Re: [BUG] infinite loop in find_get_pages()

[Shaohua Li <shaohua.li@intel.com>]

On Thu, 2011-09-15 at 06:37 +0800, Linus Torvalds wrote:
> On Wed, Sep 14, 2011 at 2:53 PM, Hugh Dickins <hughd@google.com> wrote:
> >
> > Thanks, Eric, though it may not be worth spending your time on it.
> > It occurred to me over lunch that it may take painfully longer than
> > expected to invalidate_mapping_pages() on a single-swapped-out-page
> > 1TB sparse tmpfs file - all those "start += 1" restarts until it
> > reaches the end.
> 
> So can we have a stop-gap patch to just fixes it for now? I assume
> that would be Shaohua's patch with the "nr_found > nr_skip" change?
> 
> Can you guys send whatever patch is appropriate for now with a nice
> changelog and the appropriate sign-offs, please? So that we can at
> least close the issue...
here is my patch if you want to close the issue at hand.

Subject: mm: account skipped entries to avoid looping in find_get_pages

The found entries by find_get_pages() could be all swap entries. In
this case we skip the entries, but make sure the skipped entries are
accounted, so we don't keep looping.
Using nr_found > nr_skip to simplify code as suggested by Eric.

Reported-and-tested-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Shaohua Li <shaohua.li@intel.com>

diff --git a/mm/filemap.c b/mm/filemap.c
index 645a080..7771871 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -827,13 +827,14 @@ unsigned find_get_pages(struct address_space *mapping, pgoff_t start,
 {
 	unsigned int i;
 	unsigned int ret;
-	unsigned int nr_found;
+	unsigned int nr_found, nr_skip;
 
 	rcu_read_lock();
 restart:
 	nr_found = radix_tree_gang_lookup_slot(&mapping->page_tree,
 				(void ***)pages, NULL, start, nr_pages);
 	ret = 0;
+	nr_skip = 0;
 	for (i = 0; i < nr_found; i++) {
 		struct page *page;
 repeat:
@@ -856,6 +857,7 @@ repeat:
 			 * here as an exceptional entry: so skip over it -
 			 * we only reach this from invalidate_mapping_pages().
 			 */
+			nr_skip++;
 			continue;
 		}
 
@@ -876,7 +878,7 @@ repeat:
 	 * If all entries were removed before we could secure them,
 	 * try again, because callers stop trying once 0 is returned.
 	 */
-	if (unlikely(!ret && nr_found))
+	if (unlikely(!ret && nr_found > nr_skip))
 		goto restart;
 	rcu_read_unlock();
 	return ret;