RE: RE: Re: [PATCH] ext4: change sequential discard handling on commit complete phase into parallel manner

[Daeho Jeong <daeho.jeong@samsung.com>]

Hi Jan,

> Hi Jan,

 

> > Hum, these games with several callbacks, lists, etc. look awkward and

> > unnecessary. It think they mostly come from the fact that we call separate

> > freeing callback for each extent to free which doesn't fit the needs of

> > async discard well.

 

> > So instead of adding post_cb_list and several callback functions, it would

> > seem easier to have just one callback structure instead of one for every

> > extent. Then the structure would contain a list of extents that need to be

> > freed freed. So something like:

 

> > struct ext4_free_data {

> >         struct ext4_journal_cb_entry efd_jce;

> >         struct list_head efd_extents;

> > }

 

> > struct ext4_freed_extent {

> >         struct list_head efe_list;

> >         struct rb_node efe_node;

> >         ext4_group_t efe_group;

> >         ext4_grpblk_t efe_start_cluster;

> >         ext4_grpblk_t efe_count;

> >         tid_t efe_tid;

> > }

 

> > When commit happens, we can just walk the efd_extents list while efe_tid is

> > equal tid of the transaction for which the callback was called and submit all

> > discard requests. You can use bio chaining implemented in

> > __blkdev_issue_discard() which XFS already uses and so the result of all

> > the discards you submit will be just one bio. Then you walk the list of

> > extents again and free them in the buddy bitmaps. And finally, you wait for

> > the bio to complete. All will be then happening in one function and it will

> > be much easier to understand.

 

> It's right. the patch didn't look neat because of a few callbacks and the

> post callback list. I will modify the patch as your suggestion. It will

> look better.

 

> Thank you very much. :-)



It's a little difficult to decide when we have to add new ext4_free_data entry for

a transaction for the first time and how do we know whether the ext4_free_data entry

for a transaction is already added or not? I think that it is a bad idea to search in

t_private_list of the transaction for that, because there might be the different

type of callback entries in the future.



And how do we find the exact ext4_free_data entry for a newly created ext4_freed_extent?

We only know which transcation is related to the ext4_freed_extent, so we could use this

but I don't have any good idea for that. 



Do you have any idea?



Thank you.