Re: [PATCH 1/2] mm,vmscan: Kill global shrinker lock.

[Eric Wheeler <linux-mm@lists.ewheeler.net>]

On Thu, 25 Jan 2018, Tetsuo Handa wrote:

> Using a debug patch and a reproducer shown below, we can trivially form
> a circular locking dependency shown below.
> 
> ----------------------------------------
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index 8219001..240efb1 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -950,7 +950,7 @@ static void oom_kill_process(struct oom_control *oc, const char *message)
>  	}
>  	task_unlock(p);
>  
> -	if (__ratelimit(&oom_rs))
> +	if (0 && __ratelimit(&oom_rs))
>  		dump_header(oc, p);
>  
>  	pr_err("%s: Kill process %d (%s) score %u or sacrifice child\n",
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 1afb2af..9858449 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -410,6 +410,9 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
>  	return freed;
>  }
>  
> +struct lockdep_map __shrink_slab_map =
> +	STATIC_LOCKDEP_MAP_INIT("shrink_slab", &__shrink_slab_map);
> +
>  /**
>   * shrink_slab - shrink slab caches
>   * @gfp_mask: allocation context
> @@ -453,6 +456,8 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
>  		goto out;
>  	}
>  
> +	lock_map_acquire(&__shrink_slab_map);
> +
>  	list_for_each_entry(shrinker, &shrinker_list, list) {
>  		struct shrink_control sc = {
>  			.gfp_mask = gfp_mask,
> @@ -491,6 +496,8 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid,
>  		}
>  	}
>  
> +	lock_map_release(&__shrink_slab_map);
> +
>  	up_read(&shrinker_rwsem);
>  out:
>  	cond_resched();
> ----------------------------------------
> 
> ----------------------------------------
> #include <stdlib.h>
> 
> int main(int argc, char *argv[])
> {
> 	unsigned long long size;
> 	char *buf = NULL;
> 	unsigned long long i;
> 	for (size = 1048576; size < 512ULL * (1 << 30); size *= 2) {
> 		char *cp = realloc(buf, size);
> 		if (!cp) {
> 			size /= 2;
> 			break;
> 		}
> 		buf = cp;
> 	}
> 	for (i = 0; i < size; i += 4096)
> 		buf[i] = 0;
> 	return 0;
> }

Hi Tetsuo,

Thank you for looking into this!

I tried running this C program in 4.14.15 but did not get a deadlock, just 
OOM kills. Is the patch required to induce the deadlock?

Also, what are you doing to XFS to make it trigger?

