Re: [4.10, panic, regression] iscsi: null pointer deref at iscsi_tcp_segment_done+0x20d/0x2e0

[Jan Kara <jack@suse.cz>]

On Mon 02-01-17 16:11:36, Johannes Weiner wrote:
> On Fri, Dec 23, 2016 at 03:33:29AM -0500, Johannes Weiner wrote:
> > On Fri, Dec 23, 2016 at 02:32:41AM -0500, Johannes Weiner wrote:
> > > On Thu, Dec 22, 2016 at 12:22:27PM -0800, Hugh Dickins wrote:
> > > > On Wed, 21 Dec 2016, Linus Torvalds wrote:
> > > > > On Wed, Dec 21, 2016 at 9:13 PM, Dave Chinner <david@fromorbit.com> wrote:
> > > > > > I unmounted the fs, mkfs'd it again, ran the
> > > > > > workload again and about a minute in this fired:
> > > > > >
> > > > > > [628867.607417] ------------[ cut here ]------------
> > > > > > [628867.608603] WARNING: CPU: 2 PID: 16925 at mm/workingset.c:461 shadow_lru_isolate+0x171/0x220
> > > > > 
> > > > > Well, part of the changes during the merge window were the shadow
> > > > > entry tracking changes that came in through Andrew's tree. Adding
> > > > > Johannes Weiner to the participants.
> > > > > 
> > > > > > Now, this workload does not touch the page cache at all - it's
> > > > > > entirely an XFS metadata workload, so it should not really be
> > > > > > affecting the working set code.
> > > > > 
> > > > > Well, I suspect that anything that creates memory pressure will end up
> > > > > triggering the working set code, so ..
> > > > > 
> > > > > That said, obviously memory corruption could be involved and result in
> > > > > random issues too, but I wouldn't really expect that in this code.
> > > > > 
> > > > > It would probably be really useful to get more data points - is the
> > > > > problem reliably in this area, or is it going to be random and all
> > > > > over the place.
> > > > 
> > > > Data point: kswapd got WARNING on mm/workingset.c:457 in shadow_lru_isolate,
> > > > soon followed by NULL pointer deref in list_lru_isolate, one time when
> > > > I tried out Sunday's git tree.  Not seen since, I haven't had time to
> > > > investigate, just set it aside as something to worry about if it happens
> > > > again.  But it looks like shadow_lru_isolate() has issues beyond Dave's
> > > > case (I've no XFS and no iscsi), suspect unrelated to his other problems.
> > > 
> > > This seems consistent with what Dave observed: we encounter regular
> > > pages in radix tree nodes on the shadow LRU that should only contain
> > > nodes full of exceptional shadow entries. It could be an issue in the
> > > new slot replacement code and the node tracking callback.
> > 
> > Both encounters seem to indicate use-after-free. Dave's node didn't
> > warn about an unexpected node->count / node->exceptional state, but
> > had entries that were inconsistent with that. Hugh got the counter
> > warning but crashed on a list_head that's not NULLed in a live node.
> > 
> > workingset_update_node() should be called on page cache radix tree
> > leaf nodes that go empty. I must be missing an update_node callback
> > where a leaf node gets freed somewhere.
> 
> Sorry for dropping silent on this. I'm traveling over the holidays
> with sporadic access to my emails and no access to real equipment.
> 
> The times I managed to sneak away to look at the code didn't turn up
> anything useful yet.
> 
> Andrea encountered the warning as well and I gave him a debugging
> patch (attached below), but he hasn't been able to reproduce this
> condition. I've personally never seen the warning trigger, even though
> the patches have been running on my main development machine for quite
> a while now. Albeit against an older base; I've updated to Linus's
> master branch now in case it's an interaction with other new code.
> 
> If anybody manages to reproduce this, that would be helpful. Any extra
> eyes on this would be much appreciated too until I'm back at my desk.

I was looking into this but I didn't find a way how we could possibly leave
radix tree node on LRU. So your debug patch looks like a good way forward.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR