Re: [PATCH] mm/migrate: fix deadlock in migrate_pages_batch() on large folios

[Gao Xiang <hsiangkao@linux.alibaba.com>]

Hi Ying,

On 2024/7/29 09:38, Huang, Ying wrote:
> Hi, Xiang,
> 
> Gao Xiang <hsiangkao@linux.alibaba.com> writes:
> 
>> Currently, migrate_pages_batch() can lock multiple locked folios
>> with an arbitrary order.  Although folio_trylock() is used to avoid
>> deadlock as commit 2ef7dbb26990 ("migrate_pages: try migrate in batch
>> asynchronously firstly") mentioned, it seems try_split_folio() is
>> still missing.
>>
>> It was found by compaction stress test when I explicitly enable EROFS
>> compressed files to use large folios, which case I cannot reproduce with
>> the same workload if large folio support is off (current mainline).
>> Typically, filesystem reads (with locked file-backed folios) could use
>> another bdev/meta inode to load some other I/Os (e.g. inode extent
>> metadata or caching compressed data), so the locking order will be:
>>
>>    file-backed folios  (A)
>>       bdev/meta folios (B)
>>
>> The following calltrace shows the deadlock:
>>     Thread 1 takes (B) lock and tries to take folio (A) lock
>>     Thread 2 takes (A) lock and tries to take folio (B) lock
>>
>> [Thread 1]
>> INFO: task stress:1824 blocked for more than 30 seconds.
>>        Tainted: G           OE      6.10.0-rc7+ #6
>> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>> task:stress          state:D stack:0     pid:1824  tgid:1824  ppid:1822   flags:0x0000000c
>> Call trace:
>>   __switch_to+0xec/0x138
>>   __schedule+0x43c/0xcb0
>>   schedule+0x54/0x198
>>   io_schedule+0x44/0x70
>>   folio_wait_bit_common+0x184/0x3f8
>> 			<-- folio mapping ffff00036d69cb18 index 996  (**)
>>   __folio_lock+0x24/0x38
>>   migrate_pages_batch+0x77c/0xea0	// try_split_folio (mm/migrate.c:1486:2)
>> 						// migrate_pages_batch (mm/migrate.c:1734:16)
>> 		<--- LIST_HEAD(unmap_folios) has
>> 			..
>> 			folio mapping 0xffff0000d184f1d8 index 1711;   (*)
>> 			folio mapping 0xffff0000d184f1d8 index 1712;
>> 			..
>>   migrate_pages+0xb28/0xe90
>>   compact_zone+0xa08/0x10f0
>>   compact_node+0x9c/0x180
>>   sysctl_compaction_handler+0x8c/0x118
>>   proc_sys_call_handler+0x1a8/0x280
>>   proc_sys_write+0x1c/0x30
>>   vfs_write+0x240/0x380
>>   ksys_write+0x78/0x118
>>   __arm64_sys_write+0x24/0x38
>>   invoke_syscall+0x78/0x108
>>   el0_svc_common.constprop.0+0x48/0xf0
>>   do_el0_svc+0x24/0x38
>>   el0_svc+0x3c/0x148
>>   el0t_64_sync_handler+0x100/0x130
>>   el0t_64_sync+0x190/0x198
>>
>> [Thread 2]
>> INFO: task stress:1825 blocked for more than 30 seconds.
>>        Tainted: G           OE      6.10.0-rc7+ #6
>> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>> task:stress          state:D stack:0     pid:1825  tgid:1825  ppid:1822   flags:0x0000000c
>> Call trace:
>>   __switch_to+0xec/0x138
>>   __schedule+0x43c/0xcb0
>>   schedule+0x54/0x198
>>   io_schedule+0x44/0x70
>>   folio_wait_bit_common+0x184/0x3f8
>> 			<-- folio = 0xfffffdffc6b503c0 (mapping == 0xffff0000d184f1d8 index == 1711) (*)
>>   __folio_lock+0x24/0x38
>>   z_erofs_runqueue+0x384/0x9c0 [erofs]
>>   z_erofs_readahead+0x21c/0x350 [erofs]       <-- folio mapping 0xffff00036d69cb18 range from [992, 1024] (**)
>>   read_pages+0x74/0x328
>>   page_cache_ra_order+0x26c/0x348
>>   ondemand_readahead+0x1c0/0x3a0
>>   page_cache_sync_ra+0x9c/0xc0
>>   filemap_get_pages+0xc4/0x708
>>   filemap_read+0x104/0x3a8
>>   generic_file_read_iter+0x4c/0x150
>>   vfs_read+0x27c/0x330
>>   ksys_pread64+0x84/0xd0
>>   __arm64_sys_pread64+0x28/0x40
>>   invoke_syscall+0x78/0x108
>>   el0_svc_common.constprop.0+0x48/0xf0
>>   do_el0_svc+0x24/0x38
>>   el0_svc+0x3c/0x148
>>   el0t_64_sync_handler+0x100/0x130
>>   el0t_64_sync+0x190/0x198
>>
>> Fixes: 5dfab109d519 ("migrate_pages: batch _unmap and _move")
>> Cc: "Huang, Ying" <ying.huang@intel.com>
>> Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
>> ---
>>   mm/migrate.c | 3 ++-
>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index 20cb9f5f7446..a912e4b83228 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -1483,7 +1483,8 @@ static inline int try_split_folio(struct folio *folio, struct list_head *split_f
>>   {
>>   	int rc;
>>   
>> -	folio_lock(folio);
>> +	if (!folio_trylock(folio))
>> +		return -EAGAIN;
>>   	rc = split_folio_to_list(folio, split_folios);
>>   	folio_unlock(folio);
>>   	if (!rc)
> 
> Good catch!  Thanks for the fixing!
> 
> The deadlock is similar as the one we fixed in commit fb3592c41a44
> ("migrate_pages: fix deadlock in batched migration").  But apparently,
> we missed this case.
> 
> For the fix, I think that we should still respect migrate_mode because
> users may prefer migration success over blocking.
> 
> @@ -1492,11 +1492,17 @@ static int unmap_and_move_huge_page(new_folio_t get_new_folio,
>   	return rc;
>   }
>   
> -static inline int try_split_folio(struct folio *folio, struct list_head *split_folios)
> +static inline int try_split_folio(struct folio *folio, struct list_head *split_folios,
> +				  enum migrate_mode mode)
>   {
>   	int rc;
>   
> -	folio_lock(folio);
> +	if (mode == MIGRATE_ASYNC) {
> +		if (!folio_trylock(folio))
> +			return -EAGAIN;
> +	} else {
> +		folio_lock(folio);
> +	}
>   	rc = split_folio_to_list(folio, split_folios);
>   	folio_unlock(folio);
>   	if (!rc)

Okay, yeah it looks better since it seems I missed the fallback
part in migrate_pages_sync().

Let me send the next version to follow your advice, thanks.

Thanks,
Gao Xiang


> 
> 
> --
> Best Regards,
> Huang, Ying