Re: [PATCH 1/5] liveupdate: Remove limit on the number of sessions

yanjun.zhu
2026-04-15T00:06:08Z

On 4/14/26 1:02 PM, Pasha Tatashin wrote:
> Currently, the number of LUO sessions is limited by a fixed number of
> pre-allocated pages for serialization (16 pages, allowing for ~819
> sessions).
>
> This limitation is problematic if LUO is used to support things such as
> systemd file descriptor store, and would be used not just as VM memory
> but to save other states on the machine.
>
> Remove this limit by transitioning to a linked-block approach for
> session metadata serialization. Instead of a single contiguous block,
> session metadata is now stored in a chain of 16-page blocks. Each block
> starts with a header containing the physical address of the next block
> and the number of session entries in the current block.
>
> - Bump session ABI version to v3.
> - Update struct luo_session_header_ser to include a 'next' pointer.
> - Implement dynamic block allocation in luo_session_insert().
> - Update setup, serialization, and deserialization logic to traverse
> the block chain.
> - Remove LUO_SESSION_MAX limit.
>
> Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
> ---
> include/linux/kho/abi/luo.h | 19 +--
> kernel/liveupdate/luo_internal.h | 12 +-
> kernel/liveupdate/luo_session.c | 237 +++++++++++++++++++++++--------
> 3 files changed, 197 insertions(+), 71 deletions(-)
>
> diff --git a/include/linux/kho/abi/luo.h b/include/linux/kho/abi/luo.h
> index 46750a0ddf88..f5732958545e 100644
> --- a/include/linux/kho/abi/luo.h
> +++ b/include/linux/kho/abi/luo.h
> @@ -57,9 +57,10 @@
> * - compatible: "luo-session-v1"
> * Identifies the session ABI version.
> * - luo-session-header: u64
> - * The physical address of a `struct luo_session_header_ser`. This structure
> - * is the header for a contiguous block of memory containing an array of
> - * `struct luo_session_ser`, one for each preserved session.
> + * The physical address of the first `struct luo_session_header_ser`.
> + * This structure is the header for a block of memory containing an array
> + * of `struct luo_session_ser` entries. Multiple blocks are linked via
> + * the `next` field in the header.
> *
> * File-Lifecycle-Bound Node (luo-flb):
> * This node describes all preserved global objects whose lifecycle is bound
> @@ -77,9 +78,9 @@
> * `__packed` structures. These structures contain the actual preserved state.
> *
> * - struct luo_session_header_ser:
> - * Header for the session array. Contains the total page count of the
> - * preserved memory block and the number of `struct luo_session_ser`
> - * entries that follow.
> + * Header for the session data block. Contains the physical address of the
> + * next session data block and the number of `struct luo_session_ser`
> + * entries that follow this header in the current block.
> *
> * - struct luo_session_ser:
> * Metadata for a single session, including its name and a physical pointer
> @@ -153,21 +154,23 @@ struct luo_file_set_ser {
> * luo_session_header_ser
> */
> #define LUO_FDT_SESSION_NODE_NAME "luo-session"
> -#define LUO_FDT_SESSION_COMPATIBLE "luo-session-v2"
> +#define LUO_FDT_SESSION_COMPATIBLE "luo-session-v3"
> #define LUO_FDT_SESSION_HEADER "luo-session-header"
>
> /**
> * struct luo_session_header_ser - Header for the serialized session data block.
> + * @next: Physical address of the next struct luo_session_header_ser.
> * @count: The number of `struct luo_session_ser` entries that immediately
> * follow this header in the memory block.
> *
> - * This structure is located at the beginning of a contiguous block of
> + * This structure is located at the beginning of a block of
> * physical memory preserved across the kexec. It provides the necessary
> * metadata to interpret the array of session entries that follow.
> *
> * If this structure is modified, `LUO_FDT_SESSION_COMPATIBLE` must be updated.
> */
> struct luo_session_header_ser {
> + u64 next;
> u64 count;
> } __packed;
>
> diff --git a/kernel/liveupdate/luo_internal.h b/kernel/liveupdate/luo_internal.h
> index 875844d7a41d..a73f42069301 100644
> --- a/kernel/liveupdate/luo_internal.h
> +++ b/kernel/liveupdate/luo_internal.h
> @@ -11,6 +11,16 @@
> #include <linux/liveupdate.h>
> #include <linux/uaccess.h>
>
> +/*
> + * Safeguard limit for the number of serialization blocks. This is used to
> + * prevent infinite loops and excessive memory allocation in case of memory
> + * corruption in the preserved state.
> + *
> + * This limit allows for ~8.1 million sessions and ~1.2 million files per
> + * session, which is more than enough for all realistic use cases.
> + */
> +#define LUO_MAX_BLOCKS 10000
> +
> struct luo_ucmd {
> void __user *ubuffer;
> u32 user_size;
> @@ -59,7 +69,6 @@ struct luo_file_set {
> * struct luo_session - Represents an active or incoming Live Update session.
> * @name: A unique name for this session, used for identification and
> * retrieval.
> - * @ser: Pointer to the serialized data for this session.
> * @list: A list_head member used to link this session into a global list
> * of either outgoing (to be preserved) or incoming (restored from
> * previous kernel) sessions.
> @@ -70,7 +79,6 @@ struct luo_file_set {
> */
> struct luo_session {