--
Eric Wheeler

> ----------------------------------------
> 
> ----------------------------------------
> CentOS Linux 7 (Core)
> Kernel 4.15.0-rc8-next-20180119+ on an x86_64
> 
> localhost login: [   36.954893] cp (2850) used greatest stack depth: 10816 bytes left
> [   89.216085] Out of memory: Kill process 6981 (a.out) score 876 or sacrifice child
> [   89.225853] Killed process 6981 (a.out) total-vm:4264020kB, anon-rss:3346832kB, file-rss:8kB, shmem-rss:0kB
> [   89.313597] oom_reaper: reaped process 6981 (a.out), now anon-rss:0kB, file-rss:0kB, shmem-rss:0kB
> [   92.640566] Out of memory: Kill process 6983 (a.out) score 876 or sacrifice child
> [   92.642153] 
> [   92.643464] Killed process 6983 (a.out) total-vm:4264020kB, anon-rss:3348624kB, file-rss:4kB, shmem-rss:0kB
> [   92.644416] ======================================================
> [   92.644417] WARNING: possible circular locking dependency detected
> [   92.644418] 4.15.0-rc8-next-20180119+ #222 Not tainted
> [   92.644419] ------------------------------------------------------
> [   92.644419] kworker/u256:29/401 is trying to acquire lock:
> [   92.644420]  (shrink_slab){+.+.}, at: [<0000000040040aca>] shrink_slab.part.42+0x73/0x350
> [   92.644428] 
> [   92.644428] but task is already holding lock:
> [   92.665257]  (&xfs_nondir_ilock_class){++++}, at: [<00000000ae515ec8>] xfs_ilock+0xa3/0x180 [xfs]
> [   92.668490] 
> [   92.668490] which lock already depends on the new lock.
> [   92.668490] 
> [   92.672781] 
> [   92.672781] the existing dependency chain (in reverse order) is:
> [   92.676310] 
> [   92.676310] -> #1 (&xfs_nondir_ilock_class){++++}:
> [   92.679519]        xfs_free_eofblocks+0x9d/0x210 [xfs]
> [   92.681716]        xfs_fs_destroy_inode+0x9e/0x220 [xfs]
> [   92.683962]        dispose_list+0x30/0x40
> [   92.685822]        prune_icache_sb+0x4d/0x70
> [   92.687961]        super_cache_scan+0x136/0x180
> [   92.690017]        shrink_slab.part.42+0x205/0x350
> [   92.692109]        shrink_node+0x313/0x320
> [   92.694177]        kswapd+0x386/0x6d0
> [   92.695951]        kthread+0xeb/0x120
> [   92.697889]        ret_from_fork+0x3a/0x50
> [   92.699800] 
> [   92.699800] -> #0 (shrink_slab){+.+.}:
> [   92.702676]        shrink_slab.part.42+0x93/0x350
> [   92.704756]        shrink_node+0x313/0x320
> [   92.706660]        do_try_to_free_pages+0xde/0x350
> [   92.708737]        try_to_free_pages+0xc5/0x100
> [   92.710734]        __alloc_pages_slowpath+0x41c/0xd60
> [   92.712470] oom_reaper: reaped process 6983 (a.out), now anon-rss:0kB, file-rss:0kB, shmem-rss:0kB
> [   92.712978]        __alloc_pages_nodemask+0x22a/0x270
> [   92.713013]        xfs_buf_allocate_memory+0x16b/0x2d0 [xfs]
> [   92.721378]        xfs_buf_get_map+0xaf/0x140 [xfs]
> [   92.723562]        xfs_buf_read_map+0x1f/0xc0 [xfs]
> [   92.726105]        xfs_trans_read_buf_map+0xf5/0x2d0 [xfs]
> [   92.728461]        xfs_btree_read_buf_block.constprop.36+0x69/0xc0 [xfs]
> [   92.731321]        xfs_btree_lookup_get_block+0x82/0x180 [xfs]
> [   92.733739]        xfs_btree_lookup+0x118/0x450 [xfs]
> [   92.735982]        xfs_alloc_ag_vextent_near+0xb2/0xb80 [xfs]
> [   92.738380]        xfs_alloc_ag_vextent+0x1cc/0x320 [xfs]
> [   92.740646]        xfs_alloc_vextent+0x416/0x480 [xfs]
> [   92.743023]        xfs_bmap_btalloc+0x340/0x8b0 [xfs]
> [   92.745597]        xfs_bmapi_write+0x6c1/0x1270 [xfs]
