Re: [PATCH v4 09/13] liveupdate: Remove limit on the number of sessions

[Pratyush Yadav <pratyush@kernel.org>]

On Sat, May 30 2026, 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.
>
> Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
> ---
[...]
> @@ -63,13 +58,15 @@
>  #define _LINUX_KHO_ABI_LUO_H
>  
>  #include <linux/align.h>
> +#include <linux/kho/abi/block.h>
>  #include <uapi/linux/liveupdate.h>
>  
>  /*
>   * The LUO state is registered under this KHO entry name.
>   */
>  #define LUO_KHO_ENTRY_NAME	"LUO"
> -#define LUO_ABI_COMPATIBLE	"luo-v3"
> +#define LUO_COMPAT_BASE		"luo-v3"
> +#define LUO_ABI_COMPATIBLE	LUO_COMPAT_BASE "-" KHO_BLOCK_ABI_COMPATIBLE

That's clever :-)

[...]
>  int luo_session_serialize(void)
>  {
>  	struct luo_session_header *sh = &luo_session_global.outgoing;
>  	struct luo_session *session;
> -	int i = 0;
> +	struct kho_block_it it;
>  	int err;
>  
>  	down_write(&luo_session_serialize_rwsem);
>  	down_write(&sh->rwsem);
>  	*sh->sessions_pa = 0;
>  
> +	kho_block_it_init(&it, &sh->block_set);
> +
>  	list_for_each_entry(session, &sh->list, list) {
> -		err = luo_session_freeze_one(session, &sh->ser[i]);
> -		if (err)
> +		struct luo_session_ser *ser = kho_block_it_next(&it);
> +
> +		if (!ser) {
> +			err = -ENOSPC;
>  			goto err_undo;
> +		}
>  
> -		strscpy(sh->ser[i].name, session->name,
> -			sizeof(sh->ser[i].name));
> -		i++;
> -	}
> +		err = luo_session_freeze_one(session, ser);
> +		if (err) {
> +			kho_block_it_prev(&it);
> +			goto err_undo;
> +		}
>  
> -	if (sh->header_ser && sh->count > 0) {
> -		sh->header_ser->count = sh->count;
> -		*sh->sessions_pa = virt_to_phys(sh->header_ser);
> +		strscpy(ser->name, session->name, sizeof(ser->name));
>  	}
> +
> +	kho_block_it_finalize(&it);
> +
> +	if (sh->sessions_pa && sh->count > 0)

Nit: Why check for sh->sessions_pa? It can never be NULL.

Other than this, 

Reviewed-by: Pratyush Yadav (Google) <pratyush@kernel.org>

> +		*sh->sessions_pa = sh->block_set.head_pa;
>  	up_write(&sh->rwsem);
>  
>  	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));
> +		struct luo_session_ser *ser = kho_block_it_prev(&it);
> +
> +		luo_session_unfreeze_one(session, ser);
> +		memset(ser->name, 0, sizeof(ser->name));
>  	}
>  	up_write(&sh->rwsem);
>  	up_write(&luo_session_serialize_rwsem);

-- 
Regards,
Pratyush Yadav