Re: [Ext2-devel] Bufferheads & page-cache reference

[Sonny Rao <sonny@burdell.org>]

On Mon, Feb 14, 2005 at 11:30:16AM -0800, Badari Pulavarty wrote:
> Hi,,
> 
> I am trying to understand interactions between filesystem pagecache
> pages & bufferhead associated with them. 
> 
> I was wondering if someone could help me clarify this..
> 
> I see that as part of bufferheads to page association, we get a
> ref. on the page.
> 
>   create_empty_buffers() -> attach_page_buffers() -> page_cache_get()
> 
> I also see that this reference get dropped by ..
> 
>   shrink_list() -> try_to_release_page() ->
>         try_to_free_buffers() -> drop_buffers() ->
>                  __clear_page_buffers()-> page_cache_release();
> 
> So, it looks like we drop the reference on the page and disassociate
> bufferheads from the page when VM wants to re-use the page. Only other
> path, I see this can happen is through invalidate_mapping_pages().
> Is this true ?
> 
> If I do fsync(), we flush the data - still leave the page & bufferhead
> association. If I see lots of bufferheads even after fsync() is normal.
> Correct ?

Also Badari, a minor addition in the same vein, if a_ops->releasepage
gets called journal_try_to_free_buffers() will be called by
ext3_releasepage, which may actually release the buffers before
try_to_free_buffers() does.  I believe this should happen in the
shrink_list->try_to_release_page case.

I can't find any other place where the buffers would be released after
a normal ext3 write.

Here's a list of the callers of try_to_free_buffers:


release_buffer_page: 		fs/jbd/commit.c:72
journal_try_to_free_bufers:	fs/jbd/transaction.c:1633
mpage_writepage:		fs/mpage.c:563
try_to_release_page:		fs/buffer.c:1584
grow_dev_page:			fs/buffer.c:1137

XFS only
linvfs_release_page:		fs/xfs/linux-2.6/xfs_aops.c:1250

I looked at all of these and callers to "free_buffer_head" but didn't
see any obvious freeing until we release the whole page under memory
pressure, etc.

Sonny