On Tue, Jun 02, 2026 at 12:16:50PM +1000, Balbir Singh wrote:
> On Sun, May 24, 2026 at 09:50:06PM -0400, Gregory Price wrote:
> >
> > I'm debating on whether to include OPS_MEMPOLICY in the initial version
> > if only because it's not intuitive how it interacts with pagecache. That
> > needs more time to bake.
> >
>
> It makes sense to look at it and then decide if it makes sense.
>
I am thinking i will ship without any OPS flags at all for now and the
have the introduction of ops as a separate series.
> > alloc_pages_node() is the kernel interface
>
> I was think we wouldn't need explicit flags and that allocations would
> happen from user space using __GFP_THISNODE to the node or via a nodemask
> based on nodes of interest. Is there a reason to add this flag, a system
> might have more than one source of N_MEMORY_PRIVATE?
>
There's a few things to unpack here. I discussed this many times on
list and at LSF, but to reiterate.
1) __GFP_THISNODE is insufficient to enforce isolation and otherwise
not particularly useful. Additionally, from userland, it's not
something you can actually set.
for node in possible_nodes:
alloc_pages_node(private_node, __GFP_THISNODE)
In fact it's the opposite semantic of what we want.
THISNODE says: "Do not fallback back to OTHER nodes".
The semantic we want is "Do not allow allocations from private
nodes UNLESS we specifically request" (__GFP_PRIVATE).
__GFP_THISNODE does not actually buy you anything here, AND it's
worse, in the scenario where a private node makes its way into the
preferred slot (via possible_nodes or some other nodemask), the
allocator cannot fall back to a node it can access.
__GFP_THISNODE cannot be overloaded to do anything useful here.
2) We're trying not to expose *ANY* userland APIs for this, at all.
The ultimate goal here should be one of two things:
1) fd = open(/dev/xxx, ...);
mem = mmap(fd, ...);
mem[0] = 0xDEADBEEF; /* Fault device page into page table */
In this case, the driver is responsible for doing the
alloc_pages_node() call.
or
2) mem = mmap(NULL, ..., ANON);
mbind(mem, ..., private_node);
mem[0] = 0xDEADBEEF; /* Fault device page into page table */
in this case mempolicy.c is responsible for doing the
alloc_pages_node() call via the _mpol() alloc variants.
Addition OPT flags (reclaim, compaction, whatever), would
(optionally) allow mm/ to operate on the device memory with, for
example, mmu_notifier callbacks to tell the device to invalidate
whatever it's caching about that page.
This would all be relatively transparent the userland, all userland
"knows" is that it's getting memory from a device (/dev/xxx) or a
node it's otherwise aware of hosting device memory somehow.
~Gregory
