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

[Pasha Tatashin <pasha.tatashin@soleen.com>]

On 06-01 16:03, Pratyush Yadav wrote:
> 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.

Good point, I will remove it.

> 
> 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