Re: smp race fix between invalidate_inode_pages* and do_no_page

[Andrea Arcangeli <andrea@suse.de>]

On Wed, Jan 11, 2006 at 12:51:34AM -0800, Andrew Morton wrote:
> Andrea Arcangeli <andrea@suse.de> wrote:
> >
> >  On Wed, Jan 11, 2006 at 03:08:31PM +1100, Nick Piggin wrote:
> >  > I'd be inclined to think a lock_page is not a big SMP scalability
> >  > problem because the struct page's cacheline(s) will be written to
> >  > several times in the process of refcounting anyway. Such a workload
> >  > would also be running into tree_lock as well.
> > 
> >  I seem to recall you wanted to make the tree_lock a readonly lock for
> >  readers for the exact same scalability reason? do_no_page is quite a
> >  fast path for the tree lock too. But I totally agree the unavoidable is
> >  the atomic_inc though, good point, so it worth more to remove the
> >  tree_lock than to remove the page lock, the tree_lock can be avoided the
> >  atomic_inc on page->_count not.
> > 
> >  The other bonus that makes this attractive is that then we can drop the
> >  *whole* vm_truncate_count mess... vm_truncate_count and
> >  inode->trunate_count exists for the only single reason that do_no_page
> >  must not map into the pte a page that is under truncation.
> 
> I think you'll find this hard - filemap_nopage() is the first to find the
> page but we need lock coverage up in do_no_page().  So the ->nopage
> protocol will need to be changed to "must return with the page locked".  Or
> we add a new ->nopage_locked and call that if the vm_ops implements it.

Can't we avoid to change the protocol and use lock_page in do_no_page
instead? All we need to check before mailing out trying again is that
page->mapping is still there and then we have to set "page_mapping() ==
True" before unlocking (then the other side will have to block the pte
pte_lock running unmap_mapping_pages a second time with the page lock
held).

The main scary thing as far as I can tell, is the blocking lock_page. We
can't just do TryLockPage...

> But I agree it's a good change if we can pull it off.

Ok good thanks!