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

From: Javier GonzÃlez
Date: Mon Nov 25 2019 - 14:03:27 EST


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