Re: [PATCH] f2fs: Fix deadlock under storage almost full/dirty condition

[Chao Yu]

Hi Sahitya,

On 2019/11/11 14:44, Sahitya Tummala wrote:
> Hi Chao,
> 
> On Mon, Nov 11, 2019 at 02:28:47PM +0800, Chao Yu wrote:
>> Hi Sahitya,
>>
>> On 2019/11/11 11:40, Sahitya Tummala wrote:
>>> Hi Chao,
>>>
>>> On Mon, Nov 11, 2019 at 10:51:10AM +0800, Chao Yu wrote:
>>>> On 2019/11/8 19:03, Sahitya Tummala wrote:
>>>>> There could be a potential deadlock when the storage capacity
>>>>> is almost full and theren't enough free segments available, due
>>>>> to which FG_GC is needed in the atomic commit ioctl as shown in
>>>>> the below callstack -
>>>>>
>>>>> schedule_timeout
>>>>> io_schedule_timeout
>>>>> congestion_wait
>>>>> f2fs_drop_inmem_pages_all
>>>>> f2fs_gc
>>>>> f2fs_balance_fs
>>>>> __write_node_page
>>>>> f2fs_fsync_node_pages
>>>>> f2fs_do_sync_file
>>>>> f2fs_ioctl
>>>>>
>>>>> If this inode doesn't have i_gc_failures[GC_FAILURE_ATOMIC] set,
>>>>> then it waits forever in f2fs_drop_inmem_pages_all(), for this
>>>>> atomic inode to be dropped. And the rest of the system is stuck
>>>>> waiting for sbi->gc_mutex lock, which is acquired by f2fs_balance_fs()
>>>>> in the stack above.
>>>>
>>>> I think the root cause of this issue is there is potential infinite loop in
>>>> f2fs_drop_inmem_pages_all() for the case of gc_failure is true, because once the
>>>> first inode in inode_list[ATOMIC_FILE] list didn't suffer gc failure, we will
>>>> skip dropping its in-memory cache and calling iput(), and traverse the list
>>>> again, most possibly there is the same inode in the head of that list.
>>>>
>>>
>>> I thought we are expecting for those atomic updates (without any gc failures) to be
>>> committed by doing congestion_wait() and thus retrying again. Hence, I just
>>
>> Nope, we only need to drop inode which encounter gc failures, and keep the rest
>> inodes.
>>
>>> fixed only if we are ending up waiting for commit to happen in the atomic
>>> commit path itself, which will be a deadlock.
>>
>> Look into call stack you provide, I don't think it's correct to drop such inode,
>> as its dirty pages should be committed before f2fs_fsync_node_pages(), so
>> calling f2fs_drop_inmem_pages won't release any inmem pages, and won't help
>> looped GC caused by skipping due to inmem pages.
>>
>> And then I figure out below fix...
>>
> 
> Thanks for the explanation.
> The fix below looks good to me.

So could you submit v2? :)

