Re: possible bug in find_get_pages

[Minchan Kim <minchan.kim@gmail.com>]

Nick already found and solved this problem .
It can help you. 

http://patchwork.kernel.org/patch/860/


> On Fri, 6 Mar 2009 11:26:25 -0800
> mark gross <mgross@linux.intel.com> wrote:
>
> I'm looking at a system hang (note: new hardware going under stress
> tests using a ubuntu 2.6.27-11-generic)
> 
> It seems that page->_count == 0 at some point on some overnight runs
> with locks the system into a tight loop from the repeat: and a goto
> repeat in find_get_pages. 
> 
> Code inserted for convenience:
> 
> unsigned find_get_pages(struct address_space *mapping, pgoff_t start,
> 			    unsigned int nr_pages, struct page **pages)
> {
> 	unsigned int i;
> 	unsigned int ret;
> 	unsigned int nr_found;
> 
> 	rcu_read_lock();
> restart:
> 	nr_found = radix_tree_gang_lookup_slot(&mapping->page_tree,
> 				(void ***)pages, start, nr_pages);
> 	ret = 0;
> 	for (i = 0; i < nr_found; i++) {
> 		struct page *page;
> repeat:
> 		page = radix_tree_deref_slot((void **)pages[i]);
> 		if (unlikely(!page))
> 			continue;
> 		/*
> 		 * this can only trigger if nr_found == 1, making
> 		 * livelock
> 		 * a non issue.
> 		 */
> 		if (unlikely(page == RADIX_TREE_RETRY))
> 			goto restart;
> 
> 		if (!page_cache_get_speculative(page))
> 			goto repeat; <---------_always_hits_ 
> 
> 		/* Has the page moved? */
> 		if (unlikely(page != *((void **)pages[i]))) {
> 			page_cache_release(page);
> 			goto repeat;
> 		}
> 
> 		pages[ret] = page;
> 		ret++;
> 	}
> 	rcu_read_unlock();
> 	return ret;
> }
> 
> My question is that as I look at this code I don't see any way out of it
> once I get a page with zero _count from radix_tree_deref_slot, then I
> will get the same page forever.  The input to radix_tree_deref_slot
> never changes so I assume the output should be the same crappy page with
> zero _count that drops me on the goto repeat line.
> 
> Is this a bug?
> 
> Also, is having a page->_count == 0 an unexpected or invalid state?
> 
> Thanks!
> 
> --mgross
> 
> 
> 
> 
> 
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-- 
Kinds Regards
Minchan Kim

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