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

[Mike Rapoport <rppt@kernel.org>]

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.