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Subject[Patch 6/8] 2.4.20-pre4/ext3: Performance fix for O_SYNC behaviour
Remove the unconditional mark_inode_dirty() which was being done on O_SYNC
inodes, as that is a major performance hit if we are updating files in place
and don't need an inode timestamp update to hit the journal (Linux really
implements O_DATASYNC instead of O_SYNC).

Cater for the data=journal case by forcing a commit after the write if needed;
also do this for IS_SYNC() case. (Simply setting the inode dirty before the
write is not sufficient in the data=journal case --- the write might get split
over multiple transactions and there is no guarantee that later transactions
would get flushed.)

Add a bit of documentation around the ext3_file_write --- the interactions
between the various sync modes and the core VFS (especially
generic_osync_inode) are subtle.

--- linux-ext3-2.4merge/fs/ext3/file.c.=K0007=.orig Tue Aug 27 23:17:59 2002
+++ linux-ext3-2.4merge/fs/ext3/file.c Tue Aug 27 23:19:57 2002
@@ -61,19 +61,52 @@
static ssize_t
ext3_file_write(struct file *file, const char *buf, size_t count, loff_t *ppos)
{
+ int ret, err;
struct inode *inode = file->f_dentry->d_inode;

- /*
- * Nasty: if the file is subject to synchronous writes then we need
- * to force generic_osync_inode() to call ext3_write_inode().
- * We do that by marking the inode dirty. This adds much more
- * computational expense than we need, but we're going to sync
- * anyway.
- */
- if (IS_SYNC(inode) || (file->f_flags & O_SYNC))
- mark_inode_dirty(inode);
+ ret = generic_file_write(file, buf, count, ppos);

- return generic_file_write(file, buf, count, ppos);
+ /* Skip file flushing code if there was an error, or if nothing
+ was written. */
+ if (ret <= 0)
+ return ret;
+
+ /* If the inode is IS_SYNC, or is O_SYNC and we are doing
+ data-journaling, then we need to make sure that we force the
+ transaction to disk to keep all metadata uptodate
+ synchronously. */
+
+ if (file->f_flags & O_SYNC) {
+ /* If we are non-data-journaled, then the dirty data has
+ already been flushed to backing store by
+ generic_osync_inode, and the inode has been flushed
+ too if there have been any modifications other than
+ mere timestamp updates.
+
+ Open question --- do we care about flushing
+ timestamps too if the inode is IS_SYNC? */
+ if (!ext3_should_journal_data(inode))
+ return ret;
+
+ goto force_commit;
+ }
+
+ /* So we know that there has been no forced data flush. If the
+ inode is marked IS_SYNC, we need to force one ourselves. */
+ if (!IS_SYNC(inode))
+ return ret;
+
+ /* Open question #2 --- should we force data to disk here too?
+ If we don't, the only impact is that data=writeback
+ filesystems won't flush data to disk automatically on
+ IS_SYNC, only metadata (but historically, that is what ext2
+ has done.) */
+
+force_commit:
+ err = ext3_force_commit(inode->i_sb);
+ if (err)
+ return err;
+ return ret;
}

struct file_operations ext3_file_operations = {
-
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