> char name[LIVEUPDATE_SESSION_NAME_LENGTH];
> - struct luo_session_ser *ser;
> struct list_head list;
> bool retrieved;
> struct luo_file_set file_set;
> diff --git a/kernel/liveupdate/luo_session.c b/kernel/liveupdate/luo_session.c
> index 92b1af791889..007ca34eba79 100644
> --- a/kernel/liveupdate/luo_session.c
> +++ b/kernel/liveupdate/luo_session.c
> @@ -69,30 +69,39 @@
> #include <uapi/linux/liveupdate.h>
> #include "luo_internal.h"
>
> -/* 16 4K pages, give space for 744 sessions */
> +/* 16 4K pages, give space for 819 sessions per block */
> #define LUO_SESSION_PGCNT 16ul
> -#define LUO_SESSION_MAX (((LUO_SESSION_PGCNT << PAGE_SHIFT) - \
> +#define LUO_SESSION_BLOCK_MAX (((LUO_SESSION_PGCNT << PAGE_SHIFT) - \
> sizeof(struct luo_session_header_ser)) / \
> sizeof(struct luo_session_ser))
>
> +/**
> + * struct luo_session_block - Internal representation of a session serialization block.
> + * @list: List head for linking blocks in memory.
> + * @ser: Pointer to the serialized header in preserved memory.
> + */
> +struct luo_session_block {
> + struct list_head list;
> + struct luo_session_header_ser *ser;
> +};
> +
> /**
> * struct luo_session_header - Header struct for managing LUO sessions.
> * @count: The number of sessions currently tracked in the @list.
> + * @nblocks: The number of allocated serialization blocks.
> * @list: The head of the linked list of `struct luo_session` instances.
> * @rwsem: A read-write semaphore providing synchronized access to the
> * session list and other fields in this structure.
> - * @header_ser: The header data of serialization array.
> - * @ser: The serialized session data (an array of
> - * `struct luo_session_ser`).
> + * @blocks: The list of serialization blocks (struct luo_session_block).
> * @active: Set to true when first initialized. If previous kernel did not
> * send session data, active stays false for incoming.
> */
> struct luo_session_header {
> long count;
> + long nblocks;
> struct list_head list;
> struct rw_semaphore rwsem;
> - struct luo_session_header_ser *header_ser;
> - struct luo_session_ser *ser;
> + struct list_head blocks;
> bool active;
> };
>
> @@ -110,10 +119,12 @@ static struct luo_session_global luo_session_global = {
> .incoming = {
> .list = LIST_HEAD_INIT(luo_session_global.incoming.list),
> .rwsem = __RWSEM_INITIALIZER(luo_session_global.incoming.rwsem),
> + .blocks = LIST_HEAD_INIT(luo_session_global.incoming.blocks),
> },
> .outgoing = {
> .list = LIST_HEAD_INIT(luo_session_global.outgoing.list),
> .rwsem = __RWSEM_INITIALIZER(luo_session_global.outgoing.rwsem),
> + .blocks = LIST_HEAD_INIT(luo_session_global.outgoing.blocks),
> },
> };
>
> @@ -140,6 +151,70 @@ static void luo_session_free(struct luo_session *session)
> kfree(session);
> }
>
> +static int luo_session_add_block(struct luo_session_header *sh,
> + struct luo_session_header_ser *ser)
> +{
> + struct luo_session_block *block;
> +
> + if (sh->nblocks >= LUO_MAX_BLOCKS)
> + return -ENOSPC;
> +
> + block = kzalloc_obj(*block);
> + if (!block)
> + return -ENOMEM;
> +
> + block->ser = ser;
> + list_add_tail(&block->list, &sh->blocks);
> + sh->nblocks++;
> +
> + return 0;
> +}
> +
> +static int luo_session_create_ser_block(struct luo_session_header *sh)
> +{
> + struct luo_session_block *last = NULL;
> + struct luo_session_header_ser *ser;
> + int err;
> +
> + ser = kho_alloc_preserve(LUO_SESSION_PGCNT << PAGE_SHIFT);
> + if (IS_ERR(ser))
> + return PTR_ERR(ser);
> +
> + if (!list_empty(&sh->blocks))
> + last = list_last_entry(&sh->blocks, struct luo_session_block, list);
> +
> + err = luo_session_add_block(sh, ser);
> + if (err)
> + goto err_unpreserve;
> +
> + if (last)
> + last->ser->next = virt_to_phys(ser);
> +
> + return 0;
> +
> +err_unpreserve:
> + kho_unpreserve_free(ser);
> + return err;
> +}
> +
> +static void luo_session_destroy_ser_blocks(struct luo_session_header *sh,
> + bool unpreserve)
> +{
> + struct luo_session_block *block, *tmp;
> +
> + list_for_each_entry_safe(block, tmp, &sh->blocks, list) {
> + if (block->ser) {
> + if (unpreserve)
> + kho_unpreserve_free(block->ser);
> + else
> + kho_restore_free(block->ser);
> + }
> + list_del(&block->list);
> + kfree(block);
> + sh->nblocks--;
> + }
> +}
> +
> static int luo_session_insert(struct luo_session_header *sh,
> struct luo_session *session)
> {
> @@ -147,15 +222,6 @@ static int luo_session_insert(struct luo_session_header *sh,
>
> guard(rwsem_write)(&sh->rwsem);
>
> - /*
> - * For outgoing we should make sure there is room in serialization array
> - * for new session.
> - */
> - if (sh == &luo_session_global.outgoing) {
> - if (sh->count == LUO_SESSION_MAX)
> - return -ENOMEM;
> - }
> -
> /*
> * For small number of sessions this loop won't hurt performance
> * but if we ever start using a lot of sessions, this might
> @@ -166,6 +232,20 @@ static int luo_session_insert(struct luo_session_header *sh,
> if (!strncmp(it->name, session->name, sizeof(it->name)))
> return -EEXIST;
> }
> +
> + /*
> + * For outgoing we should make sure there is room in serialization array
> + * for new session. If not, allocate a new block.
> + */
> + if (sh == &luo_session_global.outgoing) {
> + if (sh->count == sh->nblocks * LUO_SESSION_BLOCK_MAX) {

