Date:	Thu, 10 Sep 2009 16:06:36 +0200
From:	Jan Kara <jack@...e.cz>
To:	Theodore Tso <tytso@....edu>
Cc:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Jan Kara <jack@...e.cz>, linux-ext4@...r.kernel.org
Subject: Re: fsync on ext[34] working only by an accident

On Thu 10-09-09 09:10:07, 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.
  Yes, this is exactly what I was trying to point out.

> 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.
  Actually no. mark_inode_dirty() dirties inode with I_DIRTY ==
(I_DIRTY_SYNC | I_DIRTY_DATASYNC | I_DIRTY_PAGES) so it happens to work.
The fact that quota *could* dirty the inode with I_DIRTY_SYNC only
is right but that's a separate issue.

> I think the right thing to do is to create an
> _ext[34]_mark_inode_dirty() which takes an extra argument, which
> controls whether or not we set I_DIRTY_SYNC or I_DIRTY_DATASYNC.  In
> fact, most of the time, we want to be setting I_DIRTY_DATASYNC, so we
> should probably have ext[34]_mark_inode_dirty() call
> _ext[34]_mark_inode_dirty() with I_DIRTY_DATASYNC, and then create a
> ext[34]_mark_inode_nodatasync() version passes in I_DIRTY_SYNC.
> 
> This will cause pdflush to call ext4_write_inode() more frequently,
> but pdflush calls write_inode with wait=0, and ext4_write_inode() is a
> no-op in that case.
  Thinking about it, it won't work so easily. The problem is that when
pdflush decides to write the inode, it unconditionally clears dirty flags.
We could redirty the inode in write_inode() but that's IMHO too ugly to
bear it.
  We could use ext[34] internal inode dirty flags to track when transaction
commit is needed on fsync and fdatasync... That would provide the integrity
guarantees. The performance problem with this is that because these flags
won't get cleared on transaction commit, we'd force transaction commit
unnecessarily when it already happened before fsync. So either we can force
commit only if inode buffer is part of the running transaction (but that's
slightly hacky as in fact we want to force a commit whetever some metadata
is part of the running transaction) or we can let inode track TIDs
(transaction ID) which must be committed to return from fsync / fdatasync.
  The last possibility looks the best to me so I'd go for it. I can write a
patch next week...

> BTW, while I was looking into this, I noted that the comments ahead of
> ext[34]_mark_inode_dirty are out of date; they date back to a time
> when when prune_icache actually was responsible for cleaning dirty
> inodes; these days, that honor is owned by fs-writeback.c and
> pdflush.)  Also, the second half of the comments in
> ext4_write_inode(), where they reference mark_inode_dirty() are also
> painfully out of date, and somewhat misleading as a result.
  Yeah, I also couldn't make sence from some of those comments probably
because I lack enough history context ;).

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