Re: [RFC PATCH 00/11] mm/mempolicy: Make task->mempolicy externally modifiable via syscall and procfs

[Michal Hocko]

Sorry, didn't have much time to do a proper review. Couple of points
here at least.

On Wed 22-11-23 17:24:10, Gregory Price wrote:
> On Wed, Nov 22, 2023 at 01:33:48PM -0800, Andrew Morton wrote:
> > On Wed, 22 Nov 2023 16:11:49 -0500 Gregory Price <gourry.memverge@xxxxxxxxx> wrote:
> > 
> > > The patch set changes task->mempolicy to be modifiable by tasks other
> > > than just current.
> > > 
> > > The ultimate goal is to make mempolicy more flexible and extensible,
> > > such as adding interleave weights (which may need to change at runtime
> > > due to hotplug events).  Making mempolicy externally modifiable allows
> > > for userland daemons to make runtime performance adjustments to running
> > > tasks without that software needing to be made numa-aware.
> > 
> > Please add to this [0/N] a full description of the security aspect: who
> > can modify whose mempolicy, along with a full description of the
> > reasoning behind this decision.
> > 
> 
> Will do. For the sake of v0 for now:
> 
> 1) the task itself (task == current)
>    for obvious reasons: it already can
> 
> 2) from external interfaces: CAP_SYS_NICE

Makes sense.

[...]
> > > 3. Add external interfaces which allow for a task mempolicy to be
> > >    modified by another task.  This is implemented in 4 syscalls
> > >    and a procfs interface:
> > >         sys_set_task_mempolicy
> > >         sys_get_task_mempolicy
> > >         sys_set_task_mempolicy_home_node
> > >         sys_task_mbind
> > >         /proc/[pid]/mempolicy
> > 
> > Why is the procfs interface needed?  Doesn't it simply duplicate the
> > syscall interface?  Please update [0/N] with a description of this
> > decision.
> > 
> 
> Honestly I wrote the procfs interface first, and then came back around
> to just implement the syscalls.  mbind is not friendly to being procfs'd
> so if the preference is to have only one, not both, then it should
> probably be the syscalls.
> 
> That said, when I introduce weighted interleave on top of this, having a
> simple procfs interface to those weights would be valuable, so I
> imagined something like `proc/mempolicy` to determine if interleave was
> being used and something like `proc/mpol_interleave_weights` for a clean
> interface to update weights.
> 
> However, in the same breath, I have a prior RFC with set/get_mempolicy2
> which could probably take all future mempolicy extensions and wrap them
> up into one pair of syscalls, instead of us ending up with 200 more
> sys_mempolicy_whatever as memory attached fabrics become more common.
> 
> So... yeah... the is one area I think the community very much needs to
> comment:  set/get_mempolicy2, many new mempolicy syscalls, procfs? All
> of the above?

I think we should actively avoid using proc interface. The most
reasonable way would be to add get_mempolicy2 interface that would allow
extensions and then create a pidfd counterpart to allow acting on a
remote task. The latter would require some changes to make mempolicy
code less current oriented.
-- 
Michal Hocko
SUSE Labs