I am not sure if "LUO_SESSION_BLOCK_MAX 10000" has special meaning or not.

If not, we can use the following:

#define LUO_SESSION_BLOCK_SHIFT 13
#define LUO_SESSION_BLOCK_MAX (1UL << LUO_SESSION_BLOCK_SHIFT) // 8192
#define LUO_SESSION_BLOCK_MASK (LUO_SESSION_BLOCK_MAX - 1) // 0x1FFF

if (!(sh->count & LUO_SESSION_BLOCK_MASK) && sh->count > 0) {
...
}

Zhu Yanjun

> + int err = luo_session_create_ser_block(sh);
> +
> + if (err)
> + return err;
> + }
> + }
> +
> list_add_tail(&session->list, &sh->list);
> sh->count++;
>
> @@ -444,9 +524,12 @@ int __init luo_session_setup_outgoing(void *fdt_out)
> u64 header_ser_pa;
> int err;
>
> - header_ser = kho_alloc_preserve(LUO_SESSION_PGCNT << PAGE_SHIFT);
> - if (IS_ERR(header_ser))
> - return PTR_ERR(header_ser);
> + err = luo_session_create_ser_block(&luo_session_global.outgoing);
> + if (err)
> + return err;
> +
> + header_ser = list_first_entry(&luo_session_global.outgoing.blocks,
> + struct luo_session_block, list)->ser;
> header_ser_pa = virt_to_phys(header_ser);
>
> err = fdt_begin_node(fdt_out, LUO_FDT_SESSION_NODE_NAME);
> @@ -459,19 +542,18 @@ int __init luo_session_setup_outgoing(void *fdt_out)
> if (err)
> goto err_unpreserve;
>
> - luo_session_global.outgoing.header_ser = header_ser;
> - luo_session_global.outgoing.ser = (void *)(header_ser + 1);
> luo_session_global.outgoing.active = true;
>
> return 0;
>
> err_unpreserve:
> - kho_unpreserve_free(header_ser);
> + luo_session_destroy_ser_blocks(&luo_session_global.outgoing, true);
> return err;
> }
>
> int __init luo_session_setup_incoming(void *fdt_in)
> {
> + struct luo_session_header *sh = &luo_session_global.incoming;
> struct luo_session_header_ser *header_ser;
> int err, header_size, offset;
> u64 header_ser_pa;
> @@ -501,11 +583,14 @@ int __init luo_session_setup_incoming(void *fdt_in)
> }
>
> 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);
> - luo_session_global.incoming.active = true;
> + while (header_ser_pa) {
> + header_ser = phys_to_virt(header_ser_pa);
> + err = luo_session_add_block(sh, header_ser);
> + if (err)
> + return err;
> + header_ser_pa = header_ser->next;
> + }
> + sh->active = true;
>
> return 0;
> }
> @@ -513,6 +598,7 @@ int __init luo_session_setup_incoming(void *fdt_in)
> int luo_session_deserialize(void)
> {
> struct luo_session_header *sh = &luo_session_global.incoming;
> + struct luo_session_block *block;
> static bool is_deserialized;
> static int err;
>
> @@ -539,40 +625,49 @@ int luo_session_deserialize(void)
> * userspace to detect the failure and trigger a reboot, which will
> * reliably reset devices and reclaim memory.
> */
> - 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",
> - (int)sizeof(sh->ser[i].name),
> - sh->ser[i].name, session);
> - err = PTR_ERR(session);
> - return err;
> - }
> + list_for_each_entry(block, &sh->blocks, list) {
> + struct luo_session_ser *ser = (void *)(block->ser + 1);
>
> - err = luo_session_insert(sh, session);
> - if (err) {
> - pr_warn("Failed to insert session [%s] %pe\n",
