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

[Christoph Hellwig <hch@lst.de>]

On Thu, Jul 09, 2026 at 07:46:42AM -0600, Keith Busch wrote:
> 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.

Ah, makes sense.  But besides the missing documentation I think
max_segments is a bit of a misleading name for that.

Something like max_vecs_per_block (although we don't expose blocks
in the UAPI) or max_vecs_per_granularity (I think grammar wants a word
with me for that, though...) might be a bit more suitable.

> > > @@ -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.

Ah, right.