Re: [PATCH v2] ext4: fix a possible ABBA deadlock dued to busy PA

[brookxu <brookxu.cn@gmail.com>]

Hi Jan & riteshh:

Thanks for your time to review this patch. But kernel test robot has
reported an warning for the v2 version. I have fixed it and submitted
the v3 version, maybe we should goto v3 version, thanks all.

Jan Kara wrote on 2021/11/23 1:50:
> Hello,
> 
> it seems this patch has fallen through the cracks.
> 
> On Wed 29-09-21 09:12:25, brookxu wrote:
>> From: Chunguang Xu <brookxu@tencent.com>
>>
>> We found on older kernel (3.10) that in the scenario of insufficient
>> disk space, system may trigger an ABBA deadlock problem, it seems that
>> this problem still exists in latest kernel, try to fix it here. The
>> main process triggered by this problem is that task A occupies the PA
>> and waits for the jbd2 transaction finish, the jbd2 transaction waits
>> for the completion of task B's IO (plug_list), but task B waits for
>> the release of PA by task A to finish discard, which indirectly forms
>> an ABBA deadlock. The related calltrace is as follows:
>>
>>     Task A
>>     vfs_write
>>     ext4_mb_new_blocks()
>>     ext4_mb_mark_diskspace_used()       JBD2
>>     jbd2_journal_get_write_access()  -> jbd2_journal_commit_transaction()
>>   ->schedule()                          filemap_fdatawait()
>>  |                                              |
>>  | Task B                                       |
>>  | do_unlinkat()                                |
>>  | ext4_evict_inode()                           |
>>  | jbd2_journal_begin_ordered_truncate()        |
>>  | filemap_fdatawrite_range()                   |
>>  | ext4_mb_new_blocks()                         |
>>   -ext4_mb_discard_group_preallocations() <-----
>>
>> Here, try to cancel ext4_mb_discard_group_preallocations() internal
>> retry due to PA busy, and do a limited number of retries inside
>> ext4_mb_discard_preallocations(), which can circumvent the above
>> problems, but also has some advantages:
>>
>> 1. Since the PA is in a busy state, if other groups have free PAs,
>>    keeping the current PA may help to reduce fragmentation.
>> 2. Continue to traverse forward instead of waiting for the current
>>    group PA to be released. In most scenarios, the PA discard time
>>    can be reduced.
>>
>> However, in the case of smaller free space, if only a few groups have
>> space, then due to multiple traversals of the group, it may increase
>> CPU overhead. But in contrast, I feel that the overall benefit is
>> better than the cost.
>>
>> Signed-off-by: Chunguang Xu <brookxu@tencent.com>
> 
> Thanks for the patch! I guess this is a reasonable middle-ground so feel
> free to add:
> 
> Reviewed-by: Jan Kara <jack@suse.cz>
> 
> I think Ritesh was last touching this logic so let's check he doesn't see
> anything wrong with this change. Ritesh?
> 
> 								Honza
> 
> 
>> ---
>> v2: reset busy to zero before goto repeat.
>>
>>  fs/ext4/mballoc.c | 35 ++++++++++++++++-------------------
>>  1 file changed, 16 insertions(+), 19 deletions(-)
>>
>> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
>> index 72bfac2..72de6c1 100644
>> --- a/fs/ext4/mballoc.c
>> +++ b/fs/ext4/mballoc.c
>> @@ -4814,7 +4814,7 @@ static void ext4_mb_new_preallocation(struct ext4_allocation_context *ac)
>>   */
>>  static noinline_for_stack int
>>  ext4_mb_discard_group_preallocations(struct super_block *sb,
>> -					ext4_group_t group, int needed)
>> +				     ext4_group_t group, int needed, int *busy)
>>  {
>>  	struct ext4_group_info *grp = ext4_get_group_info(sb, group);
>>  	struct buffer_head *bitmap_bh = NULL;
>> @@ -4822,8 +4822,7 @@ static void ext4_mb_new_preallocation(struct ext4_allocation_context *ac)
>>  	struct list_head list;
>>  	struct ext4_buddy e4b;
>>  	int err;
>> -	int busy = 0;
>> -	int free, free_total = 0;
>> +	int free = 0;
>>  
>>  	mb_debug(sb, "discard preallocation for group %u\n", group);
>>  	if (list_empty(&grp->bb_prealloc_list))
>> @@ -4850,15 +4849,13 @@ static void ext4_mb_new_preallocation(struct ext4_allocation_context *ac)
>>  		needed = EXT4_CLUSTERS_PER_GROUP(sb) + 1;
>>  
>>  	INIT_LIST_HEAD(&list);
>> -repeat:
>> -	free = 0;
>>  	ext4_lock_group(sb, group);
>>  	list_for_each_entry_safe(pa, tmp,
>>  				&grp->bb_prealloc_list, pa_group_list) {
>>  		spin_lock(&pa->pa_lock);
>>  		if (atomic_read(&pa->pa_count)) {
>>  			spin_unlock(&pa->pa_lock);
>> -			busy = 1;
>> +			*busy = 1;
>>  			continue;
>>  		}
>>  		if (pa->pa_deleted) {
>> @@ -4898,22 +4895,13 @@ static void ext4_mb_new_preallocation(struct ext4_allocation_context *ac)
>>  		call_rcu(&(pa)->u.pa_rcu, ext4_mb_pa_callback);
>>  	}
>>  
>> -	free_total += free;
>> -
>> -	/* if we still need more blocks and some PAs were used, try again */
>> -	if (free_total < needed && busy) {
>> -		ext4_unlock_group(sb, group);
>> -		cond_resched();
>> -		busy = 0;
>> -		goto repeat;
>> -	}
>>  	ext4_unlock_group(sb, group);
>>  	ext4_mb_unload_buddy(&e4b);
>>  	put_bh(bitmap_bh);
>>  out_dbg:
>>  	mb_debug(sb, "discarded (%d) blocks preallocated for group %u bb_free (%d)\n",
>> -		 free_total, group, grp->bb_free);
>> -	return free_total;
>> +		 free, group, grp->bb_free);
>> +	return free;
>>  }
>>  
>>  /*
>> @@ -5455,13 +5443,22 @@ static int ext4_mb_discard_preallocations(struct super_block *sb, int needed)
>>  {
>>  	ext4_group_t i, ngroups = ext4_get_groups_count(sb);
>>  	int ret;
>> -	int freed = 0;
>> +	int freed = 0, busy = 0;
>> +	int retry = 0;
>>  
>>  	trace_ext4_mb_discard_preallocations(sb, needed);
>> + repeat:
>> +	retry++;
>>  	for (i = 0; i < ngroups && needed > 0; i++) {
>> -		ret = ext4_mb_discard_group_preallocations(sb, i, needed);
>> +		ret = ext4_mb_discard_group_preallocations(sb, i, needed, &busy);
>>  		freed += ret;
>>  		needed -= ret;
>> +		cond_resched();
>> +	}
>> +
>> +	if (needed > 0 && busy && retry < 3) {
>> +		busy = 0;
>> +		goto repeat;
>>  	}
>>  
>>  	return freed;
>> -- 
>> 1.8.3.1
>>