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

["Javier González" <javier@javigon.com>]

On 26.11.2019 06:20, Damien Le Moal wrote:
>+ 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@samsung.com>
>>>>>>>
>>>>>>> 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@samsung.com>
>>>>>>> ---
>>>>>>>  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.
>>

Yes! This is it. I originally though that the problem was on
f2fs_force_buffered_io(), but could not hit the problem there. Thanks
for the analysis; it makes sense now.

Just tested your patch on our drives and the problem is gone too. Guess
you can send a new patch an ignore this one. You can set my reviewed-by
on it.

Thanks Damien!
Javier


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