Re: BUG: soft lockup - is this XFS problem?

Nick Piggin
2009-01-05T01:48:35Z

On Sat, Jan 03, 2009 at 04:44:43PM -0500, Christoph Hellwig wrote:
> On Tue, Dec 30, 2008 at 05:23:33AM +0100, Nick Piggin wrote:
> > On Tue, Dec 23, 2008 at 12:12:59PM -0500, Christoph Hellwig wrote:
> > >
> > > Nick, I've seen various reports like this by Roman. It seems to be
> > > caused by an interaction of the lockless pagecache with the xfs
> > > I/O code. Any idea what might be wrong here:
> >
> > Hmm, it could get into a loop here if there is a page in the pagecache
> > with a zero refcount, which might be a problem with XFS... other looping
> > conditions might indicate a problem iwth lockless pagecache or radix
> > tree. It would be very helpful to know what condition it is looping on...
>
> See http://oss.sgi.com/bugzilla/show_bug.cgi?id=805

OK.. Hmm, well here is a modification to your patch which might help further.
I'll see if I can reproduce it here meanwhile.

---
mm/filemap.c | 29 +++++++++++++++++++++++++----
1 file changed, 25 insertions(+), 4 deletions(-)

Index: linux-2.6/mm/filemap.c
===================================================================
--- linux-2.6.orig/mm/filemap.c
+++ linux-2.6/mm/filemap.c
@@ -770,11 +770,13 @@ EXPORT_SYMBOL(find_or_create_page);
* find_get_pages() returns the number of pages which were found.
*/
unsigned find_get_pages(struct address_space *mapping, pgoff_t start,
- unsigned int nr_pages, struct page **pages)
+ unsigned int nr_pages,
+ struct page **pages)
{
unsigned int i;
unsigned int ret;
unsigned int nr_found;
+ int locked = 0;

rcu_read_lock();
restart:
@@ -785,27 +787,46 @@ restart:
struct page *page;
repeat:
page = radix_tree_deref_slot((void **)pages[i]);
- if (unlikely(!page))
+ if (unlikely(!page)) {
+ if (printk_ratelimit())
+ printk(KERN_INFO "unable to deref page\n");
continue;
+ }
+
/*
* this can only trigger if nr_found == 1, making livelock
* a non issue.
*/
- if (unlikely(page == RADIX_TREE_RETRY))
+ if (unlikely(page == RADIX_TREE_RETRY)) {
+ printk(KERN_INFO "got RADIX_TREE_RETRY\n");
goto restart;
+ }

- if (!page_cache_get_speculative(page))
+ if (!page_cache_get_speculative(page)) {
+ /* If the page is in the radix-tree, and the radix-tree
+ * is locked, the page must have a non-zero refcount */
+ BUG_ON(locked);
+ printk(KERN_INFO "page_cache_get failed\n");
+ spin_lock_irq(&mapping->tree_lock);
+ locked = 1;
goto repeat;
+ }

/* Has the page moved? */
if (unlikely(page != *((void **)pages[i]))) {
+ BUG_ON(locked);
+ printk(KERN_INFO "page moved\n");
page_cache_release(page);
+ spin_lock_irq(&mapping->tree_lock);
+ locked = 1;
goto repeat;
}

pages[ret] = page;
ret++;
}
+ if (locked)
+ spin_unlock_irq(&mapping->tree_lock);
rcu_read_unlock();
return ret;
}