Re: [RFC 1/7] iomap: Don't fall back to buffered write if the write is atomic

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

On 30/11/2023 21:10, Dave Chinner wrote:
> On Thu, Nov 30, 2023 at 07:23:09PM +0530, Ojaswin Mujoo wrote:
>> Currently, iomap only supports atomic writes for direct IOs and there is
>> no guarantees that a buffered IO will be atomic. Hence, if the user has
>> explicitly requested the direct write to be atomic and there's a
>> failure, return -EIO instead of falling back to buffered IO.
>>
>> Signed-off-by: Ojaswin Mujoo<ojaswin@linux.ibm.com>
>> ---
>>   fs/iomap/direct-io.c | 8 +++++++-
>>   1 file changed, 7 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
>> index 6ef25e26f1a1..3e7cd9bc8f4d 100644
>> --- a/fs/iomap/direct-io.c
>> +++ b/fs/iomap/direct-io.c
>> @@ -662,7 +662,13 @@ __iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
>>   			if (ret != -EAGAIN) {
>>   				trace_iomap_dio_invalidate_fail(inode, iomi.pos,
>>   								iomi.len);
>> -				ret = -ENOTBLK;
>> +				/*
>> +				 * if this write was supposed to be atomic,
>> +				 * return the err rather than trying to fall
>> +				 * back to buffered IO.
>> +				 */
>> +				if (!atomic_write)
>> +					ret = -ENOTBLK;
> This belongs in the caller when it receives an -ENOTBLK from
> iomap_dio_rw(). The iomap code is saying "this IO cannot be done
> with direct IO" by returning this value, and then the caller can
> make the determination of whether to run a buffered IO or not.
> 
> For example, a filesystem might still be able to perform an atomic
> IO via a COW-based buffered IO slow path. Sure, ext4 can't do this,
> but the above patch would prevent filesystems that could from being
> able to implement such a fallback....

Sure, and I think that we need a better story for supporting buffered IO 
for atomic writes.

Currently we have:
- man pages tell us RWF_ATOMIC is only supported for direct IO
- statx gives atomic write unit min/max, not explicitly telling us it's 
for direct IO
- RWF_ATOMIC is ignored for !O_DIRECT

So I am thinking of expanding statx support to enable querying of atomic 
write capabilities for buffered IO and direct IO separately.

Thanks,
John