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

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Message-ID: <CA+CK2bC96fxHBb78DvNhyfdjsDfPCLY5J5cN8W0hUDt9KAPBJQ@mail.gmail.com>
Date: Mon, 1 Sep 2025 16:54:15 +0000
From: Pasha Tatashin <pasha.tatashin@...een.com>
To: Mike Rapoport <rppt@...nel.org>
Cc: Jason Gunthorpe <jgg@...dia.com>, pratyush@...nel.org, jasonmiu@...gle.com, 
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Subject: Re: [PATCH v3 29/30] luo: allow preserving memfd

On Mon, Sep 1, 2025 at 4:23 PM 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:
> >
> > > +   /*
> > > +    * Most of the space should be taken by preserved folios. So take its
> > > +    * size, plus a page for other properties.
> > > +    */
> > > +   fdt = memfd_luo_create_fdt(PAGE_ALIGN(preserved_size) + PAGE_SIZE);
> > > +   if (!fdt) {
> > > +           err = -ENOMEM;
> > > +           goto err_unpin;
> > > +   }
> >
> > This doesn't seem to have any versioning scheme, it really should..
> >
> > > +   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'
>
> We can just add kho_{preserve,restore}_vmalloc(). I've drafted it here:
> https://git.kernel.org/pub/scm/linux/kernel/git/rppt/linux.git/log/?h=kho/vmalloc/v1

The patch looks okay to me, but it doesn't support holes in vmap
areas. While that is likely acceptable for vmalloc, it could be a
problem if we want to preserve memfd with holes and using vmap
preservation as a method, which would require a different approach.
Still, this would help with preserving memfd.

However, I wonder if we should add a separate preservation library on
top of the kho and not as part of kho (or at least keep them in a
separate file from core logic). This would allow us to preserve more
advanced data structures such as this and define preservation version
control, similar to Jason's store_object/restore_object proposal.

>
> Will wait for kbuild and then send proper patches.
>
>
> --
> Sincerely yours,
> Mike.