Re: [RFC v2 01/11] file: add callback for pre-mapping dmabuf

[Christoph Hellwig <hch@lst.de>]

On Fri, Jan 09, 2026 at 10:10:57AM +0800, Ming Lei wrote:
> On Thu, Jan 08, 2026 at 11:17:03AM +0100, Christoph Hellwig wrote:
> > On Thu, Jan 08, 2026 at 10:19:18AM +0800, Ming Lei wrote:
> > > > The feature is in no way nvme specific.  nvme is just the initial
> > > > underlying driver.  It makes total sense to support this for any high
> > > > performance block device, and to pass it through file systems.
> > > 
> > > But why does FS care the dma buffer attachment? Since high performance
> > > host controller is exactly the dma buffer attachment point.
> > 
> > I can't parse what you're trying to say here.
> 
> dma buffer attachment is simply none of FS's business.

The file systems should indeed never do a dma buffer attachment itself,
but that's not the point.

> > But even when not stacking, the registration still needs to go
> > through the file system even for a single device, never mind multiple
> > controlled by the file system.
> 
> dma_buf can have multiple importers, so why does it have to go through FS for
> single device only?
>
> If the registered buffer is attached to single device before going
> through FS, it can not support stacking block device, and it can't or not
> easily to use for multiple block device, no matter if they are behind same
> host controller or multiple.

Because the file system, or the file_operations instance to be more
specific, is the only entity that known what block device(s) or other DMA
capable device(s) like (R)NIC a file maps to.