Re: [PATCH] f2fs: remove broken support for allocating DIO writes

[Chao Yu <chao@kernel.org>]

On 2021/8/21 2:11, Eric Biggers wrote:
> On Fri, Aug 20, 2021 at 05:35:21PM +0800, Chao Yu wrote:
>>>>>>
>>>>>> Hmm, I'm still trying to deal with this as a corner case where the writes
>>>>>> haven't completed due to an error. How about keeping the preallocated block
>>>>>> offsets and releasing them if we get an error? Do we need to handle EIO right?
>>>>>
>>>>> What about the case that CP + SPO following DIO preallocation? User will
>>>>> encounter uninitialized block after recovery.
>>>>
>>>> I think buffered writes as a workaround can expose the last unwritten block as
>>>> well, if SPO happens right after block allocation. We may need to compromise
>>>> at certain level?
>>>>
>>>
>>> Freeing preallocated blocks on error would be better than nothing, although note
>>> that the preallocated blocks may have filled an arbitrary sequence of holes --
>>> so simply truncating past EOF would *not* be sufficient.
>>>
>>> But really filesystems need to be designed to never expose uninitialized data,
>>> even if I/O errors or a sudden power failure occurs.  It is unfortunate that
>>> f2fs apparently wasn't designed with that goal in mind.
>>>
>>> In any case, I don't think we can proceed with any other f2fs direct I/O
>>> improvements until this data leakage bug can be solved one way or another.  If
>>> my patch to remove support for allocating writes isn't acceptable and the
>>> desired solution is going to require some more invasive f2fs surgery, are you or
>>> Chao going to work on it?  I'm not sure there's much I can do here.
>>
>> I may have time to take look into the implementation as I proposed above, maybe
>> just enabling this in FSYNC_MODE_STRICT mode if user concerns unwritten data?
>> thoughts?
>>
> 
> What does this have to do with fsync?

Oops, maybe a separate option is more appropriate.

> 
> - Eric
>