Re: [RFC 4/4] xfs: support dma-buf registered buffers for direct I/O

[Pavel Begunkov <asml.silence@gmail.com>]

On 10/16/26 10:28, Christoph Hellwig wrote:
> On Fri, Oct 16, 2026 at 02:49:14PM +0530, Anuj Gupta wrote:
>> From: Nitesh Shetty <nj.shetty@samsung.com>
>>
>> Add the init_dma_buf_io_ctx fop for XFS files.
>> XFS picks the device and calls the iomap layer with
>> the relevant device.
>>
>> Refuse the iterator from every CPU-copy path: DAX, buffered
>> fallback from a -ENOTBLK direct write, and read/writes
>> without O_DIRECT.
>>
>> If a file is switched between the data and realtime devices
>> after registration, the block layer cross-device check refuses
>> the I/O.
>>
>> Signed-off-by: Nitesh Shetty <nj.shetty@samsung.com>
>> ---
>>   fs/xfs/xfs_file.c | 30 ++++++++++++++++++++++++++++++
>>   1 file changed, 30 insertions(+)
>>
>> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
>> index 426a67b813a7..f97d5acfaf68 100644
>> --- a/fs/xfs/xfs_file.c
>> +++ b/fs/xfs/xfs_file.c
>> @@ -340,6 +340,11 @@ xfs_file_read_iter(
>>   	if (xfs_is_shutdown(mp))
>>   		return -EIO;
>>   
>> +	/* a device mapping can only be read by a direct I/O bio */
>> +	if (iov_iter_is_dmabuf_map(to) &&
>> +	    (IS_DAX(inode) || !(iocb->ki_flags & IOCB_DIRECT)))
>> +		return -EOPNOTSUPP;
> 
> Maybe move this into the actual methods for dax/buffered instead
> to avoid extra conditionals?  Same for the write side.
> 
>>   
>> @@ -1209,6 +1219,9 @@ xfs_file_write_iter(
>>   		ret = xfs_file_dio_write(iocb, from);
>>   		if (ret != -ENOTBLK)
>>   			return ret;
>> +		/* the page cache cannot copy from a device mapping */
>> +		if (iov_iter_is_dmabuf_map(from))
>> +			return -EOPNOTSUPP;
> 
> I suspect these random fallbacks are going to cause trouble, but
> I'm not really sure what to do about it either.

Folks did mention it to me before. For a slow path we can try
do something like amdxdna_cbuf_clear() does:

dma_buf_vmap(dbuf, &vmap);
if (!vmap.vaddr)
	return -EFAULT;
memset(vmap.vaddr, 0, dbuf->size);
dma_buf_vunmap(dbuf, &vmap);

But I can already imagine it triggering some interesting
behaviour like buffer migration, which will be too penalising
for things like clear on a hole. And we'll need to be careful
about deadlocking here.

-- 
Pavel Begunkov