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Date:   Fri, 23 Dec 2016 03:33:29 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Dave Chinner <david@...morbit.com>,
        Chris Leech <cleech@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Lee Duncan <lduncan@...e.com>, open-iscsi@...glegroups.com,
        Linux SCSI List <linux-scsi@...r.kernel.org>,
        linux-block@...r.kernel.org, Christoph Hellwig <hch@....de>
Subject: Re: [4.10, panic, regression] iscsi: null pointer deref at
 iscsi_tcp_segment_done+0x20d/0x2e0

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@...morbit.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.

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