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:

quoted

quoted

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