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Message-ID: <20191126035726.xj7pierxsck6adow@mpHalley>
Date: Tue, 26 Nov 2019 04:57:26 +0100
From: Javier González <javier@...igon.com>
To: Damien Le Moal <Damien.LeMoal@....com>
Cc: "jaegeuk@...nel.org" <jaegeuk@...nel.org>,
"yuchao0@...wei.com" <yuchao0@...wei.com>,
"linux-f2fs-devel@...ts.sourceforge.net"
<linux-f2fs-devel@...ts.sourceforge.net>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
Javier González <javier.gonz@...sung.com>
Subject: Re: [PATCH] f2fs: disble physical prealloc in LSF mount
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@...sung.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@...sung.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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