Re: ext3 leaking buffer_heads

[Theodore Tso <tytso@mit.edu>]

On Tue, Mar 24, 2009 at 05:43:10PM -0500, Don Porter wrote:
> Thanks for the tip, Eric.
>
> I spent some time tracing through this in a debugger.
>
> I believe the lingering buffer_heads are allocated by  
> journal_write_commit_record()->journal_get_descriptor_buffer()->__getblk(). 
>  The reference count ends up at zero, but the buffer head is never freed.
>
> As best I can tell, this is correct as long as the associated page is  
> cached, so that the associated buffer_head can be looked up and reused  
> later.
>
> I plan to look into this more, but perhaps the issue is just that the  
> kernel should be more aggressive about freeing cached journal pages?   
> They are highly unlikely to be used again once a committed transaction  
> is completely out on disk, as the journal is roughly an append-only log,  
> right?

The journal is a fixed sized, circular log, so the buffer_head will be
reused eventually.  In the case of memory pressure the unreferenced
buffer_head will be freed, but since we never reference the contents
of the journal, we could be more aggressive about freeing the
buffer_head just to avoid pushing out more valuable memory contents
when we start getting put under memory pressure.

					- Ted