Re: [RFC][PATCH 1/3] radix priority search tree - objrmapcomplexity fix

[Andrew Morton]

Andrea Arcangeli <andrea@xxxxxxx> wrote:
>
> > @@ -149,8 +149,14 @@ int rw_swap_page_sync(int rw, swp_entry_
> >  	};
> >  
> >  	lock_page(page);
> > -
> > +	/*
> > +	 * This library call can be only used to do I/O
> > +	 * on _private_ pages just allocated with alloc_pages().
> > +	 */
> >  	BUG_ON(page->mapping);
> > +	BUG_ON(PageSwapCache(page));
> > +	BUG_ON(PageAnon(page));
> > +	BUG_ON(PageLRU(page));
> >  	ret = add_to_page_cache(page, &swapper_space, entry.val, GFP_KERNEL);
> >  	if (unlikely(ret)) {
> >  		unlock_page(page);
> 
> 
> the good thing is that I believe this fix will make it work with the -mm
> writeback changes. However this fix now collides with anon-vma since
> swapsuspend passes compound pages to rw_swap_page_sync and
> add_to_page_cache overwrites page->private and the kernel crashes at the
> next page_cache_get() since page->private is now the swap entry and not
> a page_t pointer.

Why do swapcache pages have their ->index in ->private?  That should have
been commented.

(hugetlb pages are also added to pagecache, and they are compound, but the
code looks OK).

> So I guess I've a good reason now to giveup trying to
> add the page to the swapcache, and to just fake the radix tree like I
> did in my original fix. That way the page won't be swapcache either so I
> don't even need to use get_page to avoid remove_exclusive_swap_page to
> mess with it.

The BUG_ON in radix_tree_tag_set() is a fairly arbitrary sanity check:
"hey, why are you tagging a non-existent item?".

We could simply replace it with a `return NULL;'?
-
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