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

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Message-ID: <20090908132601.GA17778@duck.suse.cz>
Date:	Tue, 8 Sep 2009 15:26:01 +0200
From:	Jan Kara <jack@...e.cz>
To:	linux-ext4@...r.kernel.org
Subject: fsync on ext[34] working only by an accident

  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!
  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.

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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