Re: smp race fix between invalidate_inode_pages* and do_no_page

[Andrea Arcangeli <andrea@suse.de>]

On Wed, Jan 11, 2006 at 01:06:38AM -0800, Andrew Morton wrote:
> Andrea Arcangeli <andrea@suse.de> wrote:
> >
> >  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?
> 
> Confused.  do_no_page() doesn't have a page to lock until it has called
> ->nopage.

yes, I mean doing lock_page after ->nopage returned it here:


	lock_page(page);
	if (mapping && !page->mapping)
		goto bail_out;
	page_table = pte_offset_map_lock(mm, pmd, address, &ptl);
[..]
			page_add_file_rmap()
			unlock_page()

That should be enough no? Imagine the truncate side implemented exactly
like invalidate_inode_pages2:

	lock_page(page)
	if (page_mapped(page))	
		unmap_mapping_pages()
	truncate_full_page(page)
	unlock_page(page)

Either the pte is dropped by unmap_mapping_pages and we're safe, or
->nopage returns an already truncated page and page->mapping is null and
we bail out.
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