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

[Javier GonzÃlez]

On 25.11.2019 00:48, Damien Le Moal wrote:
> On 2019/11/22 18:00, Javier GonzÃlez wrote:
> > From: Javier GonzÃlez <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 GonzÃlez <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.

I did test the code you propose and, as expected, it still triggered the
corruption.

> > -
> >  	if (is_inode_flag_set(inode, FI_NO_PREALLOC))
> >  		return 0;
>
>
> --
> Damien Le Moal
> Western Digital Research

Thanks,
Javier