Re: swap_cluster_info lockdep splat

["Huang\, Ying" <ying.huang@intel.com>]

Hugh Dickins <hughd@google.com> writes:

> 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?
>> 
>> 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).

Thanks for your patch.  cluster_lock is bit_spinlock before, the wrapper
made it easier to be converted to normal spinlock.  But especially after
splitting the function into 2 variants, the wrapper looks pure
redundant.  Thanks for fixing that too.

Best Regards,
Huang, Ying

> [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>
> ---
>
>  mm/swapfile.c |    9 ++-------
>  1 file changed, 2 insertions(+), 7 deletions(-)
>
> --- 4.10-rc7-mm1/mm/swapfile.c	2017-02-08 10:56:23.359358518 -0800
> +++ linux/mm/swapfile.c	2017-02-08 11:25:55.513241067 -0800
> @@ -258,11 +258,6 @@ static inline void cluster_set_null(stru
>  	info->data = 0;
>  }
>  
> -static inline void __lock_cluster(struct swap_cluster_info *ci)
> -{
> -	spin_lock(&ci->lock);
> -}
> -
>  static inline struct swap_cluster_info *lock_cluster(struct swap_info_struct *si,
>  						     unsigned long offset)
>  {
> @@ -271,7 +266,7 @@ static inline struct swap_cluster_info *
>  	ci = si->cluster_info;
>  	if (ci) {
>  		ci += offset / SWAPFILE_CLUSTER;
> -		__lock_cluster(ci);
> +		spin_lock(&ci->lock);
>  	}
>  	return ci;
>  }
> @@ -336,7 +331,7 @@ static void cluster_list_add_tail(struct
>  		 * only acquired when we held swap_info_struct->lock
>  		 */
>  		ci_tail = ci + tail;
> -		__lock_cluster(ci_tail);
> +		spin_lock_nested(&ci_tail->lock, SINGLE_DEPTH_NESTING);
>  		cluster_set_next(ci_tail, idx);
>  		unlock_cluster(ci_tail);
>  		cluster_set_next_flag(&list->tail, idx, 0);