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) <[email protected]>
> Signed-off-by: Pasha Tatashin <[email protected]>
> ---
[...]
> @@ -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) <[email protected]>
> + *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
