Re: [Lsf-pc] [LSF/MM/BPF TOPIC][RFC PATCH v4 00/27] Private Memory Nodes (w/ Compressed RAM)

[Vlastimil Babka (SUSE)]

On 6/12/26 17:29, Gregory Price wrote:
> On Wed, Jun 10, 2026 at 04:12:52PM -0400, Gregory Price wrote:
>> On Wed, Jun 10, 2026 at 08:59:59PM +0200, David Hildenbrand (Arm) wrote:
>> > > 
>> > > I understand this question in two ways:
>> > > 
>> > >   1) Can we disallow PAGE allocation and limit this to FOLIO allocation
>> > 
>> > Yes. Can we only allow folios to be allocated from private memory nodes. So let
>> > me reply to that one below.
>> > 
>> ... snip ...
>> > 
>> > At LSF/MM we talked about how GFP flags are bad and how deriving stuff from the
>> > context might be better. I think there was also talk about how the memalloc_*
>> > interface might be a better way forward. Maybe we would start giving the
>> > allocator more context ("we are allocating a folio").
>> > 
>> > The following is incomplete (esp. hugetlb stuff I assume), just as some idea:
>> >
>> 
>> I will still probably send the next RFC version tomorrow or friday,
>> as I want to get some eyes on the __GFP_PRIVATE-less pattern.
>> 
>> Also, I made a new `anondax` driver which enables userland testing
>> of this functionality without any specialty hardware.
>> 
> 
> (apologies for the length of this email: this will all be covered in
> the coming cover letter, but I just wanted to share a bit of a preview)
> 
> ===
> 
> Just another small update - I am planning to post the RFC today once i
> get some mild cleanup done.  It will be based on the dax atomic hotplug
> 
> https://lore.kernel.org/linux-mm/20260605211911.2160954-1-gourry@xxxxxxxxxx/
> 
> But a couple specific details regarding the memalloc pieces that i've
> learned the past couple of days playing with it.
> 
> 1) memalloc_folio is required to ensure non-folio allocations don't land
>    on the private node, even if it happens within a memalloc_private
>    context.  Since memalloc_folio may be useful in contexts outside of
>    private nodes, I kept this as a separate flag.
> 
>    If we think there will *never* be additional users of memalloc_folio,
>    then we could fold _folio into _private to save the flag for now and
>    add it back when we actually need it.
> 
> 2) memalloc_private is needed to unlock private nodes, but in the
>    original NOFALLBACK-only design, you also needed __GFP_THISNODE.
> 
>    This is *highly* restrictive.  I found when playing with mbind that
>    MPOL_BIND + __GFP_THISNODE generates a WARN (valid WARN, it normally
>    implies a bug). 
> 
>    That leads me to #3

I think the memalloc approach is dangerous due to unexpected nesting. There
might be nested page allocations in page allocation itself (due to some
debugging option). But also interrupts do not change what "current" points
to. Suddenly those could start requesting folios and/or private nodes and be
surprised, I'm afraid.

The memalloc scopes only work well when they restrict the context wrt
reclaim, and allocations in IRQ have to be already restricted heavily
(atomic) so further memalloc restrictions don't do anything in practice. But
to make them change other aspects of the allocations like this won't work.

> 3) If a private node is opted into something like Demotion (the node is
>    a demotion target) or mbind(), such that normal kernel operation can
>    place memory there - it's *pseudo-private*, and should actually land
>    in it's own FALLBACK list (reachable without __GFP_THISNODE, but not
>    reachable as a normal fallback allocation target).
> 
> I'm still playing with this, but I think we can even omit the
> __GFP_THISNODE requirement (my initial feeling that __GFP_THISNODE
> didn't buy us anything in particular seems to have panned out).
> 
> At the end of the day, this makes the whole memalloc_private_save()
> pattern a heck of a lot cleaner than trying fiddle with GFP.
> 
> I think you will all enjoy how clean the code ends up, and how easily
> testable it is.
> 
> As a testbed I've implement an anondax (we can discuss naming) that
> adds some sample NODE_PRIVATE_OPT_* flags so you can do the following.
> 
> I'm including this in the next RFC - but we can hack the entire thing
> off (including the OPT flags) if we prefer to just get the base set in
> without a new driver as a start.
> 
> echo 1 > dax0.0/reclaim   # kswapd and reclaim run normally on this node
> echo 1 > dax0.0/demotion  # it is a demotion target
> echo 1 > dax0.0/mbind     # mbind() can target this node for anon-vma's
> echo 1 > dax0.0/madvise   # allow madvise() to operate on its folios
> echo 1 > dax0.0/numa_balance  # allow numa balancing for this node
> echo 1 > dax0.0/ltpin     # allow GUP longterm pin to operate normally
> echo * > dax0.0/adistance # set the adistance for hotplug time
> echo * > dax0.0/hotplug   # same as kmem/hotplug
> 
> This also means *existing hardware* can leverage private nodes if
> they're capable of generating a dax device.
> 
> I've even gotten it such that you can put a private node above dram in
> the adistance heirarchy - which means demotion flows downward from
> device to CPU, but allocations don't default or fallback there.
> 
> This seems *immediately* useful for a variety of use cases.
> 
> ~Gregory
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