> [   92.747749]        xfs_iomap_write_allocate+0x16c/0x360 [xfs]
> [   92.750317]        xfs_map_blocks+0x175/0x230 [xfs]
> [   92.752745]        xfs_do_writepage+0x232/0x6e0 [xfs]
> [   92.754843]        write_cache_pages+0x1d1/0x3b0
> [   92.756801]        xfs_vm_writepages+0x60/0xa0 [xfs]
> [   92.758838]        do_writepages+0x12/0x60
> [   92.760822]        __writeback_single_inode+0x2c/0x170
> [   92.762895]        writeback_sb_inodes+0x267/0x460
> [   92.764851]        __writeback_inodes_wb+0x82/0xb0
> [   92.766821]        wb_writeback+0x203/0x210
> [   92.768676]        wb_workfn+0x266/0x2e0
> [   92.770494]        process_one_work+0x253/0x460
> [   92.772378]        worker_thread+0x42/0x3e0
> [   92.774153]        kthread+0xeb/0x120
> [   92.775775]        ret_from_fork+0x3a/0x50
> [   92.777513] 
> [   92.777513] other info that might help us debug this:
> [   92.777513] 
> [   92.781361]  Possible unsafe locking scenario:
> [   92.781361] 
> [   92.784382]        CPU0                    CPU1
> [   92.786276]        ----                    ----
> [   92.788130]   lock(&xfs_nondir_ilock_class);
> [   92.790048]                                lock(shrink_slab);
> [   92.792256]                                lock(&xfs_nondir_ilock_class);
> [   92.794756]   lock(shrink_slab);
> [   92.796251] 
> [   92.796251]  *** DEADLOCK ***
> [   92.796251] 
> [   92.799521] 6 locks held by kworker/u256:29/401:
> [   92.801573]  #0:  ((wq_completion)"writeback"){+.+.}, at: [<0000000087382bbf>] process_one_work+0x1f0/0x460
> [   92.804947]  #1:  ((work_completion)(&(&wb->dwork)->work)){+.+.}, at: [<0000000087382bbf>] process_one_work+0x1f0/0x460
> [   92.808596]  #2:  (&type->s_umount_key#31){++++}, at: [<0000000048ea98d7>] trylock_super+0x11/0x50
> [   92.811957]  #3:  (sb_internal){.+.+}, at: [<0000000058532c48>] xfs_trans_alloc+0xe4/0x120 [xfs]
> [   92.815280]  #4:  (&xfs_nondir_ilock_class){++++}, at: [<00000000ae515ec8>] xfs_ilock+0xa3/0x180 [xfs]
> [   92.819075]  #5:  (shrinker_rwsem){++++}, at: [<0000000039dd500e>] shrink_slab.part.42+0x3c/0x350
> [   92.822354] 
> [   92.822354] stack backtrace:
> [   92.824820] CPU: 1 PID: 401 Comm: kworker/u256:29 Not tainted 4.15.0-rc8-next-20180119+ #222
> [   92.827894] Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 05/19/2017
> [   92.831609] Workqueue: writeback wb_workfn (flush-8:0)
> [   92.833759] Call Trace:
> [   92.835144]  dump_stack+0x7d/0xb6
> [   92.836761]  print_circular_bug.isra.37+0x1d7/0x1e4
> [   92.838908]  __lock_acquire+0x10da/0x15b0
> [   92.840744]  ? __lock_acquire+0x390/0x15b0
> [   92.842603]  ? lock_acquire+0x51/0x70
> [   92.844308]  lock_acquire+0x51/0x70
> [   92.845980]  ? shrink_slab.part.42+0x73/0x350
> [   92.847896]  shrink_slab.part.42+0x93/0x350
> [   92.849799]  ? shrink_slab.part.42+0x73/0x350
> [   92.851710]  ? mem_cgroup_iter+0x140/0x530
> [   92.853890]  ? mem_cgroup_iter+0x158/0x530
> [   92.855897]  shrink_node+0x313/0x320
> [   92.857609]  do_try_to_free_pages+0xde/0x350
> [   92.859502]  try_to_free_pages+0xc5/0x100
> [   92.861328]  __alloc_pages_slowpath+0x41c/0xd60
> [   92.863298]  __alloc_pages_nodemask+0x22a/0x270
> [   92.865285]  xfs_buf_allocate_memory+0x16b/0x2d0 [xfs]
> [   92.867621]  xfs_buf_get_map+0xaf/0x140 [xfs]
> [   92.869562]  xfs_buf_read_map+0x1f/0xc0 [xfs]
> [   92.871494]  xfs_trans_read_buf_map+0xf5/0x2d0 [xfs]
