Re: [PATCH 4/6] ext4: Warn if we ever fallback to buffered-io for DIO atomic writes

[Ritesh Harjani (IBM) <ritesh.list@gmail.com>]

Hi Dave, 

Dave Chinner <david@fromorbit.com> writes:

> On Fri, Oct 25, 2024 at 09:15:53AM +0530, Ritesh Harjani (IBM) wrote:
>> iomap will not return -ENOTBLK in case of dio atomic writes. But let's
>> also add a WARN_ON_ONCE and return -EIO as a safety net.
>> 
>> Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
>> ---
>>  fs/ext4/file.c | 10 +++++++++-
>>  1 file changed, 9 insertions(+), 1 deletion(-)
>> 
>> diff --git a/fs/ext4/file.c b/fs/ext4/file.c
>> index f9516121a036..af6ebd0ac0d6 100644
>> --- a/fs/ext4/file.c
>> +++ b/fs/ext4/file.c
>> @@ -576,8 +576,16 @@ static ssize_t ext4_dio_write_iter(struct kiocb *iocb, struct iov_iter *from)
>>  		iomap_ops = &ext4_iomap_overwrite_ops;
>>  	ret = iomap_dio_rw(iocb, from, iomap_ops, &ext4_dio_write_ops,
>>  			   dio_flags, NULL, 0);
>> -	if (ret == -ENOTBLK)
>> +	if (ret == -ENOTBLK) {
>>  		ret = 0;
>> +		/*
>> +		 * iomap will never return -ENOTBLK if write fails for atomic
>> +		 * write. But let's just add a safety net.
>> +		 */
>> +		if (WARN_ON_ONCE(iocb->ki_flags & IOCB_ATOMIC))
>> +			ret = -EIO;
>> +	}
>
> Why can't the iomap code return EIO in this case for IOCB_ATOMIC?
> That way we don't have to put this logic into every filesystem.

This was origially intended as a safety net hence the WARN_ON_ONCE.
Later Darrick pointed out that we still might have an unconverted
condition in iomap which can return ENOTBLK for DIO atomic writes (page
cache invalidation).

You pointed it right that it should be fixed in iomap. However do you
think filesystems can still keep this as safety net (maybe no need of
WARN_ON_ONCE).

>
> When/if we start supporting atomic writes for buffered IO, then it's
> worth pushing this out to filesystems, but right now it doesn't seem
> necessary...
>
> -Dave.
> -- 
> Dave Chinner
> david@fromorbit.com