Re: [PATCH 1/3] migrate_pages: fix deadlock in batched migration

[Hugh Dickins <hughd@google.com>]

On Fri, 24 Feb 2023, Huang Ying wrote:

> Two deadlock bugs were reported for the migrate_pages() batching
> series.  Thanks Hugh and Pengfei!  For example, in the following
> deadlock trace snippet,
> 
>  INFO: task kworker/u4:0:9 blocked for more than 147 seconds.
>        Not tainted 6.2.0-rc4-kvm+ #1314
>  "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>  task:kworker/u4:0    state:D stack:0     pid:9     ppid:2      flags:0x00004000
>  Workqueue: loop4 loop_rootcg_workfn
>  Call Trace:
>   <TASK>
>   __schedule+0x43b/0xd00
>   schedule+0x6a/0xf0
>   io_schedule+0x4a/0x80
>   folio_wait_bit_common+0x1b5/0x4e0
>   ? __pfx_wake_page_function+0x10/0x10
>   __filemap_get_folio+0x73d/0x770
>   shmem_get_folio_gfp+0x1fd/0xc80
>   shmem_write_begin+0x91/0x220
>   generic_perform_write+0x10e/0x2e0
>   __generic_file_write_iter+0x17e/0x290
>   ? generic_write_checks+0x12b/0x1a0
>   generic_file_write_iter+0x97/0x180
>   ? __sanitizer_cov_trace_const_cmp4+0x1a/0x20
>   do_iter_readv_writev+0x13c/0x210
>   ? __sanitizer_cov_trace_const_cmp4+0x1a/0x20
>   do_iter_write+0xf6/0x330
>   vfs_iter_write+0x46/0x70
>   loop_process_work+0x723/0xfe0
>   loop_rootcg_workfn+0x28/0x40
>   process_one_work+0x3cc/0x8d0
>   worker_thread+0x66/0x630
>   ? __pfx_worker_thread+0x10/0x10
>   kthread+0x153/0x190
>   ? __pfx_kthread+0x10/0x10
>   ret_from_fork+0x29/0x50
>   </TASK>
> 
>  INFO: task repro:1023 blocked for more than 147 seconds.
>        Not tainted 6.2.0-rc4-kvm+ #1314
>  "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>  task:repro           state:D stack:0     pid:1023  ppid:360    flags:0x00004004
>  Call Trace:
>   <TASK>
>   __schedule+0x43b/0xd00
>   schedule+0x6a/0xf0
>   io_schedule+0x4a/0x80
>   folio_wait_bit_common+0x1b5/0x4e0
>   ? compaction_alloc+0x77/0x1150
>   ? __pfx_wake_page_function+0x10/0x10
>   folio_wait_bit+0x30/0x40
>   folio_wait_writeback+0x2e/0x1e0
>   migrate_pages_batch+0x555/0x1ac0
>   ? __pfx_compaction_alloc+0x10/0x10
>   ? __pfx_compaction_free+0x10/0x10
>   ? __this_cpu_preempt_check+0x17/0x20
>   ? lock_is_held_type+0xe6/0x140
>   migrate_pages+0x100e/0x1180
>   ? __pfx_compaction_free+0x10/0x10
>   ? __pfx_compaction_alloc+0x10/0x10
>   compact_zone+0xe10/0x1b50
>   ? lock_is_held_type+0xe6/0x140
>   ? check_preemption_disabled+0x80/0xf0
>   compact_node+0xa3/0x100
>   ? __sanitizer_cov_trace_const_cmp8+0x1c/0x30
>   ? _find_first_bit+0x7b/0x90
>   sysctl_compaction_handler+0x5d/0xb0
>   proc_sys_call_handler+0x29d/0x420
>   proc_sys_write+0x2b/0x40
>   vfs_write+0x3a3/0x780
>   ksys_write+0xb7/0x180
>   __x64_sys_write+0x26/0x30
>   do_syscall_64+0x3b/0x90
>   entry_SYSCALL_64_after_hwframe+0x72/0xdc
>  RIP: 0033:0x7f3a2471f59d
>  RSP: 002b:00007ffe567f7288 EFLAGS: 00000217 ORIG_RAX: 0000000000000001
>  RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f3a2471f59d
>  RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000005
>  RBP: 00007ffe567f72a0 R08: 0000000000000010 R09: 0000000000000010
>  R10: 0000000000000010 R11: 0000000000000217 R12: 00000000004012e0
