Re: 2.6.5-rc2-aa5

[Andrea Arcangeli <andrea@suse.de>]

On Mon, Mar 29, 2004 at 12:48:03PM -0800, Andrew Morton wrote:
> Andrea Arcangeli <andrea@suse.de> wrote:
> >
> > Notably there is a BUG_ON(page->mapping) triggering in
> > page_remove_rmap in the pagecache case. that could be ex-pagecache being
> > removed from pagecache before all ptes have been zapped, infact the
> > page_remove_rmap triggers in the vmtruncate path.
> 
> Confused.  vmtruncate zaps the ptes before removing pages from pagecache,
> so I'd expect a non-null ->mapping in page_remove_rmap() is a very common

the bugcheck was for NULL ->mapping in page_remove_rmap:

	BUG_ON(!page->mapping);

I tend to forget the ! in the pseudocode in emails sorry (today I did it
twice, luckily I didn't get it wrong in the actual patches ;).

> thing.  truncate a file which someone has mmapped and it'll happen every
> time, will it not?

as you say vmtruncate zaps the pte _first_, so the page->mapcount should
be down to 0 by the time we set page->mapping = NULL. 

the thing I was wondering about is the controlled race where some page
can go out of pagecache despite still being mapped somewhere, that could
happen in the past IIRC.