Re: [f2fs-dev] F2fs panic | update_sit_entry() | no free segment | se invalid

[Chao Yu <yuchao0@huawei.com>]

On 2020/2/17 13:08, 王矛 wrote:
> 
> Hi Chao,
> 
> Thanks for your prompt reply.
> 1. Is it possible that when the issue occurred, there is indeed all free segment
> used up?

No, f2fs reserved some sections for foreground GC, no one should use up those
reserved sections, though foreground GC may use them temporarily to migrate blocks.

>     (End user used up all free segments, system core service(uid root) used up
> all reserved segments)
> 
>     If this maybe true, it would lead such crash issue, is this right?
>     I mean if we can add more detailed check to see if there is free segment and
> return ENOSPC in time before hit crash.
> 2. In data/node write path, it will call has_not_enough_free_secs(sbi, 0, 0) to
> make sure there is enough segments.

That is for reclaim path in where wbc->for_reclaim should be true.

But anyway, in data/node write path, we may call f2fs_balance_fs() to check
foreground GC water line.

> But in this crash callstack, it crashed in
> do_write_page()->f2fs_allocate_data_block().
> Is there any corner case that has_not_enough_free_secs() is missed during this
> write procedure?

I've no idea, one way is to check whether there is a determinate method to
reproduce this issue, if there is, maybe I can check this issue in more details.

Thanks,

> 
> Thanks,
> Mao
> 
> 
> At 2020-02-17 11:50:56, "Chao Yu" <yuchao0@huawei.com> wrote:
>>Hi 王矛，
>>
>>On 2020/2/16 19:39, 王矛 wrote:
>>> *So the problem is:*
>>> 
>>> 1. in new_curseg(), if the segno allocated is invalid(no free segment, max segno
>>> is returned).
>>> 
>>> F2fs should do something to indicate this exception.
>>> 
>>> 2. otherwise, we may hit the f2fs panic(se invalid).
>>> 
>>> Maybe we should do sanity check in update_sit_entry() to see if segno is really
>>> out of range and caused this panic.
>>
>>I'm afraid it's too late to handle such error in update_sit_entry(), since we
>>expect all procedures in do_write_page() will be successful, it's a little hard
>>to handle such error in that context.
>>
>>So the problem here is why we can not find any free segments w/ LFS allocation,
>>because in case of lack of free segments (check via has_not_enough_free_secs()),
>>f2fs will force to trigger f2fs_gc() to recycle free sections.
>>
>>I doubt there may be some corner case we haven't considered, result all free
>>segments (including reserved free segments) was exhausted by data/node writes
>>when last checkpoint is triggered during umount.
>>
>>If this issue can be reproduced (during umount, free_segments() <
>>reserved_segments()), we can add some logs to see why f2fs_balance_fs() fail to
>>recycle enough free segments previously.
>>
>>Thanks,
>>
>>> 
>>> 
>>> Thanks,
>>> Mao
>>> 
>>> 
>>> 
>>>  
>>> 
> 
> 
> 
>  
> 


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