linux-kernel - Re: [PATCH v3 29/30] luo: allow preserving memfd

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Message-ID: <CAF8kJuN+CsXo2QwrMvcSkn=_WB+zqikgLK9=ydqUDj+8Osmf6Q@mail.gmail.com>
Date: Fri, 29 Aug 2025 11:47:17 -0700
From: Chris Li <chrisl@...nel.org>
To: Mike Rapoport <rppt@...nel.org>
Cc: Jason Gunthorpe <jgg@...dia.com>, Pasha Tatashin <pasha.tatashin@...een.com>, pratyush@...nel.org, 
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Subject: Re: [PATCH v3 29/30] luo: allow preserving memfd

On Thu, Aug 28, 2025 at 12:14 AM Mike Rapoport <rppt@...nel.org> wrote:
>
> On Tue, Aug 26, 2025 at 01:20:19PM -0300, Jason Gunthorpe wrote:
> > On Thu, Aug 07, 2025 at 01:44:35AM +0000, Pasha Tatashin wrote:
> >
> > > +   err = fdt_property_placeholder(fdt, "folios", preserved_size,
> > > +                                  (void **)&preserved_folios);
> > > +   if (err) {
> > > +           pr_err("Failed to reserve folios property in FDT: %s\n",
> > > +                  fdt_strerror(err));
> > > +           err = -ENOMEM;
> > > +           goto err_free_fdt;
> > > +   }
> >
> > Yuk.
> >
> > This really wants some luo helper
> >
> > 'luo alloc array'
> > 'luo restore array'
> > 'luo free array'
> >
> > Which would get a linearized list of pages in the vmap to hold the
> > array and then allocate some structure to record the page list and
> > return back the u64 of the phys_addr of the top of the structure to
> > store in whatever.
> >
> > Getting fdt to allocate the array inside the fds is just not going to
> > work for anything of size.
>
> I agree that we need a side-car structure for preserving large (potentially
> sparse) arrays, but I think it should be a part of KHO rather than LUO.

I agree this can be used by components outside of LUO as well. Ideally
as some helper library so every component can use it. I don't have a
strong opinion on KHO or the stand alone library. I am fine with both.

Chris