Re: linux-next: slab shrinkers: BUG at mm/list_lru.c:92

[Glauber Costa <glommer@gmail.com>]

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On Sun, Jun 23, 2013 at 03:51:29PM +0400, Glauber Costa wrote:
> On Fri, Jun 21, 2013 at 11:00:21AM +0200, Michal Hocko wrote:
> > On Thu 20-06-13 17:12:01, Michal Hocko wrote:
> > > I am bisecting it again. It is quite tedious, though, because good case
> > > is hard to be sure about.
> > 
> > OK, so now I converged to 2d4fc052 (inode: convert inode lru list to generic lru
> > list code.) in my tree and I have double checked it matches what is in
> > the linux-next. This doesn't help much to pin point the issue I am
> > afraid :/
> > 
> Can you revert this patch (easiest way ATM is to rewind your tree to a point
> right before it) and apply the following patch?
> 
> As Dave has mentioned, it is very likely that this bug was already there, we
> were just not ever checking imbalances. The attached patch would tell us at
> least if the imbalance was there before. If this is the case, I would suggest
> turning the BUG condition into a WARN_ON_ONCE since we would be officially
> not introducing any regression. It is no less of a bug, though, and we should
> keep looking for it.
> 
> The main change from before / after the patch is that we are now keeping things
> per node. One possibility of having this BUGing would be to have an inode to be
> inserted into one node-lru and removed from another. I cannot see how it could
> happen, because kernel pages are stable in memory and are not moved from node
> to node. We could still have some sort of weird bug in the node calculation
> function. In any case, would it be possible for you to artificially restrict
> your setup to a single node ? Although I have no idea how to do that, we seem
> to have no parameter to disable numa. Maybe booting with less memory, enough to
> fit a single node?
> 
The patch:

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diff --git a/fs/inode.c b/fs/inode.c
index 1ddaa2e..0b5c3fa 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -427,6 +427,7 @@ static void inode_lru_list_del(struct inode *inode)
 	if (!list_empty(&inode->i_lru)) {
 		list_del_init(&inode->i_lru);
 		inode->i_sb->s_nr_inodes_unused--;
+		BUG_ON(sb->s_nr_inodes_unused < 0);
 		this_cpu_dec(nr_unused);
 	}
 	spin_unlock(&inode->i_sb->s_inode_lru_lock);
@@ -739,6 +740,7 @@ long prune_icache_sb(struct super_block *sb, unsigned long nr_to_scan)
 			list_del_init(&inode->i_lru);
 			spin_unlock(&inode->i_lock);
 			sb->s_nr_inodes_unused--;
+			BUG_ON(sb->s_nr_inodes_unused < 0);
 			this_cpu_dec(nr_unused);
 			continue;
 		}
@@ -777,6 +779,7 @@ long prune_icache_sb(struct super_block *sb, unsigned long nr_to_scan)
 
 		list_move(&inode->i_lru, &freeable);
 		sb->s_nr_inodes_unused--;
+		BUG_ON(sb->s_nr_inodes_unused < 0);
 		this_cpu_dec(nr_unused);
 		freed++;
 	}