Re: [PATCH 04/10 v5] ext4: track all extent status in extent status tree

[Zheng Liu <gnehzuil.liu@gmail.com>]

On Mon, Feb 11, 2013 at 01:21:27PM +0100, Jan Kara wrote:
> On Fri 08-02-13 16:44:00, Zheng Liu wrote:
> > From: Zheng Liu <wenqing.lz@taobao.com>
> > 
> > By recording the phycisal block and status, extent status tree is able
> > to track the status of every extents.  When we call _map_blocks
> > functions to lookup an extent or create a new written/unwritten/delayed
> > extent, this extent will be inserted into extent status tree.  The hole
> > extent is inserted in ext4_ext_put_gap_in_cache().  If there is no any
> > extent, we will not insert a hole extent [0, ~0] into the extent status
> > tree in order to reduce the complextiy of code.
> > 
> > We don't load all extents from disk in alloc_inode() because it costs
> > too much memory, and if a file is opened and closed frequently it will
> > takes too much time to load all extent information.  So currently when
> > we create/lookup an extent, this extent will be inserted into extent
> > status tree.  Hence, the extent status tree may not comprehensively
> > contain all of the extents found in the file.
> > 
> > Signed-off-by: Zheng Liu <wenqing.lz@taobao.com>
> > Cc: "Theodore Ts'o" <tytso@mit.edu>
> > Cc: Jan kara <jack@suse.cz>
> > ---
> >  fs/ext4/extents.c           |  4 +--
> >  fs/ext4/extents_status.c    | 27 ++++++++++++------
> >  fs/ext4/extents_status.h    |  4 +--
> >  fs/ext4/file.c              |  4 +--
> >  fs/ext4/inode.c             | 68 ++++++++++++++++++++++++++++-----------------
> >  include/trace/events/ext4.h |  4 +--
> >  6 files changed, 70 insertions(+), 41 deletions(-)
> > 
> ...
> > diff --git a/fs/ext4/extents_status.c b/fs/ext4/extents_status.c
> > index 5093cee..71cb75a 100644
> > --- a/fs/ext4/extents_status.c
> > +++ b/fs/ext4/extents_status.c
> > @@ -239,14 +239,15 @@ static struct extent_status *__es_tree_search(struct rb_root *root,
> >   * EXT_MAX_BLOCKS if no extent is found.
> >   * Delayed extent is returned via @es.
> >   */
> > -ext4_lblk_t ext4_es_find_extent(struct inode *inode, struct extent_status *es)
> > +ext4_lblk_t ext4_es_find_delayed_extent(struct inode *inode,
> > +					struct extent_status *es)
> >  {
>   I have to say I'm still not very happy about this function (but it's much
> better than it used to be so thanks for that!). I have two suggestions for
> improvement:
> 1) 'es' is both input and output argument where for input only es_lblk is
> used. That's a bit confusing so how about making the function like:
> 
> ext4_es_find_delayed_extent(struct inode *inode, ext4_lblk_t offset,
> 			    struct extent_status *out);
> 
>   to separate input and output? Also you can comment that we use the 'out'
> parameter instead of returning the extent_status from the tree because that
> can be freed once we drop the spinlock protecting status tree.
> 
> 2) The returned value is somewhat surprisingly the logical offset of the
> *next* delalloc extent. It's used only in ext4_fill_fiemap_extents()
> AFAICS. It would be easier to understand if the function didn't return
> anything. ext4_fill_fiemap_extents() would use
> ext4_es_find_delayed_extent() to find both current and next delalloc extent
> (which would become the 'current' one in the next iteration). As a bonus
> you would also save some iteration of the extent status tree...

I have seen that Ted has sent out two patches that split this patch.  I
will pick them up and polish them according to your comments.  But
before this, I need to fix the problem that triggers a failure of
xfstests #13 with bigalloc.

Thanks,
                                                - Zheng