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

[IBM]

Hi Dave, 

Dave Chinner <david@xxxxxxxxxxxxx> writes:

> On Mon, Oct 28, 2024 at 06:39:36AM +0530, Ritesh Harjani wrote:
>> 
>> Hi Dave, 
>> 
>> Dave Chinner <david@xxxxxxxxxxxxx> 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@xxxxxxxxx>
>> >> ---
>> >>  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).
>
> Yes. That's my point - iomap knows that it's an atomic write, it
> knows that invalidation failed, and it knows that there is no such
> thing as buffered atomic writes. So there is no possible fallback
> here, and it should be returning EIO in the page cache invalidation
> failure case and not ENOTBLK.
>

So the iomap DIO can return following as return values which can make
some filesystems fallback to buffered-io (if they implement fallback
logic) - 
(1) -ENOTBLK -> this is only returned for pagecache invalidation failure.
(2) 0 or partial write size -> This can never happen for atomic writes
(since we are only allowing for single fsblock as of now).

Now looking at XFS, it never fallsback to buffered-io ever except just 2
cases - 
1. When pagecache invalidation fails in iomap (can never happen for
atomic writes)
2. On unaligned DIO writes to reflinked CoW (not possible for atomic writes)

So it anyways should never happen that XFS ever fallback to buffered-io
for DIO atomic writes. Even today it does not fallback to buffered-io
for non-atomic short DIO writes.

>> 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).
>
> I don't see any point in adding "impossible to hit" checks into
> filesystems just in case some core infrastructure has a bug
> introduced....

Yes, that is true for XFS. EXT4 however can return -ENOTBLK for short
writes, though it should not happen for current atomic write case where
we are only allowing for 1 fsblock. 

However given there are several places in EXT4 which has got fallback logic,
I would still like to go with a WARN_ON_ONCE where we are calling ext4
buffered-io handling for DIO fallback writes. This is to catch any bugs
even in future when we move to multi-fsblock case (until we have atomic
write support for buffered-io).

Thanks!
-ritesh