Re: [LSF/MM/BPF BoF Session] Numa-Aware Placement for Page Cache Pages

[Matthew Wilcox <willy@infradead.org>]

On Sat, May 02, 2026 at 03:57:19PM +0100, Gregory Price wrote:
> On Thu, Apr 30, 2026 at 02:15:19PM +0100, Matthew Wilcox wrote:
> > Ideally, no, the kernel should observe the task and get it right.
> 
> Out of curiosity, a use case i've been exploring is something like
> 
> fd = open()
> buf = mmap(fd, ...)
> mbind(buf, device_node)
> /* fault file pages directly onto device memory */

Could you be explicit what _kind_ of node you're talking about here?
My initial thought went to something like DAX where the node actually
contains persistent memory and the fd is a reference to some chunk
of that storage.  But I don't think that's what you're referring to.
You might be thinking about a presentation of DRAM over the CXL fabric.
Or you might be thinking about memory presented by a graphics card
(perhaps over CXL, perhaps some other way).  The meaning of "device
memory" has become thoroughly confused (thanks Jerome!) so I sincerely
don't know what you're talking about.

> this obviously breaks if there are concurrent accessors of said file
> with read() (filemap will just fault onto the local node - clear race).
> 
> Do you think there's a world where we can hang a mempolicy off the
> address_space via an fctrl() call with CAP_SYS_NICE?

I don't hate the idea.  Presumably we'd document that it overrides any
mempolicy applied to the process?