Re: [PATCH -v2] rmap: make anon_vma_prepare link in all the anon_vmasof a mergeable VMA

[Linus Torvalds]

On Mon, 12 Apr 2010, Rik van Riel wrote:

> On 04/12/2010 12:01 PM, Peter Zijlstra wrote:
> 
> > @@ -864,15 +889,8 @@ void page_remove_rmap(struct page *page)
> >   		__dec_zone_page_state(page, NR_FILE_MAPPED);
> >   		mem_cgroup_update_file_mapped(page, -1);
> >   	}
> > -	/*
> > -	 * It would be tidy to reset the PageAnon mapping here,
> > -	 * but that might overwrite a racing page_add_anon_rmap
> > -	 * which increments mapcount after us but sets mapping
> > -	 * before us: so leave the reset to free_hot_cold_page,
> > -	 * and remember that it's only reliable while mapped.
> > -	 * Leaving it set also helps swapoff to reinstate ptes
> > -	 * faster for those pages still in swapcache.
> > -	 */
> > +
> > +	page->mapping = NULL;
> >   }
> 
> That would be a bug for file pages :)
> 
> I could see how it could work for anonymous memory, though.

I think it's scary for anonymous pages too. The _common_ case of 
page_remove_rmap() is from unmap/exit, which holds no locks on the page 
what-so-ever. So assuming the page could be reachable some other way (swap 
cache etc), I think the above is pretty scary. 

Also do note that the bug we've been chasing has _always_ had that test 
for "page_mapped(page)". See my other email about why the unmapped case 
isn't even interesting, because it's so easy to see how page->mapping can 
be stale for unmapped pages.

It's the _mapped_ case that is interesting, not the unmapped one. So 
setting page->mapping to NULL when unmapping is perhaps a nice consistency 
issue ("never have stale pointers"), but it's missing the fact that it's 
not really the case we care about.

			Linus
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