>  R13: 00007ffe567f73e0 R14: 0000000000000000 R15: 0000000000000000
>   </TASK>
> 
> The page migration task has held the lock of the shmem folio A, and is
> waiting the writeback of the folio B of the file system on the loop
> block device to complete.  While the loop worker task which writes
> back the folio B is waiting to lock the shmem folio A, because the
> folio A backs the folio B in the loop device.  Thus deadlock is
> triggered.
> 
> In general, if we have locked some other folios except the one we are
> migrating, it's not safe to wait synchronously, for example, to wait
> the writeback to complete or wait to lock the buffer head.
> 
> To fix the deadlock, in this patch, we avoid to batch the page
> migration except for MIGRATE_ASYNC mode.  In MIGRATE_ASYNC mode,
> synchronous waiting is avoided.
> 
> The fix can be improved further.  We will do that as soon as possible.
> 
> Link: https://lore.kernel.org/linux-mm/87a6c8c-c5c1-67dc-1e32-eb30831d6e3d@google.com/
> Link: https://lore.kernel.org/linux-mm/874jrg7kke.fsf@yhuang6-desk2.ccr.corp.intel.com/
> Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
> Reported-by: Hugh Dickins <hughd@google.com>
> Reported-by: "Xu, Pengfei" <pengfei.xu@intel.com>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Stefan Roesch <shr@devkernel.io>
> Cc: Tejun Heo <tj@kernel.org>
> Cc: Xin Hao <xhao@linux.alibaba.com>
> Cc: Zi Yan <ziy@nvidia.com>
> Cc: Yang Shi <shy828301@gmail.com>
> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Mike Kravetz <mike.kravetz@oracle.com>
> ---
>  mm/migrate.c | 62 ++++++++++++++++------------------------------------
>  1 file changed, 19 insertions(+), 43 deletions(-)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 37865f85df6d..7ac37dbbf307 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1106,7 +1106,7 @@ static void migrate_folio_done(struct folio *src,
>  /* Obtain the lock on page, remove all ptes. */
>  static int migrate_folio_unmap(new_page_t get_new_page, free_page_t put_new_page,
>  			       unsigned long private, struct folio *src,
> -			       struct folio **dstp, int force, bool avoid_force_lock,
> +			       struct folio **dstp, int force,
>  			       enum migrate_mode mode, enum migrate_reason reason,
>  			       struct list_head *ret)
>  {
> @@ -1157,17 +1157,6 @@ static int migrate_folio_unmap(new_page_t get_new_page, free_page_t put_new_page
>  		if (current->flags & PF_MEMALLOC)
>  			goto out;
>  
> -		/*
> -		 * We have locked some folios and are going to wait to lock
> -		 * this folio.  To avoid a potential deadlock, let's bail
> -		 * out and not do that. The locked folios will be moved and
> -		 * unlocked, then we can wait to lock this folio.
> -		 */
> -		if (avoid_force_lock) {
> -			rc = -EDEADLOCK;
> -			goto out;
> -		}
> -
>  		folio_lock(src);
>  	}
>  	locked = true;
> @@ -1247,7 +1236,7 @@ static int migrate_folio_unmap(new_page_t get_new_page, free_page_t put_new_page
>  		/* Establish migration ptes */
>  		VM_BUG_ON_FOLIO(folio_test_anon(src) &&
>  			       !folio_test_ksm(src) && !anon_vma, src);
> -		try_to_migrate(src, TTU_BATCH_FLUSH);
> +		try_to_migrate(src, mode == MIGRATE_ASYNC ? TTU_BATCH_FLUSH : 0);

