Re: [PATCH] f2fs: disble physical prealloc in LSF mount

[Damien Le Moal]

+ Shin'Ichiro

On 2019/11/26 15:19, Damien Le Moal wrote:
> On 2019/11/26 12:58, Javier González wrote:
>> On 26.11.2019 02:06, Damien Le Moal wrote:
>>> On 2019/11/26 4:03, Javier González wrote:
>>>> On 25.11.2019 00:48, Damien Le Moal wrote:
>>>>> On 2019/11/22 18:00, Javier González wrote:
>>>>>> From: Javier González <javier.gonz@xxxxxxxxxxx>
>>>>>>
>>>>>> Fix file system corruption when using LFS mount (e.g., in zoned
>>>>>> devices). Seems like the fallback into buffered I/O creates an
>>>>>> inconsistency if the application is assuming both read and write DIO. I
>>>>>> can easily reproduce a corruption with a simple RocksDB test.
>>>>>>
>>>>>> Might be that the f2fs_forced_buffered_io path brings some problems too,
>>>>>> but I have not seen other failures besides this one.
>>>>>>
>>>>>> Problem reproducible without a zoned block device, simply by forcing
>>>>>> LFS mount:
>>>>>>
>>>>>>   $ sudo mkfs.f2fs -f -m /dev/nvme0n1
>>>>>>   $ sudo mount /dev/nvme0n1 /mnt/f2fs
>>>>>>   $ sudo  /opt/rocksdb/db_bench  --benchmarks=fillseq --use_existing_db=0
>>>>>>   --use_direct_reads=true --use_direct_io_for_flush_and_compaction=true
>>>>>>   --db=/mnt/f2fs --num=5000 --value_size=1048576 --verify_checksum=1
>>>>>>   --block_size=65536
>>>>>>
>>>>>> Note that the options that cause the problem are:
>>>>>>   --use_direct_reads=true --use_direct_io_for_flush_and_compaction=true
>>>>>>
>>>>>> Fixes: f9d6d0597698 ("f2fs: fix out-place-update DIO write")
>>>>>>
>>>>>> Signed-off-by: Javier González <javier.gonz@xxxxxxxxxxx>
>>>>>> ---
>>>>>>  fs/f2fs/data.c | 3 ---
>>>>>>  1 file changed, 3 deletions(-)
>>>>>>
>>>>>> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
>>>>>> index 5755e897a5f0..b045dd6ab632 100644
>>>>>> --- a/fs/f2fs/data.c
>>>>>> +++ b/fs/f2fs/data.c
>>>>>> @@ -1081,9 +1081,6 @@ int f2fs_preallocate_blocks(struct kiocb *iocb, struct iov_iter *from)
>>>>>>  			return err;
>>>>>>  	}
>>>>>>
>>>>>> -	if (direct_io && allow_outplace_dio(inode, iocb, from))
>>>>>> -		return 0;
>>>>>
>>>>> Since for LFS mode, all DIOs can end up out of place, I think that it
>>>>> may be better to change allow_outplace_dio() to always return true in
>>>>> the case of LFS mode. So may be something like:
>>>>>
>>>>> static inline int allow_outplace_dio(struct inode *inode,
>>>>> 			struct kiocb *iocb, struct iov_iter *iter)
>>>>> {
>>>>> 	struct f2fs_sb_info *sbi = F2FS_I_SB(inode);
>>>>> 	int rw = iov_iter_rw(iter);
>>>>>
>>>>> 	return test_opt(sbi, LFS) ||
>>>>> 	 	(rw == WRITE && !block_unaligned_IO(inode, iocb, iter));
>>>>> }
>>>>>
>>>>> instead of the original:
>>>>>
>>>>> static inline int allow_outplace_dio(struct inode *inode,
>>>>> 			struct kiocb *iocb, struct iov_iter *iter)
>>>>> {
>>>>> 	struct f2fs_sb_info *sbi = F2FS_I_SB(inode);
>>>>> 	int rw = iov_iter_rw(iter);
>>>>>
>>>>> 	return (test_opt(sbi, LFS) && (rw == WRITE) &&
>>>>> 				!block_unaligned_IO(inode, iocb, iter));
>>>>> }
>>>>>
>>>>> Thoughts ?
>>>>>
>>>>
>>>> I see what you mean and it makes sense. However, the problem I am seeing
>>>> occurs when allow_outplace_dio() returns true, as this is what creates
>>>> the inconsistency between the write being buffered and the read being
>>>> DIO.
>>>
>>> But if the write is switched to buffered, the DIO read should use the
>>> buffered path too, no ? Since this is all happening under VFS, the
>>> generic DIO read path will not ensure that the buffered writes are
>>> flushed to disk before issuing the direct read, I think. So that would
>>> explain your data corruption, i.e. you are reading stale data on the
>>> device before the buffered writes make it to the media.
>>>
>>
>> As far as I can see, the read is always sent DIO, so yes, I also believe
>> that we are reading stale data. This is why the corruption is not seen
>> if preventing allow_outplace_dio() from sending the write to the
>> buffered path.
>>
>> What surprises me is that this is very easy to trigger (see commit), so
>> I assume you must have seen this with SMR in the past.
> 
> We just did. Shin'Ichiro in my team finally succeeded in recreating the
> problem. The cause seems to be:
> 
> bool direct_io = iocb->ki_flags & IOCB_DIRECT;
> 
> being true on entry of f2fs_preallocate_blocks() whereas
> f2fs_direct_IO() forces buffered IO path for DIO on zoned devices with:
> 
> if (f2fs_force_buffered_io(inode, iocb, iter))
> 		return 0;
> 
> which has:
> 
> 	if (f2fs_sb_has_blkzoned(sbi))
> 		return true;
> 
> So the top DIO code says "do buffered IOs", but lower in the write path,
> the IO is still assumed to be a DIO because of the iocb flag... That's
> inconsistent.
> 
> Note that for the non-zoned device LFS case, f2fs_force_buffered_io()
> returns true only for unaligned write DIOs... But that will still trip
> on the iocb flag test. So the proper fix is likely something like:
> 
> int f2fs_preallocate_blocks(struct kiocb *iocb, struct iov_iter *from)
> {
> 	struct inode *inode = file_inode(iocb->ki_filp);
> 	struct f2fs_map_blocks map;
> 	int flag;
> 	int err = 0;
> -	bool direct_io = iocb->ki_flags & IOCB_DIRECT;
> +	bool direct_io = (iocb->ki_flags & IOCB_DIRECT) &&
> +		!2fs_force_buffered_io(inode, iocb, iter);
> 
> 	/* convert inline data for Direct I/O*/
> 	if (direct_io) {
> 		err = f2fs_convert_inline_inode(inode);
> 		if (err)
> 			return err;
> 	}
> 
> Shin'Ichiro tried this on SMR disks and the failure is gone...
> 
> Cheers.
> 
> 
>>
>> Does it make sense to leave the LFS check out of the
>> allow_outplace_dio()? Or in other words, is there a hard requirement for
>> writes to take this path on a zoned device that I am not seeing?
>> Something like:
>>
>>    static inline int allow_outplace_dio(struct inode *inode,
>>    			struct kiocb *iocb, struct iov_iter *iter)
>>    {
>>    	struct f2fs_sb_info *sbi = F2FS_I_SB(inode);
>>    	int rw = iov_iter_rw(iter);
>>
>>    	return (rw == WRITE && !block_unaligned_IO(inode, iocb, iter));
>>    }
>>
>> Thanks,
>> Javier
>>
> 
> 


-- 
Damien Le Moal
Western Digital Research