writepage(struct
err = ext3_journal_stop(handle);
if (!ret)
ret = err;
+out:
return ret;

out_fail:
@@ -1333,14 +1357,26 @@ static int ext3_writeback_writepage(stru
handle_t *handle = NULL;
int ret = 0;
int err;
+ int need_trans = 1;

if (ext3_journal_current_handle())
goto out_fail;

- handle = ext3_journal_start(inode, ext3_writepage_trans_blocks
(inode));
- if (IS_ERR(handle)) {
- ret = PTR_ERR(handle);
- goto out_fail;
+ /*
+ * Check to see if the disk blocking is already available.
+ * If so, there is no need to start a transaction.
+ */
+ if (page_has_buffers(page))
+ need_trans = walk_page_buffers(handle, page_buffers(page), 0,
+ PAGE_CACHE_SIZE, NULL, check_bmap);
+
+ if (need_trans) {
+ handle = ext3_journal_start(inode,
+ ext3_writepage_trans_blocks(inode));
+ if (IS_ERR(handle)) {
+ ret = PTR_ERR(handle);
+ goto out_fail;
+ }
}

if (test_opt(inode->i_sb, NOBH))
@@ -1348,9 +1384,11 @@ static int ext3_writeback_writepage(stru
else
ret = block_write_full_page(page, ext3_get_block, wbc);

- err = ext3_journal_stop(handle);
- if (!ret)
- ret = err;
+ if (need_trans) {
+ err = ext3_journal_stop(handle);
+ if (!ret)
+ ret = err;
+ }
return ret;

out_fail:

-
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