> - session->name, ERR_PTR(err));
> - luo_session_free(session);
> + if (block->ser->count > LUO_SESSION_BLOCK_MAX) {
> + pr_warn("Session block contains too many entries: %llu\n",
> + block->ser->count);
> + err = -EINVAL;
> return err;
> }
>
> - scoped_guard(mutex, &session->mutex) {
> - err = luo_file_deserialize(&session->file_set,
> - &sh->ser[i].file_set_ser);
> - }
> - if (err) {
> - pr_warn("Failed to deserialize files for session [%s] %pe\n",
> - session->name, ERR_PTR(err));
> - return err;
> + for (int i = 0; i < block->ser->count; i++) {
> + struct luo_session *session;
> +
> + session = luo_session_alloc(ser[i].name);
> + if (IS_ERR(session)) {
> + pr_warn("Failed to allocate session [%.*s] during deserialization %pe\n",
> + (int)sizeof(ser[i].name),
> + ser[i].name, session);
> + err = PTR_ERR(session);
> + return err;
> + }
> +
> + err = luo_session_insert(sh, session);
> + if (err) {
> + pr_warn("Failed to insert session [%s] %pe\n",
> + session->name, ERR_PTR(err));
> + luo_session_free(session);
> + return err;
> + }
> +
> + scoped_guard(mutex, &session->mutex) {
> + err = luo_file_deserialize(&session->file_set,
> + &ser[i].file_set_ser);
> + }
> + if (err) {
> + pr_warn("Failed to deserialize files for session [%s] %pe\n",
> + session->name, ERR_PTR(err));
> + return err;
> + }
> }
> }
>
> - kho_restore_free(sh->header_ser);
> - sh->header_ser = NULL;
> - sh->ser = NULL;
> + luo_session_destroy_ser_blocks(sh, false);
>
> return 0;
> }
> @@ -580,31 +675,51 @@ int luo_session_deserialize(void)
> int luo_session_serialize(void)
> {
> struct luo_session_header *sh = &luo_session_global.outgoing;
> + struct luo_session_block *block;
> struct luo_session *session;
> + struct luo_session_ser *ser;
> int i = 0;
> int err;
>
> guard(rwsem_write)(&sh->rwsem);
> +
> + if (list_empty(&sh->blocks))
> + return 0;
> +
> + block = list_first_entry(&sh->blocks, struct luo_session_block, list);
> + ser = (void *)(block->ser + 1);
> +
> list_for_each_entry(session, &sh->list, list) {
> - err = luo_session_freeze_one(session, &sh->ser[i]);
> + if (i == LUO_SESSION_BLOCK_MAX) {
> + block->ser->count = i;
> + block = list_next_entry(block, list);
> + ser = (void *)(block->ser + 1);
> + i = 0;
> + }
> +
> + err = luo_session_freeze_one(session, &ser[i]);
> if (err)
> goto err_undo;
>
> - strscpy(sh->ser[i].name, session->name,
> - sizeof(sh->ser[i].name));
> + strscpy(ser[i].name, session->name,
> + sizeof(ser[i].name));
> i++;
> }
> - sh->header_ser->count = sh->count;
> + block->ser->count = i;
>
> return 0;
>
> err_undo:
> list_for_each_entry_continue_reverse(session, &sh->list, list) {
> i--;
> - luo_session_unfreeze_one(session, &sh->ser[i]);
> - memset(sh->ser[i].name, 0, sizeof(sh->ser[i].name));
> + if (i < 0) {
> + block = list_prev_entry(block, list);
> + ser = (void *)(block->ser + 1);
> + i = LUO_SESSION_BLOCK_MAX - 1;
> + }
> + luo_session_unfreeze_one(session, &ser[i]);
> + memset(ser[i].name, 0, sizeof(ser[i].name));
> }
>
> return err;
> }
> -