Thanks,

> 
> Thanks,
> Sahitya.
> 
>>>
>>>> Could you please check below fix:
>>>>
>>>> diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
>>>> index 7bf7b0194944..8a3a35b42a37 100644
>>>> --- a/fs/f2fs/f2fs.h
>>>> +++ b/fs/f2fs/f2fs.h
>>>> @@ -1395,6 +1395,7 @@ struct f2fs_sb_info {
>>>>  	unsigned int gc_mode;			/* current GC state */
>>>>  	unsigned int next_victim_seg[2];	/* next segment in victim section */
>>>>  	/* for skip statistic */
>>>> +	unsigned int atomic_files;		/* # of opened atomic file */
>>>>  	unsigned long long skipped_atomic_files[2];	/* FG_GC and BG_GC */
>>>>  	unsigned long long skipped_gc_rwsem;		/* FG_GC only */
>>>>
>>>> diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
>>>> index ecd063239642..79f4b348951a 100644
>>>> --- a/fs/f2fs/file.c
>>>> +++ b/fs/f2fs/file.c
>>>> @@ -2047,6 +2047,7 @@ static int f2fs_ioc_start_atomic_write(struct file *filp)
>>>>  	spin_lock(&sbi->inode_lock[ATOMIC_FILE]);
>>>>  	if (list_empty(&fi->inmem_ilist))
>>>>  		list_add_tail(&fi->inmem_ilist, &sbi->inode_list[ATOMIC_FILE]);
>>>> +	sbi->atomic_files++;
>>>>  	spin_unlock(&sbi->inode_lock[ATOMIC_FILE]);
>>>>
>>>>  	/* add inode in inmem_list first and set atomic_file */
>>>> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
>>>> index 8b977bbd6822..6aa0bb693697 100644
>>>> --- a/fs/f2fs/segment.c
>>>> +++ b/fs/f2fs/segment.c
>>>> @@ -288,6 +288,8 @@ void f2fs_drop_inmem_pages_all(struct f2fs_sb_info *sbi,
>>>> bool gc_failure)
>>>>  	struct list_head *head = &sbi->inode_list[ATOMIC_FILE];
>>>>  	struct inode *inode;
>>>>  	struct f2fs_inode_info *fi;
>>>> +	unsigned int count = sbi->atomic_files;
>>>
>>> If the sbi->atomic_files decrements just after this, then the below exit condition
>>> may not work. In that case, looped will never be >= count.
>>>
>>>> +	unsigned int looped = 0;
>>>>  next:
>>>>  	spin_lock(&sbi->inode_lock[ATOMIC_FILE]);
>>>>  	if (list_empty(head)) {
>>>> @@ -296,22 +298,29 @@ void f2fs_drop_inmem_pages_all(struct f2fs_sb_info *sbi,
>>>> bool gc_failure)
>>>>  	}
>>>>  	fi = list_first_entry(head, struct f2fs_inode_info, inmem_ilist);
>>>>  	inode = igrab(&fi->vfs_inode);
>>>> +	if (inode)
>>>> +		list_move_tail(&fi->inmem_ilist, head);
>>>>  	spin_unlock(&sbi->inode_lock[ATOMIC_FILE]);
>>>>
>>>>  	if (inode) {
>>>>  		if (gc_failure) {
>>>> -			if (fi->i_gc_failures[GC_FAILURE_ATOMIC])
>>>> -				goto drop;
>>>> -			goto skip;
>>>> +			if (!fi->i_gc_failures[GC_FAILURE_ATOMIC])
>>>> +				goto skip;
>>>>  		}
>>>> -drop:
>>>>  		set_inode_flag(inode, FI_ATOMIC_REVOKE_REQUEST);
>>>>  		f2fs_drop_inmem_pages(inode);
>>>> +skip:
>>>>  		iput(inode);
>>>
>>> Does this result into f2fs_evict_inode() in this context for this inode?
>>
>> Yup, we need to call igrab/iput in pair in f2fs_drop_inmem_pages_all() anyway.
>>
>> Previously, we may have .i_count leak...
>>
>> Thanks,
>>
>>>
>>> thanks,
>>>
>>>>  	}
>>>> -skip:
>>>> +
>>>>  	congestion_wait(BLK_RW_ASYNC, HZ/50);
>>>>  	cond_resched();
>>>> +
>>>> +	if (gc_failure) {
>>>> +		if (++looped >= count)
>>>> +			return;
>>>> +	}
>>>> +
>>>>  	goto next;
>>>>  }
>>>>
>>>> @@ -334,6 +343,7 @@ void f2fs_drop_inmem_pages(struct inode *inode)
>>>>  	spin_lock(&sbi->inode_lock[ATOMIC_FILE]);
>>>>  	if (!list_empty(&fi->inmem_ilist))
>>>>  		list_del_init(&fi->inmem_ilist);
>>>> +	sbi->atomic_files--;
>>>>  	spin_unlock(&sbi->inode_lock[ATOMIC_FILE]);
>>>>  }
>>>>
>>>> Thanks,
>>>
>