Re: swap_cluster_info lockdep splat

Minchan Kim
2017-02-16T23:45:04Z

Hi Huang and Hugh,

Thanks for the quick reponse!

On Thu, Feb 16, 2017 at 11:00:00AM -0800, Hugh Dickins wrote:
> On Thu, 16 Feb 2017, Huang, Ying wrote:
>
> > Hi, Minchan,
> >
> > Minchan Kim <minchan@kernel.org> writes:
> >
> > > Hi Huang,
> > >
> > > With changing from bit lock to spinlock of swap_cluster_info, my zram
> > > test failed with below message. It seems nested lock problem so need to
> > > play with lockdep.
> >
> > Sorry, I could not reproduce the warning in my tests. Could you try the
> > patches as below? And could you share your test case?

It's a simple kernel build test in small memory system.
4-core and 750M memory with zram-4G swap.

> >
> > Best Regards,
> > Huang, Ying
> >
> > ------------------------------------------------------------->
> > From 2b9e2f78a6e389442f308c4f9e8d5ac40fe6aa2f Mon Sep 17 00:00:00 2001
> > From: Huang Ying <ying.huang@intel.com>
> > Date: Thu, 16 Feb 2017 16:38:17 +0800
> > Subject: [PATCH] mm, swap: Annotate nested locking for cluster lock
> >
> > There is a nested locking in cluster_list_add_tail() for cluster lock,
> > which caused lockdep to complain as below. The nested locking is safe
> > because both cluster locks are only acquired when we held the
> > swap_info_struct->lock. Annotated the nested locking via
> > spin_lock_nested() to fix the complain of lockdep.
> >
> > =============================================
> > [ INFO: possible recursive locking detected ]
> > 4.10.0-rc8-next-20170214-zram #24 Not tainted
> > ---------------------------------------------
> > as/6557 is trying to acquire lock:
> > (&(&((cluster_info + ci)->lock))->rlock){+.+.-.}, at: [<ffffffff811ddd03>] cluster_list_add_tail.part.31+0x33/0x70
> >
> > but task is already holding lock:
> > (&(&((cluster_info + ci)->lock))->rlock){+.+.-.}, at: [<ffffffff811df2bb>] swapcache_free_entries+0x9b/0x330
> >
> > other info that might help us debug this:
> > Possible unsafe locking scenario:
> >
> > CPU0
> > ----
> > lock(&(&((cluster_info + ci)->lock))->rlock);
> > lock(&(&((cluster_info + ci)->lock))->rlock);
> >
> > *** DEADLOCK ***
> >
> > May be due to missing lock nesting notation
> >
> > 3 locks held by as/6557:
> > #0: (&(&cache->free_lock)->rlock){......}, at: [<ffffffff811c206b>] free_swap_slot+0x8b/0x110
> > #1: (&(&p->lock)->rlock){+.+.-.}, at: [<ffffffff811df295>] swapcache_free_entries+0x75/0x330
> > #2: (&(&((cluster_info + ci)->lock))->rlock){+.+.-.}, at: [<ffffffff811df2bb>] swapcache_free_entries+0x9b/0x330
> >
> > stack backtrace:
> > CPU: 3 PID: 6557 Comm: as Not tainted 4.10.0-rc8-next-20170214-zram #24
> > Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Ubuntu-1.8.2-1ubuntu1 04/01/2014
> > Call Trace:
> > dump_stack+0x85/0xc2
> > __lock_acquire+0x15ea/0x1640
> > lock_acquire+0x100/0x1f0
> > ? cluster_list_add_tail.part.31+0x33/0x70
> > _raw_spin_lock+0x38/0x50
> > ? cluster_list_add_tail.part.31+0x33/0x70
> > cluster_list_add_tail.part.31+0x33/0x70
> > swapcache_free_entries+0x2f9/0x330
> > free_swap_slot+0xf8/0x110
> > swapcache_free+0x36/0x40
> > delete_from_swap_cache+0x5f/0xa0
> > try_to_free_swap+0x6e/0xa0
> > free_pages_and_swap_cache+0x7d/0xb0
> > tlb_flush_mmu_free+0x36/0x60
> > tlb_finish_mmu+0x1c/0x50
> > exit_mmap+0xc7/0x150
> > mmput+0x51/0x110
> > do_exit+0x2b2/0xc30
> > ? trace_hardirqs_on_caller+0x129/0x1b0
> > do_group_exit+0x50/0xd0
> > SyS_exit_group+0x14/0x20
> > entry_SYSCALL_64_fastpath+0x23/0xc6
> > RIP: 0033:0x2b9a2dbdf309
> > RSP: 002b:00007ffe71887528 EFLAGS: 00000246 ORIG_RAX: 00000000000000e7
> > RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00002b9a2dbdf309
> > RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
> > RBP: 00002b9a2ded8858 R08: 000000000000003c R09: 00000000000000e7
> > R10: ffffffffffffff60 R11: 0000000000000246 R12: 00002b9a2ded8858
> > R13: 00002b9a2dedde80 R14: 000000000255f770 R15: 0000000000000001
> >
> > Reported-by: Minchan Kim <minchan@kernel.org>
> > Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
> > ---
> > include/linux/swap.h | 6 ++++++
> > mm/swapfile.c | 8 +++++++-
> > 2 files changed, 13 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/linux/swap.h b/include/linux/swap.h
> > index 4d12b381821f..ef044ea8fe79 100644
> > --- a/include/linux/swap.h
> > +++ b/include/linux/swap.h
> > @@ -166,6 +166,12 @@ enum {
> > #define COUNT_CONTINUED 0x80 /* See swap_map continuation for full count */
> > #define SWAP_MAP_SHMEM 0xbf /* Owned by shmem/tmpfs, in first swap_map */
> >
> > +enum swap_cluster_lock_class
> > +{
> > + SWAP_CLUSTER_LOCK_NORMAL, /* implicitly used by plain spin_lock() APIs. */
> > + SWAP_CLUSTER_LOCK_NESTED,
> > +};
> > +
> > /*
> > * We use this to track usage of a cluster. A cluster is a block of swap disk
> > * space with SWAPFILE_CLUSTER pages long and naturally aligns in disk. All
> > diff --git a/mm/swapfile.c b/mm/swapfile.c
> > index 5ac2cb40dbd3..0a52e9b2f843 100644
> > --- a/mm/swapfile.c
> > +++ b/mm/swapfile.c
> > @@ -263,6 +263,12 @@ static inline void __lock_cluster(struct swap_cluster_info *ci)
> > spin_lock(&ci->lock);
> > }
> >
> > +static inline void __lock_cluster_nested(struct swap_cluster_info *ci,
> > + unsigned subclass)
> > +{
> > + spin_lock_nested(&ci->lock, subclass);
> > +}
> > +
> > static inline struct swap_cluster_info *lock_cluster(struct swap_info_struct *si,
> > unsigned long offset)
> > {
> > @@ -336,7 +342,7 @@ static void cluster_list_add_tail(struct swap_cluster_list *list,
> > * only acquired when we held swap_info_struct->lock
> > */
> > ci_tail = ci + tail;
> > - __lock_cluster(ci_tail);
> > + __lock_cluster_nested(ci_tail, SWAP_CLUSTER_LOCK_NESTED);
> > cluster_set_next(ci_tail, idx);
> > unlock_cluster(ci_tail);
> > cluster_set_next_flag(&list->tail, idx, 0);
> > --
> > 2.11.0
>
> I do not understand your zest for putting wrappers around every little
> thing, making it all harder to follow than it need be. Here's the patch
> I've been running with (but you have a leak somewhere, and I don't have
> time to search out and fix it: please try sustained swapping and swapoff).
>
> [PATCH] mm, swap: Annotate nested locking for cluster lock
>
> Fix swap cluster lockdep warnings.
>
> Reported-by: Minchan Kim <minchan@kernel.org>
> Signed-off-by: Hugh Dickins <hughd@google.com>

