Re: [PATCH v6 15/20] mm: memfd_luo: allow preserving memfd

[Pasha Tatashin]

On Mon, Nov 17, 2025 at 6:04 AM Mike Rapoport <rppt@xxxxxxxxxx> wrote:
>
> On Sat, Nov 15, 2025 at 06:34:01PM -0500, Pasha Tatashin wrote:
> > From: Pratyush Yadav <ptyadav@xxxxxxxxx>
> >
> > The ability to preserve a memfd allows userspace to use KHO and LUO to
> > transfer its memory contents to the next kernel. This is useful in many
> > ways. For one, it can be used with IOMMUFD as the backing store for
> > IOMMU page tables. Preserving IOMMUFD is essential for performing a
> > hypervisor live update with passthrough devices. memfd support provides
> > the first building block for making that possible.
> >
> > For another, applications with a large amount of memory that takes time
> > to reconstruct, reboots to consume kernel upgrades can be very
> > expensive. memfd with LUO gives those applications reboot-persistent
> > memory that they can use to quickly save and reconstruct that state.
> >
> > While memfd is backed by either hugetlbfs or shmem, currently only
> > support on shmem is added. To be more precise, support for anonymous
> > shmem files is added.
> >
> > The handover to the next kernel is not transparent. All the properties
> > of the file are not preserved; only its memory contents, position, and
> > size. The recreated file gets the UID and GID of the task doing the
> > restore, and the task's cgroup gets charged with the memory.
> >
> > Once preserved, the file cannot grow or shrink, and all its pages are
> > pinned to avoid migrations and swapping. The file can still be read from
> > or written to.
> >
> > Use vmalloc to get the buffer to hold the folios, and preserve
> > it using kho_preserve_vmalloc(). This doesn't have the size limit.
> >
> > Co-developed-by: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>
> > Signed-off-by: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>
> > Signed-off-by: Pratyush Yadav <ptyadav@xxxxxxxxx>
>
> The order of signed-offs seems wrong, Pasha's should be the last one.

Updated.


> > + * This interface is a contract. Any modification to the FDT structure,
> > + * node properties, compatible string, or the layout of the serialization
> > + * structures defined here constitutes a breaking change. Such changes require
> > + * incrementing the version number in the MEMFD_LUO_FH_COMPATIBLE string.
>
> The same comment about contract as for the generic LUO documentation
> applies here (https://lore.kernel.org/all/aRnG8wDSSAtkEI_z@xxxxxxxxxx/)

Added.

>
> > + *
> > + * FDT Structure Overview:
> > + *   The memfd state is contained within a single FDT with the following layout:
>
> ...
>
> > +static struct memfd_luo_folio_ser *memfd_luo_preserve_folios(struct file *file, void *fdt,
> > +                                                          u64 *nr_foliosp)
> > +{
>
> If we are already returning nr_folios by reference, we might do it for
> memfd_luo_folio_ser as well and make the function return int.

Done

>
> > +     struct inode *inode = file_inode(file);
> > +     struct memfd_luo_folio_ser *pfolios;
> > +     struct kho_vmalloc *kho_vmalloc;
> > +     unsigned int max_folios;
> > +     long i, size, nr_pinned;
> > +     struct folio **folios;
>
> pfolios and folios read like the former is a pointer to latter.
> I'd s/pfolios/folios_ser/

Done

> > +     int err = -EINVAL;
> > +     pgoff_t offset;
> > +     u64 nr_folios;
>
> ...
>
> > +     kvfree(folios);
> > +     *nr_foliosp = nr_folios;
> > +     return pfolios;
> > +
> > +err_unpreserve:
> > +     i--;
> > +     for (; i >= 0; i--)
>
> Maybe a single line
>
>         for (--i; i >= 0; --i)

Done, but wrote it as:
for (i = i - 1; i >= 0; i--)
Which looks a little cleaner to me.

