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Shutemov" Cc: Michal Hocko , Andrew Morton , LKML , linux-mm , linux-api@vger.kernel.org, oleksandr@redhat.com, Suren Baghdasaryan , Tim Murray , Daniel Colascione , Sandeep Patil , Sonny Rao , Brian Geffon , Johannes Weiner , Shakeel Butt , John Dias , ktkhai@virtuozzo.com, christian.brauner@ubuntu.com, sjpark@amazon.de Subject: Re: [PATCH v2 2/5] mm: introduce external memory hinting API Message-ID: <20200117173239.GB140922@google.com> References: <20200116235953.163318-1-minchan@kernel.org> <20200116235953.163318-3-minchan@kernel.org> <20200117115225.GV19428@dhcp22.suse.cz> <20200117155837.bowyjpndfiym6cgs@box> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200117155837.bowyjpndfiym6cgs@box> User-Agent: Mutt/1.10.1 (2018-07-13) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Fri, Jan 17, 2020 at 06:58:37PM +0300, Kirill A. Shutemov 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. > > I agree, it looks flawed. > > Also I don't see what System Management Software can generically do on > sub-process level. I mean how can it decide which part of address space is > less important than other. > > I see how a manager can indicate that this process (or a group of > processes) is less important than other, but on per-addres-range basis? For example, memory ranges shared by several processes or critical for the latency, we could avoid those ranges to be cold/pageout to prevent unncecessary CPU burning/paging. I also think people don't want to give an KSM hint to non-mergeable area.