Re: [PATCH] vfs: stop shrinker while fs is freezed

[Darrick J. Wong]

[adding mm to cc]

On Fri, Dec 13, 2019 at 02:24:40PM -0800, Junxiao Bi wrote:
> Shrinker could be blocked by freeze while dropping the last reference of
> some inode that had been removed. As "s_umount" lock was acquired by the
> Shrinker before blocked, the thaw will hung by this lock. This caused a
> deadlock.
> 
>  crash7latest> set 132
>      PID: 132
>  COMMAND: "kswapd0:0"
>     TASK: ffff9cdc9dfb5f00  [THREAD_INFO: ffff9cdc9dfb5f00]
>      CPU: 6
>    STATE: TASK_UNINTERRUPTIBLE
>  crash7latest> bt
>  PID: 132    TASK: ffff9cdc9dfb5f00  CPU: 6   COMMAND: "kswapd0:0"
>   #0 [ffffaa5d075bf900] __schedule at ffffffff8186487c
>   #1 [ffffaa5d075bf998] schedule at ffffffff81864e96
>   #2 [ffffaa5d075bf9b0] rwsem_down_read_failed at ffffffff818689ee
>   #3 [ffffaa5d075bfa40] call_rwsem_down_read_failed at ffffffff81859308
>   #4 [ffffaa5d075bfa90] __percpu_down_read at ffffffff810ebd38
>   #5 [ffffaa5d075bfab0] __sb_start_write at ffffffff812859ef
>   #6 [ffffaa5d075bfad0] xfs_trans_alloc at ffffffffc07ebe9c [xfs]
>   #7 [ffffaa5d075bfb18] xfs_free_eofblocks at ffffffffc07c39d1 [xfs]
>   #8 [ffffaa5d075bfb80] xfs_inactive at ffffffffc07de878 [xfs]
>   #9 [ffffaa5d075bfba0] __dta_xfs_fs_destroy_inode_3543 at ffffffffc07e885e [xfs]
>  #10 [ffffaa5d075bfbd0] destroy_inode at ffffffff812a25de
>  #11 [ffffaa5d075bfbe8] evict at ffffffff812a2b73
>  #12 [ffffaa5d075bfc10] dispose_list at ffffffff812a2c1d
>  #13 [ffffaa5d075bfc38] prune_icache_sb at ffffffff812a421a
>  #14 [ffffaa5d075bfc70] super_cache_scan at ffffffff812870a1
>  #15 [ffffaa5d075bfcc8] shrink_slab at ffffffff811eebb3
>  #16 [ffffaa5d075bfdb0] shrink_node at ffffffff811f4788
>  #17 [ffffaa5d075bfe38] kswapd at ffffffff811f58c3
>  #18 [ffffaa5d075bff08] kthread at ffffffff810b75d5
>  #19 [ffffaa5d075bff50] ret_from_fork at ffffffff81a0035e
>  crash7latest> set 31060
>      PID: 31060
>  COMMAND: "safefreeze"
>     TASK: ffff9cd292868000  [THREAD_INFO: ffff9cd292868000]
>      CPU: 2
>    STATE: TASK_UNINTERRUPTIBLE
>  crash7latest> bt
>  PID: 31060  TASK: ffff9cd292868000  CPU: 2   COMMAND: "safefreeze"
>   #0 [ffffaa5d10047c90] __schedule at ffffffff8186487c
>   #1 [ffffaa5d10047d28] schedule at ffffffff81864e96
>   #2 [ffffaa5d10047d40] rwsem_down_write_failed at ffffffff81868f18
>   #3 [ffffaa5d10047dd8] call_rwsem_down_write_failed at ffffffff81859367
>   #4 [ffffaa5d10047e20] down_write at ffffffff81867cfd
>   #5 [ffffaa5d10047e38] thaw_super at ffffffff81285d2d
>   #6 [ffffaa5d10047e60] do_vfs_ioctl at ffffffff81299566
>   #7 [ffffaa5d10047ee8] sys_ioctl at ffffffff81299709
>   #8 [ffffaa5d10047f28] do_syscall_64 at ffffffff81003949
>   #9 [ffffaa5d10047f50] entry_SYSCALL_64_after_hwframe at ffffffff81a001ad
>      RIP: 0000000000453d67  RSP: 00007ffff9c1ce78  RFLAGS: 00000206
>      RAX: ffffffffffffffda  RBX: 0000000001cbe92c  RCX: 0000000000453d67
>      RDX: 0000000000000000  RSI: 00000000c0045878  RDI: 0000000000000014
>      RBP: 00007ffff9c1cf80   R8: 0000000000000000   R9: 0000000000000012
>      R10: 0000000000000008  R11: 0000000000000206  R12: 0000000000401fb0
>      R13: 0000000000402040  R14: 0000000000000000  R15: 0000000000000000
>      ORIG_RAX: 0000000000000010  CS: 0033  SS: 002b
> 
> Signed-off-by: Junxiao Bi <junxiao.bi@xxxxxxxxxx>
> ---
>  fs/super.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/fs/super.c b/fs/super.c
> index cfadab2cbf35..adc18652302b 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -80,6 +80,11 @@ static unsigned long super_cache_scan(struct shrinker *shrink,
>  	if (!trylock_super(sb))
>  		return SHRINK_STOP;
>  
> +	if (sb->s_writers.frozen != SB_UNFROZEN) {
> +		up_read(&sb->s_umount);
> +		return SHRINK_STOP;
> +	}

Whatever happened to "let's just fsfreeze the filesystems shortly before
freezing the system?  Did someone find a reason why that wouldn't work?

Also, uh, doesn't this disable memory reclaim for frozen filesystems?

Maybe we all need to go review the xfs io-less inode reclaim series so
we can stop running transactions in reclaim... I can't merge any of it
until the mm changes go upstream.

--D

> +
>  	if (sb->s_op->nr_cached_objects)
>  		fs_objects = sb->s_op->nr_cached_objects(sb, sc);
>  
> -- 
> 2.17.1
>