linux-ext4 - Re: fsync on ext[34] working only by an accident

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Message-ID: <20090910064605.GA8690@skywalker.linux.vnet.ibm.com>
Date:	Thu, 10 Sep 2009 12:16:05 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	Jan Kara <jack@...e.cz>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: fsync on ext[34] working only by an accident

On Tue, Sep 08, 2009 at 03:26:01PM +0200, Jan Kara wrote:
>   Hi,
> 
>   When looking at how ext3/4 handles fsync, I've realized I don't
> understand how writing out inode on fsync can work. The problem is that
> ext3/4 mostly calls ext?_mark_inode_dirty() which actually does *not* dirty
> the inode. It just copies the in-memory inode content to disk buffer.
> So in particular the inode looks clean to VFS and our check in
> ext?_sync_file() shouldn't trigger.
>   The only obvious case when we call mark_inode_dirty() is from write_end
> functions when we update i_size but that's clearly not enough. Now I did
> some research why things seem to be actually working. The trick is that
> when allocating block, we call vfs_dq_alloc_block() which calls
> mark_inode_dirty(). But that's all what's keeping our fsync / writeout
> logic from breaking!

ext4_handle_dirty_metadata should do mark_inode_dirty right ?
__ext4_handle_dirty_metadata -> mark_buffer_dirty ->__set_page_dirty
-> __mark_inode_dirty ->  list_move(&inode->i_list, &sb->s_dirty);



>   There are even some cases when the logic actually is broken (I've tested
> it and it really does not work) - for example when you create an empty
> file, the inode won't get written when you fsync it.
>   So what we should IMHO do is to convert all ext?_mark_inode_dirty()
> calls to simple mark_inode_dirty() (or even maybe introduce and use
> mark_inode_dirty_datasync() where appropriate). It will cost us some more
> CPU and stack space but if we optimize ext3_dirty_inode() for the case
> where handle is already started, it shouldn't be too bad.


-aneesh
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