Re: [PATCH v3 4/4] ext4: Do not fallback to buffered-io for DIO atomic write

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

"Darrick J. Wong" <djwong@kernel.org> writes:

> On Wed, Oct 30, 2024 at 09:27:41PM +0530, Ritesh Harjani (IBM) wrote:
>> atomic writes is currently only supported for single fsblock and only
>> for direct-io. We should not return -ENOTBLK for atomic writes since we
>> want the atomic write request to either complete fully or fail
>> otherwise. We should not fallback to buffered-io in case of DIO atomic
>> write requests.
>> Let's also catch if this ever happens by adding some WARN_ON_ONCE before
>> buffered-io handling for direct-io atomic writes.
>> 
>> More details of the discussion [1].
>> 
>> [1]: https://lore.kernel.org/linux-xfs/cover.1729825985.git.ritesh.list@gmail.com/T/#m9dbecc11bed713ed0d7a486432c56b105b555f04
>> 
>> Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
>> ---
>>  fs/ext4/file.c  |  7 +++++++
>>  fs/ext4/inode.c | 14 +++++++++-----
>>  2 files changed, 16 insertions(+), 5 deletions(-)
>> 
>> diff --git a/fs/ext4/file.c b/fs/ext4/file.c
>> index 8116bd78910b..61787a37e9d4 100644
>> --- a/fs/ext4/file.c
>> +++ b/fs/ext4/file.c
>> @@ -599,6 +599,13 @@ static ssize_t ext4_dio_write_iter(struct kiocb *iocb, struct iov_iter *from)
>>  		ssize_t err;
>>  		loff_t endbyte;
>>  
>> +		/*
>> +		 * There is no support for atomic writes on buffered-io yet,
>> +		 * we should never fallback to buffered-io for DIO atomic
>> +		 * writes.
>> +		 */
>> +		WARN_ON_ONCE(iocb->ki_flags & IOCB_ATOMIC);
>> +
>>  		offset = iocb->ki_pos;
>>  		err = ext4_buffered_write_iter(iocb, from);
>>  		if (err < 0)
>> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
>> index fcdee27b9aa2..26b3c84d7f64 100644
>> --- a/fs/ext4/inode.c
>> +++ b/fs/ext4/inode.c
>> @@ -3449,12 +3449,16 @@ static int ext4_iomap_end(struct inode *inode, loff_t offset, loff_t length,
>>  {
>>  	/*
>>  	 * Check to see whether an error occurred while writing out the data to
>> -	 * the allocated blocks. If so, return the magic error code so that we
>> -	 * fallback to buffered I/O and attempt to complete the remainder of
>> -	 * the I/O. Any blocks that may have been allocated in preparation for
>> -	 * the direct I/O will be reused during buffered I/O.
>> +	 * the allocated blocks. If so, return the magic error code for
>> +	 * non-atomic write so that we fallback to buffered I/O and attempt to
>> +	 * complete the remainder of the I/O.
>> +	 * For atomic writes we will simply fail the I/O request if we coudn't
>> +	 * write anything. For non-atomic writes, any blocks that may have been
>> +	 * allocated in preparation for the direct I/O will be reused during
>> +	 * buffered I/O.
>>  	 */
>> -	if (flags & (IOMAP_WRITE | IOMAP_DIRECT) && written == 0)
>> +	if (!(flags & IOMAP_ATOMIC) && (flags & (IOMAP_WRITE | IOMAP_DIRECT))
>
> Huh.  The WRITE|DIRECT check doesn't look right to me, because the
> expression returns true for any write or any directio.  I think that's
> currently "ok" because ext4_iomap_end is only called for directio
> writes, but this bugs me anyway.  For a directio write fallback, that
> comparison really should be:
>
> 	(flags & (WRITE|DIRECT)) == (WRITE|DIRECT)
>

yes. You are right. It is working since ext4 only supports iomap
for DIRECTIO. But I agree it's better be fixed to avoid problem in future.

> static inline bool
> ext4_want_directio_fallback(unsigned flags, ssize_t written)
> {
> 	/* must be a directio to fall back to buffered */
> 	if (flags & (IOMAP_WRITE | IOMAP_DIRECT)) !=
> 		    (IOMAP_WRITE | IOMAP_DIRECT)
> 		return false;
>
> 	/* atomic writes are all-or-nothing */
> 	if (flags & IOMAP_ATOMIC)
> 		return false;
>
> 	/* can only try again if we wrote nothing */
> 	return written == 0;
> }
>
> 	if (ext4_want_directio_fallback(flags, written))
> 		return -ENOTBLK;
>

I like the above helper. Thanks for doing that. 
I will incorporate this in v4.


>> +			&& written == 0)
>
> Nit: put the '&&' operator on the previous line when there's a multiline
> expression.
>

I guess we don't need this if we do it with your above inline helper.
But sure, next time will keep in mind for any such changes.

> --D
>

Thanks for the review!
-ritesh

>>  		return -ENOTBLK;
>>  
>>  	return 0;
>> -- 
>> 2.46.0
>> 
>>