Re: [PATCH v3 17/21] iomap: Atomic write support

[John Garry <john.g.garry@oracle.com>]

On 01/05/2024 02:47, Dave Chinner wrote:
> On Mon, Apr 29, 2024 at 05:47:42PM +0000, John Garry wrote:
>> Support atomic writes by producing a single BIO with REQ_ATOMIC flag set.
>>
>> We rely on the FS to guarantee extent alignment, such that an atomic write
>> should never straddle two or more extents. The FS should also check for
>> validity of an atomic write length/alignment.
>>
>> Signed-off-by: John Garry <john.g.garry@oracle.com>
>> ---
>>   fs/iomap/direct-io.c | 10 ++++++++++
>>   1 file changed, 10 insertions(+)
>>
>> diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
>> index a3ed7cfa95bc..d7bdeb675068 100644
>> --- a/fs/iomap/direct-io.c
>> +++ b/fs/iomap/direct-io.c
>> @@ -275,6 +275,7 @@ static inline blk_opf_t iomap_dio_bio_opflags(struct iomap_dio *dio,
>>   static loff_t iomap_dio_bio_iter(const struct iomap_iter *iter,
>>   		struct iomap_dio *dio)
>>   {
>> +	bool is_atomic = dio->iocb->ki_flags & IOCB_ATOMIC;
>>   	const struct iomap *iomap = &iter->iomap;
>>   	struct inode *inode = iter->inode;
>>   	unsigned int zeroing_size, pad;
>> @@ -387,6 +388,9 @@ static loff_t iomap_dio_bio_iter(const struct iomap_iter *iter,
>>   		bio->bi_iter.bi_sector = iomap_sector(iomap, pos);
>>   		bio->bi_write_hint = inode->i_write_hint;
>>   		bio->bi_ioprio = dio->iocb->ki_ioprio;
>> +		if (is_atomic)
>> +			bio->bi_opf |= REQ_ATOMIC;
> 
> REQ_ATOMIC is only valid for write IO, isn't it?

yes, it is. We reject RWF_ATOMIC for a READ.

> 
> This should be added in iomap_dio_bio_opflags() after it is
> determined we are doing a write operation.  Regardless, it should be
> added in iomap_dio_bio_opflags(), not here. That also allows us to
> get rid of the is_atomic variable.

ok

> 
>> +
>>   		bio->bi_private = dio;
>>   		bio->bi_end_io = iomap_dio_bio_end_io;
>>   
>> @@ -403,6 +407,12 @@ static loff_t iomap_dio_bio_iter(const struct iomap_iter *iter,
>>   		}
>>   
>>   		n = bio->bi_iter.bi_size;
>> +		if (is_atomic && n != orig_count) {
>> +			/* This bio should have covered the complete length */
>> +			ret = -EINVAL;
>> +			bio_put(bio);
>> +			goto out;
>> +		}
> 
> What happens now if we've done zeroing IO before this? I suspect we
> might expose stale data if the partial block zeroing converts the
> unwritten extent in full...

We use iomap_dio.ref to ensure that __iomap_dio_rw() does not return 
until any zeroing and actual sub-io block write completes. See 
iomap_dio_zero() -> iomap_dio_submit_bio() -> atomic_inc(&dio->ref) 
callchain. I meant to add such info to the commit message, as you 
questioned this previously.

> 
>>   		if (dio->flags & IOMAP_DIO_WRITE) {
>>   			task_io_account_write(n);
>>   		} else {
> 
> Ignoring the error handling issues, this code might be better as:
> 
> 		if (dio->flags & IOMAP_DIO_WRITE) {
> 			if ((opflags & REQ_ATOMIC) && n != orig_count) {
> 				/* atomic writes are all or nothing */
> 				ret = -EIO
> 				bio_put(bio);
> 				goto out;
> 			}
> 		}
> 
> so that we are not putting atomic write error checks in the read IO
> submission path.
> 

Maybe, I'll look at a rework with the suggested change to use 
iomap_dio_bio_opflags() - I actually thought that I introduced a change 
to use iomap_dio_bio_opflags() previously...

BTW, we need to return -EINVAL, as this is what userspace expects for 
such an error.

Thanks,
John