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

[Gregory Price <gourry@gourry.net>]

On Thu, Apr 30, 2026 at 02:15:19PM +0100, Matthew Wilcox wrote:
> On Thu, Apr 30, 2026 at 05:03:37PM +0530, Ritesh Harjani (IBM) wrote:
> > Linux already supports memory tiers and there are ongoing discussions around
> > promotion of unmapped page cache pages, which lets kernel do the right thing
> > for userspace page cache pages on a tiered system.
> 
> Well, you know my opinion of that idea ...
> 
> > So the question is:
> > Do we need a userspace interface for the placement policy of page cache pages on a per file basis?
> 
> What do we do if two tasks both "know" the right NUMA placement for the
> inode's data, and they disagree?
> 
> > 1. Is there a need for an interface that allows userspace to do per-fd page
> >    placement and maybe per-fd page migration?
> 
> 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 */

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 haven't quite worked through the full lifetime, since there's a
possibility the mempolicy ends up with stale nodes (hotplug, etc)
without plumbing for that.   But it did seem like a somewhat clean
abstraction that isn't specifically a tiering use case.

(not interested in this for anything other than single node placement
policy or tiering, so no interleave or migration support or anything)

~Gregory