Re: [PATCH v7 14/22] mm: memfd_luo: allow preserving memfd

[Mike Rapoport <rppt@kernel.org>]

On Sat, Nov 22, 2025 at 05:23:41PM -0500, Pasha Tatashin wrote:
> From: Pratyush Yadav <ptyadav@amazon.de>
> 
> 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.
> 
> Signed-off-by: Pratyush Yadav <ptyadav@amazon.de>
> Co-developed-by: Pasha Tatashin <pasha.tatashin@soleen.com>
> Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
> ---

...

> +static int memfd_luo_retrieve_folios(struct file *file,
> +				     struct memfd_luo_folio_ser *folios_ser,
> +				     u64 nr_folios)
> +{
> +	struct inode *inode = file_inode(file);
> +	struct address_space *mapping = inode->i_mapping;
> +	struct folio *folio;
> +	long i = 0;
> +	int err;
> +
> +	for (; i < nr_folios; i++) {
> +		const struct memfd_luo_folio_ser *pfolio = &folios_ser[i];
> +		phys_addr_t phys;
> +		u64 index;
> +		int flags;
> +
> +		if (!pfolio->pfn)
> +			continue;
> +
> +		phys = PFN_PHYS(pfolio->pfn);
> +		folio = kho_restore_folio(phys);
> +		if (!folio) {
> +			pr_err("Unable to restore folio at physical address: %llx\n",
> +			       phys);
> +			goto put_folios;
> +		}
> +		index = pfolio->index;
> +		flags = pfolio->flags;
> +
> +		/* Set up the folio for insertion. */
> +		__folio_set_locked(folio);
> +		__folio_set_swapbacked(folio);
> +
> +		err = mem_cgroup_charge(folio, NULL, mapping_gfp_mask(mapping));
> +		if (err) {
> +			pr_err("shmem: failed to charge folio index %ld: %d\n",
> +			       i, err);
> +			goto unlock_folio;
> +		}
> +
> +		err = shmem_add_to_page_cache(folio, mapping, index, NULL,
> +					      mapping_gfp_mask(mapping));
> +		if (err) {
> +			pr_err("shmem: failed to add to page cache folio index %ld: %d\n",
> +			       i, err);
> +			goto unlock_folio;
> +		}
> +
> +		if (flags & MEMFD_LUO_FOLIO_UPTODATE)
> +			folio_mark_uptodate(folio);
> +		if (flags & MEMFD_LUO_FOLIO_DIRTY)
> +			folio_mark_dirty(folio);
> +
> +		err = shmem_inode_acct_blocks(inode, 1);
> +		if (err) {
> +			pr_err("shmem: failed to account folio index %ld: %d\n",
> +			       i, err);
> +			goto unlock_folio;
> +		}
> +
> +		shmem_recalc_inode(inode, 1, 0);
> +		folio_add_lru(folio);
> +		folio_unlock(folio);
> +		folio_put(folio);
> +	}
> +
> +	return 0;
> +
> +unlock_folio:
> +	folio_unlock(folio);
> +	folio_put(folio);
> +	i++;
 
I'd add a counter and use it int the below for loop.

> +put_folios:
> +	/*
> +	 * Note: don't free the folios already added to the file. They will be
> +	 * freed when the file is freed. Free the ones not added yet here.
> +	 */
> +	for (; i < nr_folios; i++) {
> +		const struct memfd_luo_folio_ser *pfolio = &folios_ser[i];
> +
> +		folio = kho_restore_folio(pfolio->pfn);
> +		if (folio)
> +			folio_put(folio);
> +	}
> +
> +	return err;
> +}

Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

-- 
Sincerely yours,
Mike.