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

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Message-ID: <aLbb9nOUnBo_ORT0@kernel.org>
Date: Tue, 2 Sep 2025 14:58:46 +0300
From: Mike Rapoport <rppt@...nel.org>
To: Pasha Tatashin <pasha.tatashin@...een.com>
Cc: Jason Gunthorpe <jgg@...dia.com>, pratyush@...nel.org,
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Subject: Re: [PATCH v3 29/30] luo: allow preserving memfd

On Mon, Sep 01, 2025 at 04:54:15PM +0000, Pasha Tatashin wrote:
> 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.

I can't say I understand what you mean by "holes in vmap areas". We anyway
get an array of folios in memfd_pin_folios() and at that point we know
exactly how many folios there is. So we can do something like

	preserved_folios = vmalloc_array(nr_folios, sizeof(*preserved_folios));
	memfd_luo_preserve_folios(preserved_folios, folios, nr_folios);
	kho_preserve_vmalloc(preserved_folios, &folios_info);

> 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.

kho_preserve_vmalloc() seems quite basic and I don't think it should be
separated from kho core. kho_array is already planned in a separate file :)
 
> > Will wait for kbuild and then send proper patches.
> >
> >
> > --
> > Sincerely yours,
> > Mike.
> 

-- 
Sincerely yours,
Mike.