Re: [PATCH v2] ipc: only destroy orphaned shm segments on sysctl write

[Andrew Morton]

On Mon, 15 Jun 2026 10:27:41 +0800 Jianlin Shi <shijianlin11@xxxxxxxxxxx> wrote:

> proc_ipc_dointvec_minmax_orphans() currently calls
> shm_destroy_orphaned() whenever shm_rmid_forced is set, including on
> sysctl reads. Reading /proc/sys/kernel/shm_rmid_forced should not take
> shm_ids rwsem for write and walk all segments.
> 
> Only run the cleanup when the sysctl is written and the forced RMID
> policy is enabled.
> 
> When shm_rmid_forced=1, monitoring tools that read
> /proc/sys/kernel/shm_rmid_forced trigger the cleanup on every read.
> 
> ...
>
> --- a/ipc/ipc_sysctl.c
> +++ b/ipc/ipc_sysctl.c
> @@ -28,7 +28,7 @@ static int proc_ipc_dointvec_minmax_orphans(const struct ctl_table *table, int w
>  
>  	if (err < 0)
>  		return err;
> -	if (ns->shm_rmid_forced)
> +	if (write && ns->shm_rmid_forced)
>  		shm_destroy_orphaned(ns);
>  	return err;
>  }

Seems reasonable, I guess.  Maybe.  It's not backward compatible.

Nothing in the shm_rmid_forced documentation
(Documentation/admin-guide/sysctl/kernel.rst) indicates that a read has
any such side-effect.

But there's a risk that users have discovered that a read has this
effect, so they're using reads because that's a lot less hassle than
figuring out what value to write.

Davidlohr, Eric: any thoughts on this?


btw, the doc says "if you change this from 0 to 1, already created
segments without users and with a dead originative process will be
destroyed" but the implementation will do the destruction for any write
of a non-zero value.  It should read "if a non-zero value is written
already created segments without users and with a dead originative
process will be destroyed".  Minor detail.