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

On Thu, May 30, 2019 at 11:43 PM Minchan Kim [off-list ref] 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.

How about a compromise? Let's allow all madvise hints if the process
is calling process_madvise *on itself* (which will be useful once we
wire up the atomicity cookie) and restrict the cross-process case to
the hints you've mentioned. This way, the restriction on madvise hints
isn't tied to the specific API, but to the relationship between hinter
and hintee.