Re: BTRFS: Transaction aborted (error -24)

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

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On 2020/6/11 下午9:52, David Sterba wrote:
> On Thu, Jun 11, 2020 at 08:37:11PM +0800, Qu Wenruo wrote:
>>
>>
>> On 2020/6/11 下午7:20, David Sterba wrote:
>>> On Thu, Jun 11, 2020 at 06:29:34PM +0800, Greed Rong wrote:
>>>> Hi,
>>>> I have got this error several times. Are there any suggestions to avoid this?
>>>>
>>>> # dmesg
>>>> [7142286.563596] ------------[ cut here ]------------
>>>> [7142286.564499] BTRFS: Transaction aborted (error -24)
>>>
>>> EMFILE          24      /* Too many open files */
>>>
>>> you can increase the open file limit but it's strange that this happens,
>>> first time I see this.
>>
>> Not something from btrfs code, thus it must come from the VFS/MM code.
> 
> Yeah, this is VFS. Creating a new root will need a new inode and dentry
> and the limits are applied.
> 
>> The offending abort transaction is from btrfs_read_fs_root_no_name(),
>> which is updated to btrfs_get_fs_root() in upstream kernel.
>> Overall, it's not much different between the upstream and the 5.0.10 kernel.
>>
>> But with latest btrfs_get_fs_root(), after a quick glance, there isn't
>> any obvious location to introduce the EMFILE error.
>>
>> Any extra info about the worload to trigger the bug?
> 
> I think it's from get_anon_bdev, that's called from btrfs_init_fs_root
> (in btrfs_get_fs_root):
> 
> 1073 int get_anon_bdev(dev_t *p)
> 1074 {
> 1075         int dev;
> 1076
> 1077         /*
> 1078          * Many userspace utilities consider an FSID of 0 invalid.
> 1079          * Always return at least 1 from get_anon_bdev.
> 1080          */
> 1081         dev = ida_alloc_range(&unnamed_dev_ida, 1, (1 << MINORBITS) - 1,
> 1082                         GFP_ATOMIC);
> 1083         if (dev == -ENOSPC)
> 1084                 dev = -EMFILE;
> 1085         if (dev < 0)
> 1086                 return dev;
> 1087
> 1088         *p = MKDEV(0, dev);
> 1089         return 0;
> 1090 }
> 1091 EXPORT_SYMBOL(get_anon_bdev);
> 
> And comment says "Return: 0 on success, -EMFILE if there are no
> anonymous bdevs left ".
> 
> The fs tree roots are created later than the actual command is executed,
> so all the errors are also delayed. For that reason I moved eg. the root
> item and path allocation to the first phase. We could do the same for
> the anonymous bdev.

The first question is, do we really need per-root anonymous bdev?

IMHO btrfs can shared the same anonymous bdev across the same fs, no
need for each root to own one.

The user-visible change would be, statefs() will alwasy return the same
bdev for all roots.
User would lose the ability to distinguish different roots from the same
fs, but I doubt if that would really impact the use cases.

Thanks,
Qu

> 
> The problem won't go away tough, the question is why is the IDA range
> unnamed_dev_ida exhausted.
> 


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