Re: [BUG] kmemleak on __radix_tree_preload

[Catalin Marinas]

On Thu, May 08, 2014 at 04:00:27PM +0100, Paul E. McKenney wrote:
> On Thu, May 08, 2014 at 11:24:36AM +0100, Catalin Marinas wrote:
> > On Thu, May 08, 2014 at 10:37:40AM +0100, Jaegeuk Kim wrote:
> > > 2014-05-08 (ë), 10:26 +0100, Catalin Marinas:
> > > > On Thu, May 08, 2014 at 06:16:51PM +0900, Jaegeuk Kim wrote:
> > > > > 2014-05-07 (ì), 12:39 +0100, Catalin Marinas:
> > > > > > On Wed, May 07, 2014 at 03:58:08AM +0100, Jaegeuk Kim wrote:
> > > > > > > unreferenced object 0xffff880004226da0 (size 576):
> > > > > > >   comm "fsstress", pid 14590, jiffies 4295191259 (age 706.308s)
> > > > > > >   hex dump (first 32 bytes):
> > > > > > >     01 00 00 00 81 ff ff ff 00 00 00 00 00 00 00 00  ................
> > > > > > >     50 89 34 81 ff ff ff ff b8 6d 22 04 00 88 ff ff  P.4......m".....
> > > > > > >   backtrace:
> > > > > > >     [<ffffffff816c02e8>] kmemleak_update_trace+0x58/0x80
> > > > > > >     [<ffffffff81349517>] radix_tree_node_alloc+0x77/0xa0
> > > > > > >     [<ffffffff81349718>] __radix_tree_create+0x1d8/0x230
> > > > > > >     [<ffffffff8113286c>] __add_to_page_cache_locked+0x9c/0x1b0
> > > > > > >     [<ffffffff811329a8>] add_to_page_cache_lru+0x28/0x80
> > > > > > >     [<ffffffff81132f58>] grab_cache_page_write_begin+0x98/0xf0
> > > > > > >     [<ffffffffa02e4bf4>] f2fs_write_begin+0xb4/0x3c0 [f2fs]
> > > > > > >     [<ffffffff81131b77>] generic_perform_write+0xc7/0x1c0
> > > > > > >     [<ffffffff81133b7d>] __generic_file_aio_write+0x1cd/0x3f0
> > > > > > >     [<ffffffff81133dfe>] generic_file_aio_write+0x5e/0xe0
> > > > > > >     [<ffffffff81195c5a>] do_sync_write+0x5a/0x90
> > > > > > >     [<ffffffff811968d2>] vfs_write+0xc2/0x1d0
> > > > > > >     [<ffffffff81196daf>] SyS_write+0x4f/0xb0
> > > > > > >     [<ffffffff816dead2>] system_call_fastpath+0x16/0x1b
> > > > > > >     [<ffffffffffffffff>] 0xffffffffffffffff
> > > > > >
> > > > > > OK, it shows that the allocation happens via add_to_page_cache_locked()
> > > > > > and I guess it's page_cache_tree_insert() which calls
> > > > > > __radix_tree_create() (the latter reusing the preloaded node). I'm not
> > > > > > familiar enough to this code (radix-tree.c and filemap.c) to tell where
> > > > > > the node should have been freed, who keeps track of it.
> > > > > >
> > > > > > At a quick look at the hex dump (assuming that the above leak is struct
> > > > > > radix_tree_node):
> > > > > >
> > > > > > 	.path = 1
> > > > > > 	.count = -0x7f (or 0xffffff81 as unsigned int)
> > > > > > 	union {
> > > > > > 		{
> > > > > > 			.parent = NULL
> > > > > > 			.private_data = 0xffffffff81348950
> > > > > > 		}
> > > > > > 		{
> > > > > > 			.rcu_head.next = NULL
> > > > > > 			.rcu_head.func = 0xffffffff81348950
> > > > > > 		}
> > > > > > 	}
> > > > > >
> > > > > > The count is a bit suspicious.
> > > > > >
> > > > > > From the union, it looks most likely like rcu_head information. Is
> > > > > > radix_tree_node_rcu_free() function at the above rcu_head.func?
> > > >
> > > > Thanks for the config. Could you please confirm that 0xffffffff81348950
> > > > address corresponds to the radix_tree_node_rcu_free() function in your
> > > > System.map (or something else)?
> > > 
> > > Yap, the address is matched to radix_tree_node_rcu_free().
> > 
> > Cc'ing Paul as well, not that I blame RCU ;), but maybe he could shed
> > some light on why kmemleak can't track this object.
> 
> Do we have any information on how long it has been since that data
> structure was handed to call_rcu()?  If that time is short, then it
> is quite possible that its grace period simply has not yet completed.

kmemleak scans every 10 minutes but Jaegeuk can confirm how long he has
waited.

> It might also be that one of the CPUs is stuck (e.g., spinning with
> interrupts disabled), which would prevent the grace period from
> completing, in turn preventing any memory waiting for that grace period
> from being freed.

We should get some kernel warning if it's stuck for too long but, again,
Jaegeuk can confirm. I haven't managed to reproduce this on ARM systems.

> > My summary so far:
> > 
> > - radix_tree_node reported by kmemleak as it cannot find any trace of it
> >   when scanning the memory
> > - at allocation time, radix_tree_node is memzero'ed by
> >   radix_tree_node_ctor(). Given that node->rcu_head.func ==
> >   radix_tree_node_rcu_free, my guess is that radix_tree_node_free() has
> >   been called
> > - some time later, kmemleak still hasn't received any callback for
> >   kmem_cache_free(node). Possibly radix_tree_node_rcu_free() hasn't been
> >   called either since node->count is not NULL.
> > 
> > For RCU queued objects, kmemleak should still track references to them
> > via rcu_sched_state and rcu_head members. But even if this went wrong, I
> > would expect the object to be freed eventually and kmemleak notified (so
> > just a temporary leak report which doesn't seem to be the case here).
> 
> OK, so you are saying that this memory has been in this state for quite
> some time?

These leaks don't seem to disappear (time lapsed to be confirmed) and
the object checksum not changed either (otherwise kmemleak would not
report it).

> If the system is responsive during this time, I recommend building with
> CONFIG_RCU_TRACE=y, then polling the debugfs rcu/*/rcugp files.  The value
> of "*" will be "rcu_sched" for kernels built with CONFIG_PREEMPT=n and
> "rcu_preempt" for kernels built with CONFIG_PREEMPT=y.
> 
> If the number printed does not advance, then the RCU grace period is
> stalled, which will prevent memory waiting for that grace period from
> ever being freed.

Thanks for the suggestions

> Of course, if the value of node->count is preventing call_rcu() from
> being invoked in the first place, then the needed grace period won't
> start, much less finish.  ;-)

Given the rcu_head.func value, my assumption is that call_rcu() has
already been called.

BTW, is it safe to have a union overlapping node->parent and
node->rcu_head.next? I'm still staring at the radix-tree code but a
scenario I have in mind is that call_rcu() has been raised for a few
nodes, other CPU may have some reference to one of them and set
node->parent to NULL (e.g. concurrent calls to radix_tree_shrink()),
breaking the RCU linking. I can't confirm this theory yet ;)

-- 
Catalin
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