linux-ext4 - Re: [RFC][PATCH 3/8 v2] ext4: initialize extent status tree

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Message-ID: <20120926080047.GA15618@gmail.com>
Date:	Wed, 26 Sep 2012 16:00:47 +0800
From:	Zheng Liu <gnehzuil.liu@...il.com>
To:	Theodore Ts'o <tytso@....edu>
Cc:	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Yongqiang Yang <xiaoqiangnk@...il.com>,
	Allison Henderson <achender@...ux.vnet.ibm.com>,
	Zheng Liu <wenqing.lz@...bao.com>
Subject: Re: [RFC][PATCH 3/8 v2] ext4: initialize extent status tree

On Tue, Sep 25, 2012 at 04:59:21PM -0400, Theodore Ts'o wrote:
> On Tue, Sep 25, 2012 at 08:42:52PM +0800, Zheng Liu wrote:
> > > If so, we might want to think about adding a sanity check to make sure
> > > that by the time we are done with the inode in ext4_evict_inode()
> > > (after we have forced writeback), the ext4_es_tree is empty.  Agreed?
> > 
> > Today I revise this patch again, and I find extent_status_tree is freed
> > in ext4_clear_inode().  So maybe I don't think that we need to check
> > this tree to be freed in ext4_evict_inode().  This change is in this
> > patch '[RFC][PATCH 4/8 v2] ext4: let ext4 maintain extent status tree'.
> > What's your opinion?
> 
> When you say "revise this patch again", does that mean that you would
> like to submit a new set of patch series with changes?  Or just that
> you are looking at this patch set again?
> 
> It's certainly true that ext4_evict_inode() will call
> ext4_clear_inode(), so it's not a question of worrying about a memory
> leak.  I was thinking more about doing this as a cheap sanity check
> for the data structure.  By the time we call ext4_evict_inode(), the
> mm layer all writeback should be complete.  Hence, all of the entries
> to the tree _should_ have been removed by the time we call
> ext4_evict_inode().
> 
> I don't know if this is going to change as you start using this data
> structure for other purposes (such as locking a range of pages), but
> if I understand how things are currently working, it _should_ be the
> case that when ext4_evict_inode() calls ext4_clear_inode(), the call
> to ext4_es_remove_extent() should be a no-op, since all of the nodes
> in the extent status tree should have been released by then.  If it
> isn't, then either I'm not understanding the code, or there's a bug in
> the code.

Hi Ted,

When I try to add a BUG_ON in ext4_evict_inode() to ensure that extent
status tree is empty, I will get an error, which reports that the tree
is not empty.  I find that there still has some dirty pages when we
call ext4_evict_inode() because vfs doesn't write all dirty pages back.
In iput_final(), we firstly call ext4_drop_inode().  If the return value
is true, we won't call write_inode_now() to do a writeback.  It means
that we write some data to a file and remove it immediately, and then
all dirty pages that aren't during writeback progress won't be written.
These pages will be truncated in ext4_evict_inode().  Thus, the extent
status tree is possible not to be empty when we call ext4_evict_inode().
So we cannot add a sanity check in this function.  Am I missing something?

Regards,
Zheng
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