> [   92.873589]  xfs_btree_read_buf_block.constprop.36+0x69/0xc0 [xfs]
> [   92.876322]  ? kmem_zone_alloc+0x7e/0x100 [xfs]
> [   92.878320]  xfs_btree_lookup_get_block+0x82/0x180 [xfs]
> [   92.880527]  xfs_btree_lookup+0x118/0x450 [xfs]
> [   92.882528]  ? kmem_zone_alloc+0x7e/0x100 [xfs]
> [   92.884511]  xfs_alloc_ag_vextent_near+0xb2/0xb80 [xfs]
> [   92.886974]  xfs_alloc_ag_vextent+0x1cc/0x320 [xfs]
> [   92.889088]  xfs_alloc_vextent+0x416/0x480 [xfs]
> [   92.891098]  xfs_bmap_btalloc+0x340/0x8b0 [xfs]
> [   92.893087]  xfs_bmapi_write+0x6c1/0x1270 [xfs]
> [   92.895085]  xfs_iomap_write_allocate+0x16c/0x360 [xfs]
> [   92.897277]  xfs_map_blocks+0x175/0x230 [xfs]
> [   92.899228]  xfs_do_writepage+0x232/0x6e0 [xfs]
> [   92.901218]  write_cache_pages+0x1d1/0x3b0
> [   92.903102]  ? xfs_add_to_ioend+0x290/0x290 [xfs]
> [   92.905170]  ? xfs_vm_writepages+0x4b/0xa0 [xfs]
> [   92.907182]  xfs_vm_writepages+0x60/0xa0 [xfs]
> [   92.909114]  do_writepages+0x12/0x60
> [   92.910778]  __writeback_single_inode+0x2c/0x170
> [   92.912735]  writeback_sb_inodes+0x267/0x460
> [   92.914561]  __writeback_inodes_wb+0x82/0xb0
> [   92.916413]  wb_writeback+0x203/0x210
> [   92.918050]  ? cpumask_next+0x20/0x30
> [   92.919790]  ? wb_workfn+0x266/0x2e0
> [   92.921384]  wb_workfn+0x266/0x2e0
> [   92.922908]  process_one_work+0x253/0x460
> [   92.924687]  ? process_one_work+0x1f0/0x460
> [   92.926518]  worker_thread+0x42/0x3e0
> [   92.928077]  kthread+0xeb/0x120
> [   92.929512]  ? process_one_work+0x460/0x460
> [   92.931330]  ? kthread_create_worker_on_cpu+0x70/0x70
> [   92.933313]  ret_from_fork+0x3a/0x50
> ----------------------------------------
> 
> Normally shrinker_rwsem acts like a shared lock. But when
> register_shrinker()/unregister_shrinker() called down_write(),
> shrinker_rwsem suddenly starts acting like an exclusive lock.
> 
> What is unfortunate is that down_write() is called independent of
> memory allocation requests. That is, shrinker_rwsem is essentially
> a mutex (and hence the debug patch shown above).
> 
> ----------------------------------------
> [<ffffffffac7538d3>] call_rwsem_down_write_failed+0x13/0x20
> [<ffffffffac1cb985>] register_shrinker+0x45/0xa0
> [<ffffffffac250f68>] sget_userns+0x468/0x4a0
> [<ffffffffac25106a>] mount_nodev+0x2a/0xa0
> [<ffffffffac251be4>] mount_fs+0x34/0x150
> [<ffffffffac2701f2>] vfs_kern_mount+0x62/0x120
> [<ffffffffac272a0e>] do_mount+0x1ee/0xc50
> [<ffffffffac27377e>] SyS_mount+0x7e/0xd0
> [<ffffffffac003831>] do_syscall_64+0x61/0x1a0
> [<ffffffffac80012c>] entry_SYSCALL64_slow_path+0x25/0x25
> [<ffffffffffffffff>] 0xffffffffffffffff
> ----------------------------------------
> 
> Therefore, I think that when do_shrink_slab() for GFP_KERNEL is in progress
> and down_read_trylock() starts failing because somebody else started waiting at
> down_write(), do_shrink_slab() for GFP_NOFS or GFP_NOIO cannot be called.
> Doesn't such race cause unexpected results?
> 
> Michal Hocko wrote:
> > I would rather understand the problem than speculate here. I strongly
> > suspect somebody simply didn't unlock the page.
> 
> Then, can we please please have a mechanism which tells whether somebody
> else was stuck doing memory allocation requests? It is basically
> https://lkml.kernel.org/r/1510833448-19918-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp .
> 

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