Messages in this thread | | | Subject | Re: ext2 + -osync: not as easy as it seems | From | Fernando Luis Vázquez Cao <> | Date | Fri, 16 Jan 2009 21:31:09 +0900 |
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On Fri, 2009-01-16 at 00:45 +0100, Jan Kara wrote: > On Thu 15-01-09 21:06:51, Fernando Luis Vázquez Cao wrote: > > On Wed, 2009-01-14 at 11:59 -0500, Theodore Tso wrote: > > > On Wed, Jan 14, 2009 at 03:37:56PM +0100, Jan Kara wrote: > > > > > Um, we have that already; the sync_inode() followed by > > > > > blkdev_issue_flush() is the path taken by fdatasync(), I do believe. > > > > > > > > Maybe ext4-patch-queue changes that area but in Linus's tree I see: > > > > > > > > if (datasync && !(inode->i_state & I_DIRTY_DATASYNC)) > > > > goto out; > > > > > > > > So if we just overwrite some data, we send them to disk via fdatawrite() > > > > and then we quickly bail out from ext4_sync_file() without doing > > > > blkdev_issue_flush(). > > > > > > So you're thinking about fdatawrite() being called by some code path > > > other than ext4_sync_file() before we call fsync()? Yeah, that could > > > happen.... I think that will only happen if the file is opened > > > O_SYNC, but that raises another issue, which is that we're not forcing > > > a flush for writes when the file is opened O_SYNC. > > > > Hi Jan, Ted > > > > Is something like the patch below what you had in mind? > > > > -- > > > > From: Fernando Luis Vazquez Cao <fernando@oss.ntt.co.jp> > > Subject: ext3: call blkdev_issue_flush on fsync > > > > To ensure that bits are truly on-disk after an fsync or fdatasync, we > > should call blkdev_issue_flush if barriers are supported. > > > > Signed-off-by: Fernando Luis Vazquez Cao <fernando@oss.ntt.co.jp> > > --- > > > > --- linux-2.6.29-rc1-orig/fs/ext4/fsync.c 2008-12-25 08:26:37.000000000 +0900 > > +++ linux-2.6.29-rc1/fs/ext4/fsync.c 2009-01-15 21:03:19.000000000 +0900 > > @@ -48,6 +48,7 @@ int ext4_sync_file(struct file *file, st > > { > > struct inode *inode = dentry->d_inode; > > journal_t *journal = EXT4_SB(inode->i_sb)->s_journal; > > + unsigned long i_state = inode->i_state; > > int ret = 0; > > > > J_ASSERT(ext4_journal_current_handle() == NULL); > > @@ -79,22 +80,35 @@ int ext4_sync_file(struct file *file, st > > goto out; > > } > > > > - if (datasync && !(inode->i_state & I_DIRTY_DATASYNC)) > > - goto out; > > + if (datasync && !(i_state & I_DIRTY_DATASYNC)) > > + goto flush_blkdev; > > > > /* > > * The VFS has written the file data. If the inode is unaltered > > * then we need not start a commit. > > */ > > - if (inode->i_state & (I_DIRTY_SYNC|I_DIRTY_DATASYNC)) { > > + if (i_state & (I_DIRTY_SYNC|I_DIRTY_DATASYNC)) { > > struct writeback_control wbc = { > > .sync_mode = WB_SYNC_ALL, > > .nr_to_write = 0, /* sys_fsync did this */ > > }; > > ret = sync_inode(inode, &wbc); > > - if (journal && (journal->j_flags & JBD2_BARRIER)) > > - blkdev_issue_flush(inode->i_sb->s_bdev, NULL); > > + /* > > + * When there are no blocks attached to the journal transaction > > + * some optimizations are possible, but if there were dirty > > + * pages sync_inode() should have ensured that all data gets > > + * actually written to disk. Thus, we can skip > > + * blkdev_issue_flush() below. > > + */ > > + if (!(i_state & I_DIRTY_PAGES)) > > + goto flush_blkdev; > Uh. Here I don't get it. When we did sync_inode(), blkdev_issue_flush() > is needed only if the journal does not do barriers. So I'd expect here: > if (!(journal->j_flags & JBD2_BARRIER)) > goto flush_blkdev; > goto out;
Ups, you are right. I somehow managed to mangle the logic that I intended to put here and under flush_blkdev. By the way, I think that the same check may be needed for the data==journal case too.
Thank you for the feedback, Jan.
I'll be replying to this email with new patches for ext2/ext3.
- Fernando
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