Acutually, before the reporting, I tested below hunk and confirmed it doesn't
make lockdep warn any more. But I doubted it's okay for non-nested case
(i.e., setup_swap_map_and_extends) for lockdep subclass working.
I guess it's no problem but not sure so I just reported it without fixing
by myself. :)
If it's no problem, I'm sure both patches from you guys would work well
but I prefer Hugh's patch which makes it simple/clear.

Thanks.

diff --git a/mm/swapfile.c b/mm/swapfile.c
index 5ac2cb4..348b9c5 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -263,6 +263,11 @@ static inline void __lock_cluster(struct swap_cluster_info *ci)
spin_lock(&ci->lock);
}

+static inline void __lock_cluster_nested(struct swap_cluster_info *ci)
+{
+ spin_lock_nested(&ci->lock, SINGLE_DEPTH_NESTING);
+}
+
static inline struct swap_cluster_info *lock_cluster(struct swap_info_struct *si,
unsigned long offset)
{
@@ -336,7 +341,7 @@ static void cluster_list_add_tail(struct swap_cluster_list *list,
* only acquired when we held swap_info_struct->lock
*/
ci_tail = ci + tail;
- __lock_cluster(ci_tail);
+ __lock_cluster_nested(ci_tail);
cluster_set_next(ci_tail, idx);
unlock_cluster(ci_tail);
cluster_set_next_flag(&list->tail, idx, 0);