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

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

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.

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


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