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

Mike Rapoport
2026-04-20T07:13:28Z

On Tue, Apr 14, 2026 at 08:02:33PM +0000, 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(-)

...

> +/**
> + * 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;

Don't we need some sort of locking for blocks?

> bool active;
> };

> @@ -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

For ~8.1 million sessions this comment does not seem valid anymore ;-)

--
Sincerely yours,
Mike.