Re: [RFCv2 5/6] mm: introduce external memory hinting API

[Michal Hocko]

On Fri 31-05-19 22:19:00, Minchan Kim wrote:
> On Fri, May 31, 2019 at 10:37:57AM +0200, Michal Hocko wrote:
> > On Fri 31-05-19 15:43:12, Minchan Kim wrote:
> > > There is some usecase that centralized userspace daemon want to give
> > > a memory hint like MADV_[COLD|PAGEEOUT] to other process. Android's
> > > ActivityManagerService is one of them.
> > > 
> > > It's similar in spirit to madvise(MADV_WONTNEED), but the information
> > > required to make the reclaim decision is not known to the app. Instead,
> > > it is known to the centralized userspace daemon(ActivityManagerService),
> > > and that daemon must be able to initiate reclaim on its own without
> > > any app involvement.
> > > 
> > > To solve the issue, this patch introduces new syscall process_madvise(2).
> > > It could give a hint to the exeternal process of pidfd.
> > > 
> > >  int process_madvise(int pidfd, void *addr, size_t length, int advise,
> > > 			unsigned long cookie, unsigned long flag);
> > > 
> > > Since it could affect other process's address range, only privileged
> > > process(CAP_SYS_PTRACE) or something else(e.g., being the same UID)
> > > gives it the right to ptrace the process could use it successfully.
> > > 
> > > The syscall has a cookie argument to privode atomicity(i.e., detect
> > > target process's address space change since monitor process has parsed
> > > the address range of target process so the operaion could fail in case
> > > of happening race). Although there is no interface to get a cookie
> > > at this moment, it could be useful to consider it as argument to avoid
> > > introducing another new syscall in future. It could support *atomicity*
> > > for disruptive hint(e.g., MADV_DONTNEED|FREE).
> > > flag argument is reserved for future use if we need to extend the API.
> > 
> > Providing an API that is incomplete will not fly. Really. As this really
> > begs for much more discussion and it would be good to move on with the
> > core idea of the pro active memory memory management from userspace
> > usecase. Could you split out the core change so that we can move on and
> > leave the external for a later discussion. I believe this would lead to
> > a smoother integration.
> 
> No problem but I need to understand what you want a little bit more because
> I thought this patchset is already step by step so if we reach the agreement
> of part of them like [1-5/6], it could be merged first.
> 
> Could you say how you want to split the patchset for forward progress?

I would start with new madvise modes and once they are in a shape to be
merged then we can start the remote madvise API. I believe that even
local process reclaim modes are interesting and useful. I haven't heard
anybody objecting to them without having a remote API so far.
-- 
Michal Hocko
SUSE Labs