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

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Message-ID: <20250813063407.GA3182745.vipinsh@google.com>
Date: Tue, 12 Aug 2025 23:34:37 -0700
From: Vipin Sharma <vipinsh@...gle.com>
To: Pasha Tatashin <pasha.tatashin@...een.com>
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Subject: Re: [PATCH v3 29/30] luo: allow preserving memfd

On 2025-08-07 01:44:35, Pasha Tatashin wrote:
> From: Pratyush Yadav <ptyadav@...zon.de>
> +static void memfd_luo_unpreserve_folios(const struct memfd_luo_preserved_folio *pfolios,
> +					unsigned int nr_folios)
> +{
> +	unsigned int i;
> +
> +	for (i = 0; i < nr_folios; i++) {
> +		const struct memfd_luo_preserved_folio *pfolio = &pfolios[i];
> +		struct folio *folio;
> +
> +		if (!pfolio->foliodesc)
> +			continue;
> +
> +		folio = pfn_folio(PRESERVED_FOLIO_PFN(pfolio->foliodesc));
> +
> +		kho_unpreserve_folio(folio);

This one is missing WARN_ON_ONCE() similar to the one in
memfd_luo_preserve_folios().

> +		unpin_folio(folio);
> +	}
> +}
> +
> +static void *memfd_luo_create_fdt(unsigned long size)
> +{
> +	unsigned int order = get_order(size);
> +	struct folio *fdt_folio;
> +	int err = 0;
> +	void *fdt;
> +
> +	if (order > MAX_PAGE_ORDER)
> +		return NULL;
> +
> +	fdt_folio = folio_alloc(GFP_KERNEL, order);

__GFP_ZERO should also be used here. Otherwise this can lead to
unintentional passing of old kernel memory.

> +static int memfd_luo_prepare(struct liveupdate_file_handler *handler,
> +			     struct file *file, u64 *data)
> +{
> +	struct memfd_luo_preserved_folio *preserved_folios;
> +	struct inode *inode = file_inode(file);
> +	unsigned int max_folios, nr_folios = 0;
> +	int err = 0, preserved_size;
> +	struct folio **folios;
> +	long size, nr_pinned;
> +	pgoff_t offset;
> +	void *fdt;
> +	u64 pos;
> +
> +	if (WARN_ON_ONCE(!shmem_file(file)))
> +		return -EINVAL;

This one is only check for shmem_file, whereas in
memfd_luo_can_preserve() there is check for inode->i_nlink also. Is that
not needed here?

> +
> +	inode_lock(inode);
> +	shmem_i_mapping_freeze(inode, true);
> +
> +	size = i_size_read(inode);
> +	if ((PAGE_ALIGN(size) / PAGE_SIZE) > UINT_MAX) {
> +		err = -E2BIG;
> +		goto err_unlock;
> +	}
> +
> +	/*
> +	 * Guess the number of folios based on inode size. Real number might end
> +	 * up being smaller if there are higher order folios.
> +	 */
> +	max_folios = PAGE_ALIGN(size) / PAGE_SIZE;
> +	folios = kvmalloc_array(max_folios, sizeof(*folios), GFP_KERNEL);

__GFP_ZERO?

> +static int memfd_luo_freeze(struct liveupdate_file_handler *handler,
> +			    struct file *file, u64 *data)
> +{
> +	u64 pos = file->f_pos;
> +	void *fdt;
> +	int err;
> +
> +	if (WARN_ON_ONCE(!*data))
> +		return -EINVAL;
> +
> +	fdt = phys_to_virt(*data);
> +
> +	/*
> +	 * The pos or size might have changed since prepare. Everything else
> +	 * stays the same.
> +	 */
> +	err = fdt_setprop(fdt, 0, "pos", &pos, sizeof(pos));
> +	if (err)
> +		return err;

Comment is talking about pos and size but code is only updating pos. 

> +static int memfd_luo_retrieve(struct liveupdate_file_handler *handler, u64 data,
> +			      struct file **file_p)
> +{
> +	const struct memfd_luo_preserved_folio *pfolios;
> +	int nr_pfolios, len, ret = 0, i = 0;
> +	struct address_space *mapping;
> +	struct folio *folio, *fdt_folio;
> +	const u64 *pos, *size;
> +	struct inode *inode;
> +	struct file *file;
> +	const void *fdt;
> +
> +	fdt_folio = memfd_luo_get_fdt(data);
> +	if (!fdt_folio)
> +		return -ENOENT;
> +
> +	fdt = page_to_virt(folio_page(fdt_folio, 0));
> +
> +	pfolios = fdt_getprop(fdt, 0, "folios", &len);
> +	if (!pfolios || len % sizeof(*pfolios)) {
> +		pr_err("invalid 'folios' property\n");

Print should clearly state that error is because fields is not found or
len is not multiple of sizeof(*pfolios).