Re: ext2 + -osync: not as easy as it seems

[Jan Kara]

On Wed 14-01-09 12:37:19, Fernando Luis Vázquez Cao wrote:
> (CCing Eric Sandeen)
> 
> On Tue, 2009-01-13 at 15:30 +0100, Jan Kara wrote:
> > On Tue 13-01-09 09:03:47, Theodore Tso wrote:
> > > Adding a barrier shouldn't be that hard; just a matter adding a call
> > > to blkdev_issue_flush() to ext2_sync_file() before it returns.
> >   Yes. Something like the patch below?
> > 
> >   But it's not the whole story. Strictly speaking we should also call
> > blkdev_issue_flush() whenever we write things because of O_SYNC or
> > O_DIRSYNC flags. My patch does also that (it's based on the previous ext2
> > patch I've sent a while before).
> 
> Commit d755fb384250d6bd7fd18a0930e71965acc8e72e added a call to
> blkdev_issue_flush to ext4_sync_file, and looking at its ext3
> counterpart it seems it might be needed there too.
> 
> I may be missing something, but is it possible to ensure the inode hits
> the platter without the patch below?
  Yes, I noticed that yesterday as well. But then I was puzzled why ext4
would need the flush where it has it... sync_inode() has started and
committed a transaction which issued a barrier when the commit was done.
The only reason I could imagine is that barrier (although it is usually
translated to flushing writeback caches) actually means just an ordering
requirement and hence does not necessarily mean that the caches are
properly flushed. Is that so Eric?
  BTW: We should also issue the flush in the fdatasync() case, shouldn't
we?

								Honza

> From: Fernando Luis Vazquez Cao <fernando@xxxxxxxxxxxxx>
> Subject: ext3: call blkdev_issue_flush on fsync
> 
> To ensure that bits are truly on-disk after an fsync, we should call
> blkdev_issue_flush if barriers are supported.
> 
> This is a straight port of a similar patch written by Eric Sandeen for
> ext4 (commit d755fb384250d6bd7fd18a0930e71965acc8e72e).
> 
> Signed-off-by: Fernando Luis Vazquez Cao <fernando@xxxxxxxxxxxxx>
> ---
> 
> diff -urNp linux-2.6.29-rc1-orig/fs/ext3/fsync.c linux-2.6.29-rc1/fs/ext3/fsync.c
> --- linux-2.6.29-rc1-orig/fs/ext3/fsync.c	2008-12-25 08:26:37.000000000 +0900
> +++ linux-2.6.29-rc1/fs/ext3/fsync.c	2009-01-14 11:45:47.000000000 +0900
> @@ -27,6 +27,7 @@
>  #include <linux/sched.h>
>  #include <linux/writeback.h>
>  #include <linux/jbd.h>
> +#include <linux/blkdev.h>
>  #include <linux/ext3_fs.h>
>  #include <linux/ext3_jbd.h>
>  
> @@ -45,6 +46,7 @@
>  int ext3_sync_file(struct file * file, struct dentry *dentry, int datasync)
>  {
>  	struct inode *inode = dentry->d_inode;
> +	journal_t *journal = EXT3_SB(inode->i_sb)->s_journal;
>  	int ret = 0;
>  
>  	J_ASSERT(ext3_journal_current_handle() == NULL);
> @@ -85,6 +87,9 @@ int ext3_sync_file(struct file * file, s
>  			.nr_to_write = 0, /* sys_fsync did this */
>  		};
>  		ret = sync_inode(inode, &wbc);
> +
> +		if (journal && (journal->j_flags & JFS_BARRIER))
> +			blkdev_issue_flush(inode->i_sb->s_bdev, NULL);
>  	}
>  out:
>  	return ret;
> 
> 
-- 
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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