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

[Andrea Arcangeli]

On Thu, Apr 01, 2004 at 05:15:34PM +0200, Andrea Arcangeli wrote:
> > diff -urNp --exclude CVS --exclude BitKeeper --exclude {arch} --exclude .arch-ids x-ref/mm/page_io.c x/mm/page_io.c
> > --- x-ref/mm/page_io.c	2004-04-01 17:07:10.231289760 +0200
> > +++ x/mm/page_io.c	2004-04-01 17:07:33.182800600 +0200
> > @@ -139,7 +139,7 @@ struct address_space_operations swap_aop
> >  
> >  /*
> >   * A scruffy utility function to read or write an arbitrary swap page
> > - * and wait on the I/O.
> > + * and wait on the I/O.  The caller must have a ref on the page.
> >   */
> >  int rw_swap_page_sync(int rw, swp_entry_t entry, struct page *page)
> >  {
> > @@ -151,8 +151,16 @@ int rw_swap_page_sync(int rw, swp_entry_
> >  	lock_page(page);
> >  
> >  	BUG_ON(page->mapping);
> > -	page->mapping = &swapper_space;
> > -	page->index = entry.val;
> > +	ret = add_to_page_cache(page, &swapper_space, entry.val, GFP_KERNEL);
> > +	if (unlikely(ret)) {
> > +		unlock_page(page);
> > +		return ret;
> > +	}
> > +	/*
> > +	 * get one more reference to make page non-exclusive so
> > +	 * remove_exclusive_swap_page won't mess with it.
> > +	 */
> > +	page_cache_get(page);
> >  
> >  	if (rw == READ) {
> >  		ret = swap_readpage(NULL, page);
> > @@ -161,7 +169,13 @@ int rw_swap_page_sync(int rw, swp_entry_
> >  		ret = swap_writepage(page, &swap_wbc);
> >  		wait_on_page_writeback(page);
> >  	}
> > -	page->mapping = NULL;
> > +
> > +	lock_page(page);
> > +	remove_from_page_cache(page);
> > +	unlock_page(page);
> > +	page_cache_release(page);
> > +	page_cache_release(page);	/* For add_to_page_cache() */
> > +
> >  	if (ret == 0 && (!PageUptodate(page) || PageError(page)))
> >  		ret = -EIO;
> >  	return ret;
> 
> incrementally to the above I applied this hardness checks in the
> anon-vma patch, so we're safe against the problem Andrew outlined
> (somebody attempting to do swapin/swapouts of anonymous pages through
> that interface, something that shouldn't happen since we want only the
> VM to deal with userspace mapped pages).
> 
> @@ -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. 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.
-
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