Why that change, I wonder?  The TTU_BATCH_FLUSH can still be useful for
gathering multiple cross-CPU TLB flushes into one, even when it's only
a single page in the batch.


>  		page_was_mapped = 1;
>  	}
>  
> @@ -1261,7 +1250,7 @@ static int migrate_folio_unmap(new_page_t get_new_page, free_page_t put_new_page
>  	 * A folio that has not been unmapped will be restored to
>  	 * right list unless we want to retry.
>  	 */
> -	if (rc == -EAGAIN || rc == -EDEADLOCK)
> +	if (rc == -EAGAIN)
>  		ret = NULL;
>  
>  	migrate_folio_undo_src(src, page_was_mapped, anon_vma, locked, ret);
> @@ -1634,11 +1623,9 @@ static int migrate_pages_batch(struct list_head *from, new_page_t get_new_page,
>  	LIST_HEAD(dst_folios);
>  	bool nosplit = (reason == MR_NUMA_MISPLACED);
>  	bool no_split_folio_counting = false;
> -	bool avoid_force_lock;
>  
>  retry:
>  	rc_saved = 0;
> -	avoid_force_lock = false;
>  	retry = 1;
>  	for (pass = 0;
>  	     pass < NR_MAX_MIGRATE_PAGES_RETRY && (retry || large_retry);
> @@ -1683,15 +1670,14 @@ static int migrate_pages_batch(struct list_head *from, new_page_t get_new_page,
>  			}
>  
>  			rc = migrate_folio_unmap(get_new_page, put_new_page, private,
> -						 folio, &dst, pass > 2, avoid_force_lock,
> -						 mode, reason, ret_folios);
> +						 folio, &dst, pass > 2, mode,
> +						 reason, ret_folios);
>  			/*
>  			 * The rules are:
>  			 *	Success: folio will be freed
>  			 *	Unmap: folio will be put on unmap_folios list,
>  			 *	       dst folio put on dst_folios list
>  			 *	-EAGAIN: stay on the from list
> -			 *	-EDEADLOCK: stay on the from list
>  			 *	-ENOMEM: stay on the from list
>  			 *	Other errno: put on ret_folios list
>  			 */
> @@ -1743,14 +1729,6 @@ static int migrate_pages_batch(struct list_head *from, new_page_t get_new_page,
>  					goto out;
>  				else
>  					goto move;
> -			case -EDEADLOCK:
> -				/*
> -				 * The folio cannot be locked for potential deadlock.
> -				 * Go move (and unlock) all locked folios.  Then we can
> -				 * try again.
> -				 */
> -				rc_saved = rc;
> -				goto move;
>  			case -EAGAIN:
>  				if (is_large) {
>  					large_retry++;
> @@ -1765,11 +1743,6 @@ static int migrate_pages_batch(struct list_head *from, new_page_t get_new_page,
>  				stats->nr_thp_succeeded += is_thp;
>  				break;
>  			case MIGRATEPAGE_UNMAP:
> -				/*
> -				 * We have locked some folios, don't force lock
> -				 * to avoid deadlock.
> -				 */
> -				avoid_force_lock = true;
>  				list_move_tail(&folio->lru, &unmap_folios);
>  				list_add_tail(&dst->lru, &dst_folios);
>  				break;
> @@ -1894,17 +1867,15 @@ static int migrate_pages_batch(struct list_head *from, new_page_t get_new_page,
>  		 */
>  		list_splice_init(from, ret_folios);
>  		list_splice_init(&split_folios, from);
> +		/*
> +		 * Force async mode to avoid to wait lock or bit when we have
> +		 * locked more than one folios.
> +		 */
> +		mode = MIGRATE_ASYNC;

It goes away in a later patch anyway, but I didn't understand that change -
I thought this was a point at which no locks are held.  Oh, perhaps I get
it now: because the batch of 1 is here becoming a batch of HPAGE_PMD_NR?

>  		no_split_folio_counting = true;
>  		goto retry;
>  	}
>  
> -	/*
> -	 * We have unlocked all locked folios, so we can force lock now, let's
> -	 * try again.
> -	 */
> -	if (rc == -EDEADLOCK)
> -		goto retry;
> -
>  	return rc;
>  }
>  
> @@ -1939,7 +1910,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>  		enum migrate_mode mode, int reason, unsigned int *ret_succeeded)
>  {
>  	int rc, rc_gather;
> -	int nr_pages;
> +	int nr_pages, batch;
>  	struct folio *folio, *folio2;
>  	LIST_HEAD(folios);
>  	LIST_HEAD(ret_folios);
> @@ -1953,6 +1924,11 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>  				     mode, reason, &stats, &ret_folios);
>  	if (rc_gather < 0)
>  		goto out;
> +
> +	if (mode == MIGRATE_ASYNC)
> +		batch = NR_MAX_BATCHED_MIGRATION;
> +	else
> +		batch = 1;
>  again:
>  	nr_pages = 0;
>  	list_for_each_entry_safe(folio, folio2, from, lru) {
> @@ -1963,11 +1939,11 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>  		}
>  
>  		nr_pages += folio_nr_pages(folio);
> -		if (nr_pages > NR_MAX_BATCHED_MIGRATION)
> +		if (nr_pages >= batch)
>  			break;

Yes, the off-by-one fixes look good.

>  	}
> -	if (nr_pages > NR_MAX_BATCHED_MIGRATION)
> -		list_cut_before(&folios, from, &folio->lru);
> +	if (nr_pages >= batch)
> +		list_cut_before(&folios, from, &folio2->lru);
>  	else
>  		list_splice_init(from, &folios);
>  	rc = migrate_pages_batch(&folios, get_new_page, put_new_page, private,
> -- 
> 2.39.1