On Fri 17-01-20 09:25:42, Minchan Kim wrote: > On Fri, Jan 17, 2020 at 12:52:25PM +0100, Michal Hocko wrote: > > On Thu 16-01-20 15:59:50, Minchan Kim wrote: > > > There is usecase that System Management Software(SMS) want to give > > > a memory hint like MADV_[COLD|PAGEEOUT] to other processes and > > > in the case of Android, it is the ActivityManagerService. > > > > > > 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 uses pidfd of an external processs to give the hint. > > > > > > int process_madvise(int pidfd, void *addr, size_t length, int advise, > > > 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 flag argument is reserved for future use if we need to extend the > > > API. > > > > > > I think supporting all hints madvise has/will supported/support to > > > process_madvise is rather risky. Because we are not sure all hints make > > > sense from external process and implementation for the hint may rely on > > > the caller being in the current context so it could be error-prone. > > > Thus, I just limited hints as MADV_[COLD|PAGEOUT] in this patch. > > > > > > If someone want to add other hints, we could hear hear the usecase and > > > review it for each hint. It's more safe for maintainace rather than > > > introducing a buggy syscall but hard to fix it later. > > > > I have brought this up when we discussed this in the past but there is > > no reflection on that here so let me bring that up again. > > > > I believe that the interface has an inherent problem that it is racy. > > The external entity needs to know the address space layout of the target > > process to do anyhing useful on it. The address space is however under > > the full control of the target process though and the external entity > > has no means to find out that the layout has changed. So > > time-to-check-time-to-act is an inherent problem. > > > > This is a serious design flaw and it should be explained why it doesn't > > matter or how to use the interface properly to prevent that problem. > > Sorry for the missing that part. > > It's not a particular problem of this API because other APIs already have > done with that(e.g., move_pages, process_vm_writev). I am sorry but this is not really an argument. > Point is userspace has several ways for the control of target process > like SIGSTOP, cgroup freezer or even no need to control since platform > is already aware of that the process will never run until he grant it > or it's resilient even though the race happens. If you have that level of control then you can simply inject the code via ptrace and you do not need a new syscall in the first place. > In future, if we want to support more fine-grained consistency model > like memory layout, we could provide some API to get cookie(e.g., > seq count which is updated whenever vma of the process changes). And then > we could feed the cookie to process_madvise's last argument so that > it can fail if founds it's not matched. > For that API, Daniel already posted RFC - process_getinfo[1]. > https://lore.kernel.org/lkml/20190520035254.57579-1-minchan@xxxxxxxxxx/T/#m7694416fd179b2066a2c62b5b139b14e3894e224 So why do not we start with a clean API since the beginning? I do agree that a remote madvise is an interesting feature and it opens gates to all sorts of userspace memory management which is not possible this days. But the syscall has to have a reasonable semantic to allow that. We cannot simply start with a half proken symantic first based on an Android usecase and then hit the wall as soon as others with a different user space model want to use it as well. -- Michal Hocko SUSE Labs
