Re: Bufferheads & page-cache reference

[Andrew Morton <akpm@osdl.org>]

Badari Pulavarty <pbadari@us.ibm.com> wrote:
>
> 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 ?

Seems about right.  There's also the buffer_heads_over_limit logic in
mm/vmscan.c and fs/buffer.c.  That logic has a hole in that it requires
that there be a highmem shortage before we start to reclaim the lowmem
buffer_heads, but it is somewhat helpful.