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

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

Hi, Hugh,

Thank you very much for review!

Hugh Dickins <hughd@google.com> writes:

> 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.

Firstly, I would have thought that we have no opportunities to batch the
TLB flushing now.  But as you pointed out, it is still possible to batch
if mapcount > 1.  Secondly, without TTU_BATCH_FLUSH, we may flush the
TLB for a single page (with invlpg instruction), otherwise, we will
flush the TLB for all pages.  The former is faster and will not
influence other TLB entries of the process.

Or we use TTU_BATCH_FLUSH only if mapcount > 1?

>
>>  		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?

Yes.  Now, there's HPAGE_PMD_NR folios in "from" list.

And, in the later patch, I just move the logic out of this function.
The split_folios is return to the caller, and the caller will call
migrate_pages_batch() again to migrate "split_folios" with MIGRATE_ASYNC
mode.

>>  		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.

Thanks!

>>  	}
>> -	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,
>> -- 

Best Regards,
Huang, Ying