>
> > +             kho_unpreserve_folio(folios[i]);
> > +     vfree(pfolios);
> > +err_unpin:
> > +     unpin_folios(folios, nr_folios);
> > +err_free_folios:
> > +     kvfree(folios);
> > +     return ERR_PTR(err);
> > +}
> > +
> > +static void memfd_luo_unpreserve_folios(void *fdt, struct memfd_luo_folio_ser *pfolios,
> > +                                     u64 nr_folios)
> > +{
> > +     struct kho_vmalloc *kho_vmalloc;
> > +     long i;
> > +
> > +     if (!nr_folios)
> > +             return;
> > +
> > +     kho_vmalloc = (struct kho_vmalloc *)fdt_getprop(fdt, 0, MEMFD_FDT_FOLIOS, NULL);
> > +     /* The FDT was created by this kernel so expect it to be sane. */
> > +     WARN_ON_ONCE(!kho_vmalloc);
>
> The FDT won't have FOLIOS property if size was zero, will it?
> I think that if we add kho_vmalloc handle to struct memfd_luo_private and
> pass that around it will make things easier and simpler.

I am actually thinking of removing FDTs and using versioned struct directly.

>
> > +     kho_unpreserve_vmalloc(kho_vmalloc);
> > +
> > +     for (i = 0; i < nr_folios; i++) {
> > +             const struct memfd_luo_folio_ser *pfolio = &pfolios[i];
> > +             struct folio *folio;
> > +
> > +             if (!pfolio->foliodesc)
> > +                     continue;
>
> How can this happen? Can pfolios be a sparse array?

With the current implementation of memfd_pin_folios, which populates
holes, this array will be dense. This check is defensive coding in
case we switch to a sparse preservation mechanism in the future. I
will add a comment, and add a warn_on_once.

>
> > +             folio = pfn_folio(PRESERVED_FOLIO_PFN(pfolio->foliodesc));
> > +
> > +             kho_unpreserve_folio(folio);
> > +             unpin_folio(folio);
> > +     }
> > +
> > +     vfree(pfolios);
> > +}
>
> ...
>
> > +static void memfd_luo_finish(struct liveupdate_file_op_args *args)
> > +{
> > +     const struct memfd_luo_folio_ser *pfolios;
> > +     struct folio *fdt_folio;
> > +     const void *fdt;
> > +     u64 nr_folios;
> > +
> > +     if (args->retrieved)
> > +             return;
> > +
> > +     fdt_folio = memfd_luo_get_fdt(args->serialized_data);
> > +     if (!fdt_folio) {
> > +             pr_err("failed to restore memfd FDT\n");
> > +             return;
> > +     }
> > +
> > +     fdt = folio_address(fdt_folio);
> > +
> > +     pfolios = memfd_luo_fdt_folios(fdt, &nr_folios);
> > +     if (!pfolios)
> > +             goto out;
> > +
> > +     memfd_luo_discard_folios(pfolios, nr_folios);
>
> Does not this free the actual folios that were supposed to be preserved?

It does, when memfd was not reclaimed.

>
> > +     vfree(pfolios);
> > +
> > +out:
> > +     folio_put(fdt_folio);
> > +}
>
> ...
>
> > +static int memfd_luo_retrieve(struct liveupdate_file_op_args *args)
> > +{
> > +     struct folio *fdt_folio;
> > +     const u64 *pos, *size;
> > +     struct file *file;
> > +     int len, ret = 0;
> > +     const void *fdt;
> > +
> > +     fdt_folio = memfd_luo_get_fdt(args->serialized_data);
>
> Why do we need to kho_restore_folio() twice? Here and in
> memfd_luo_finish()?

Here we retrieve memfd and give it to userspace. In finish, discard
whatever was not reclaimed.

>
> > +     if (!fdt_folio)
> > +             return -ENOENT;
> > +
> > +     fdt = page_to_virt(folio_page(fdt_folio, 0));
>
> folio_address()

Done

>

>
> --
> Sincerely yours,
> Mike.