Re: [PATCH v6 04/20] liveupdate: luo_session: add sessions support

[Mike Rapoport <rppt@kernel.org>]

On Sat, Nov 15, 2025 at 06:33:50PM -0500, Pasha Tatashin wrote:
> Introduce concept of "Live Update Sessions" within the LUO framework.
> LUO sessions provide a mechanism to group and manage `struct file *`
> instances (representing file descriptors) that need to be preserved
> across a kexec-based live update.
> 
> Each session is identified by a unique name and acts as a container
> for file objects whose state is critical to a userspace workload, such
> as a virtual machine or a high-performance database, aiming to maintain
> their functionality across a kernel transition.
> 
> This groundwork establishes the framework for preserving file-backed
> state across kernel updates, with the actual file data preservation
> mechanisms to be implemented in subsequent patches.
> 
> Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
> ---
>  include/linux/liveupdate/abi/luo.h |  83 +++++-
>  include/uapi/linux/liveupdate.h    |   3 +
>  kernel/liveupdate/Makefile         |   3 +-
>  kernel/liveupdate/luo_core.c       |  10 +
>  kernel/liveupdate/luo_internal.h   |  52 ++++
>  kernel/liveupdate/luo_session.c    | 421 +++++++++++++++++++++++++++++
>  6 files changed, 570 insertions(+), 2 deletions(-)
>  create mode 100644 kernel/liveupdate/luo_internal.h
>  create mode 100644 kernel/liveupdate/luo_session.c

...

> +/**
> + * struct luo_session_ser - Represents the serialized metadata for a LUO session.
> + * @name:    The unique name of the session, copied from the `luo_session`
> + *           structure.

I'd phase it as

		The unique name of the session provided by the userspace at
		the time of session creation.

> + * @files:   The physical address of a contiguous memory block that holds
> + *           the serialized state of files.

Maybe add                                    ^ in this session?

> + * @pgcnt:   The number of pages occupied by the `files` memory block.
> + * @count:   The total number of files that were part of this session during
> + *           serialization. Used for iteration and validation during
> + *           restoration.
> + *
> + * This structure is used to package session-specific metadata for transfer
> + * between kernels via Kexec Handover. An array of these structures (one per
> + * session) is created and passed to the new kernel, allowing it to reconstruct
> + * the session context.
> + *
> + * If this structure is modified, LUO_SESSION_COMPATIBLE must be updated.

This comment applies to the luo_session_header_ser description as well.

> + */
> +struct luo_session_ser {
> +	char name[LIVEUPDATE_SESSION_NAME_LENGTH];
> +	u64 files;
> +	u64 pgcnt;
> +	u64 count;
> +} __packed;
> +
>  #endif /* _LINUX_LIVEUPDATE_ABI_LUO_H */
> diff --git a/include/uapi/linux/liveupdate.h b/include/uapi/linux/liveupdate.h
> index df34c1642c4d..d2ef2f7e0dbd 100644
> --- a/include/uapi/linux/liveupdate.h
> +++ b/include/uapi/linux/liveupdate.h
> @@ -43,4 +43,7 @@
>  /* The ioctl type, documented in ioctl-number.rst */
>  #define LIVEUPDATE_IOCTL_TYPE		0xBA
>  
> +/* The maximum length of session name including null termination */
> +#define LIVEUPDATE_SESSION_NAME_LENGTH 56

You decided not to bump it to 64 in the end? ;-)

> +
>  #endif /* _UAPI_LIVEUPDATE_H */
> diff --git a/kernel/liveupdate/Makefile b/kernel/liveupdate/Makefile
> index 413722002b7a..83285e7ad726 100644
> --- a/kernel/liveupdate/Makefile
> +++ b/kernel/liveupdate/Makefile
> @@ -2,7 +2,8 @@
>  
>  luo-y :=								\
>  		luo_core.o						\
> -		luo_ioctl.o
> +		luo_ioctl.o						\
> +		luo_session.o
>  
>  obj-$(CONFIG_KEXEC_HANDOVER)		+= kexec_handover.o
>  obj-$(CONFIG_KEXEC_HANDOVER_DEBUG)	+= kexec_handover_debug.o

...

