Re: fsync on ext[34] working only by an accident

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

On Thu, Sep 10, 2009 at 09:10:07AM -0400, Theodore Tso wrote:
> On Thu, Sep 10, 2009 at 04:34:55PM +0530, Aneesh Kumar K.V wrote:
> > mark_buffer_dirty -> __set_page_dirty -> __mark_inode_dirty
> 
> We need to be careful here.  First of all, mark_buffer_dirty() on the
> code paths you are talking about is being passed a metadata buffer
> head.  As such, has Jan has pointed out, the bh is part of the buffer
> cache, so the page->mapping of associated with bh->b_page is the inode
> of the block device --- *not* the ext4 inode.
> 
> Secondly, __set_page_dirty calls __mark_inode_dirty passing in
> I_DIRTY_PAGES --- which should be a hint.  What Jan is talking about
> is where we set the inode flags I_DIRTY_SYNC and I_DIRTY_DATASYNC:
> 
>  * I_DIRTY_SYNC		Inode is dirty, but doesn't have to be written on
>  *			fdatasync().  i_atime is the usual cause.
>  * I_DIRTY_DATASYNC	Data-related inode changes pending. We keep track of
>  *			these changes separately from I_DIRTY_SYNC so that we
>  *			don't have to write inode on fdatasync() when only
>  *			mtime has changed in it.
> 
> This is important because ext4_sync_file() (which is called by fsync()
> and fdatasync()) uses this logic to determine whether or not to call
> sync_inode(), which is what will force a commit when wbc.sync_mode is
> set to WB_SYNC_ALL.
> 
> In fact, I think the problem is worse than Jan is pointing out,
> because it's not enough that vfs_fq_alloc_space() is calling
> mark_inode_dirty(), since that only sets I_DIRTY_SYNC.  When we touch
> i_size or i_block[], we need to make sure that I_DIRTY_DATASYNC is
> set, so that fdatasync() will force a commit.
> 

That explained it pretty nicely. Thanks

-aneesh