Re: [PATCH] fs: report direct io constraints through file_getattr

[Keith Busch <kbusch@kernel.org>]

On Thu, Jul 09, 2026 at 09:13:52AM +0200, Christoph Hellwig wrote:
> On Tue, Jul 07, 2026 at 06:18:43PM -0700, Keith Busch wrote:
> 
> > +	fa->fsx_dio_mem_align = bdev_dma_alignment(bdev) + 1;
> > +	fa->fsx_dio_offset_align = bdev_logical_block_size(bdev);
> > +	fa->fsx_dio_read_offset_align = bdev_logical_block_size(bdev);
> > +	fa->fsx_dio_virt_boundary_align = bdev_virt_boundary_alignment(bdev);
> > +	fa->fsx_max_segments = bdev_max_segments(bdev);
> 
> How is the max_segments value defined in a way that is meaningful to
> userspace?

It tells you how many sub-sector vectors you can submit in your
readv/writev before it needs to add up to a logical block size.

Ex: 4k logical block size, 4 byte DMA, 256 max segments. You can define
4-byte iov's in your command, but you'll hit the max segment count
before you have a valid IO if they're all that small.
 
> > @@ -145,6 +155,8 @@ static int file_attr_to_fileattr(const struct file_attr *fattr,
> >  
> >  	if (fattr->fa_xflags & ~mask)
> >  		return -EINVAL;
> > +	if (fattr->fa_pad)
> > +		return -EINVAL;
> 
> How is this related?

I had to add a padding field to the struct to account for the implicit
hole in 64-bit and to ensure the struct is the same size for 32-bit.
It's a reserved field, so we have to ensure the current kernel doesn't
support any value here in case we define this field for something else
in the future.