> +int luo_session_retrieve(const char *name, struct file **filep)
> +{
> +	struct luo_session_header *sh = &luo_session_global.incoming;
> +	struct luo_session *session = NULL;
> +	struct luo_session *it;
> +	int err;
> +
> +	scoped_guard(rwsem_read, &sh->rwsem) {
> +		list_for_each_entry(it, &sh->list, list) {
> +			if (!strncmp(it->name, name, sizeof(it->name))) {
> +				session = it;
> +				break;
> +			}
> +		}
> +	}
> +
> +	if (!session)
> +		return -ENOENT;
> +
> +	scoped_guard(mutex, &session->mutex) {
> +		if (session->retrieved)
> +			return -EINVAL;
> +	}
> +
> +	err = luo_session_getfile(session, filep);
> +	if (!err) {
> +		scoped_guard(mutex, &session->mutex)
> +			session->retrieved = true;

Retaking the mutex here seems a bit odd. 
Do we really have to lock session->mutex in luo_session_getfile()?

> +	}
> +
> +	return err;
> +}

...

> +int __init luo_session_setup_incoming(void *fdt_in)
> +{
> +	struct luo_session_header_ser *header_ser;
> +	int err, header_size, offset;
> +	u64 header_ser_pa;
> +	const void *ptr;
> +
> +	offset = fdt_subnode_offset(fdt_in, 0, LUO_FDT_SESSION_NODE_NAME);
> +	if (offset < 0) {
> +		pr_err("Unable to get session node: [%s]\n",
> +		       LUO_FDT_SESSION_NODE_NAME);
> +		return -EINVAL;
> +	}
> +
> +	err = fdt_node_check_compatible(fdt_in, offset,
> +					LUO_FDT_SESSION_COMPATIBLE);
> +	if (err) {
> +		pr_err("Session node incompatible [%s]\n",
> +		       LUO_FDT_SESSION_COMPATIBLE);
> +		return -EINVAL;
> +	}
> +
> +	header_size = 0;
> +	ptr = fdt_getprop(fdt_in, offset, LUO_FDT_SESSION_HEADER, &header_size);
> +	if (!ptr || header_size != sizeof(u64)) {
> +		pr_err("Unable to get session header '%s' [%d]\n",
> +		       LUO_FDT_SESSION_HEADER, header_size);
> +		return -EINVAL;
> +	}
> +
> +	header_ser_pa = get_unaligned((u64 *)ptr);
> +	header_ser = phys_to_virt(header_ser_pa);
> +
> +	luo_session_global.incoming.header_ser = header_ser;
> +	luo_session_global.incoming.ser = (void *)(header_ser + 1);
> +	INIT_LIST_HEAD(&luo_session_global.incoming.list);
> +	init_rwsem(&luo_session_global.incoming.rwsem);
> +	luo_session_global.incoming.active = true;
> +
> +	return 0;
> +}
> +
> +bool luo_session_is_deserialized(void)
> +{
> +	return luo_session_global.deserialized;
> +}
> +
> +int luo_session_deserialize(void)
> +{
> +	struct luo_session_header *sh = &luo_session_global.incoming;
> +	int err;
> +
> +	if (luo_session_is_deserialized())
> +		return 0;
> +
> +	luo_session_global.deserialized = true;
> +	if (!sh->active) {
> +		INIT_LIST_HEAD(&sh->list);
> +		init_rwsem(&sh->rwsem);
> +		return 0;

How this can happen? luo_session_deserialize() is supposed to be called
from ioctl and luo_session_global.incoming should be set up way earlier.

And, why don't we initialize ->list and ->rwsem statically?

> +	}
> +
> +	for (int i = 0; i < sh->header_ser->count; i++) {
> +		struct luo_session *session;
> +
> +		session = luo_session_alloc(sh->ser[i].name);
> +		if (IS_ERR(session)) {
> +			pr_warn("Failed to allocate session [%s] during deserialization %pe\n",
> +				sh->ser[i].name, session);
> +			return PTR_ERR(session);
> +		}

The allocated sessions still need to be freed if an insert fails ;-)

> +
> +		err = luo_session_insert(sh, session); 
> +		if (err) {
> +			luo_session_free(session);
> +			pr_warn("Failed to insert session [%s] %pe\n",
> +				session->name, ERR_PTR(err));
> +			return err;
> +		}
> +
> +		session->count = sh->ser[i].count;
> +		session->files = sh->ser[i].files ? phys_to_virt(sh->ser[i].files) : 0;
> +		session->pgcnt = sh->ser[i].pgcnt;
> +	}
> +
> +	kho_restore_free(sh->header_ser);
> +	sh->header_ser = NULL;
> +	sh->ser = NULL;
> +
> +	return 0;
> +}

-- 
Sincerely yours,
Mike.