On Tue, Nov 16, 2021 at 3:29 AM Michal Hocko <mhocko@xxxxxxxx> wrote: > > On Tue 16-11-21 02:17:09, Mina Almasry wrote: > > On Tue, Nov 16, 2021 at 1:28 AM Michal Hocko <mhocko@xxxxxxxx> wrote: > > > > > > On Mon 15-11-21 16:58:19, Mina Almasry wrote: > > > > On Mon, Nov 15, 2021 at 2:58 AM Michal Hocko <mhocko@xxxxxxxx> wrote: > > > > > > > > > > On Fri 12-11-21 09:59:22, Mina Almasry wrote: > > > > > > On Fri, Nov 12, 2021 at 12:36 AM Michal Hocko <mhocko@xxxxxxxx> wrote: > > > > > > > > > > > > > > On Fri 12-11-21 00:12:52, Mina Almasry wrote: > > > > > > > > On Thu, Nov 11, 2021 at 11:52 PM Michal Hocko <mhocko@xxxxxxxx> wrote: > > > > > > > > > > > > > > > > > > On Thu 11-11-21 15:42:01, Mina Almasry wrote: > > > > > > > > > > On remote ooms (OOMs due to remote charging), the oom-killer will attempt > > > > > > > > > > to find a task to kill in the memcg under oom, if the oom-killer > > > > > > > > > > is unable to find one, the oom-killer should simply return ENOMEM to the > > > > > > > > > > allocating process. > > > > > > > > > > > > > > > > > > This really begs for some justification. > > > > > > > > > > > > > > > > > > > > > > > > > I'm thinking (and I can add to the commit message in v4) that we have > > > > > > > > 2 reasonable options when the oom-killer gets invoked and finds > > > > > > > > nothing to kill: (1) return ENOMEM, (2) kill the allocating task. I'm > > > > > > > > thinking returning ENOMEM allows the application to gracefully handle > > > > > > > > the failure to remote charge and continue operation. > > > > > > > > > > > > > > > > For example, in the network service use case that I mentioned in the > > > > > > > > RFC proposal, it's beneficial for the network service to get an ENOMEM > > > > > > > > and continue to service network requests for other clients running on > > > > > > > > the machine, rather than get oom-killed when hitting the remote memcg > > > > > > > > limit. But, this is not a hard requirement, the network service could > > > > > > > > fork a process that does the remote charging to guard against the > > > > > > > > remote charge bringing down the entire process. > > > > > > > > > > > > > > This all belongs to the changelog so that we can discuss all potential > > > > > > > implication and do not rely on any implicit assumptions. > > > > > > > > > > > > Understood. Maybe I'll wait to collect more feedback and upload v4 > > > > > > with a thorough explanation of the thought process. > > > > > > > > > > > > > E.g. why does > > > > > > > it even make sense to kill a task in the origin cgroup? > > > > > > > > > > > > > > > > > > > The behavior I saw returning ENOMEM for this edge case was that the > > > > > > code was forever looping the pagefault, and I was (seemingly > > > > > > incorrectly) under the impression that a suggestion to forever loop > > > > > > the pagefault would be completely fundamentally unacceptable. > > > > > > > > > > Well, I have to say I am not entirely sure what is the best way to > > > > > handle this situation. Another option would be to treat this similar to > > > > > ENOSPACE situation. This would result into SIGBUS IIRC. > > > > > > > > > > The main problem with OOM killer is that it will not resolve the > > > > > underlying problem in most situations. Shmem files would likely stay > > > > > laying around and their charge along with them. Killing the allocating > > > > > task has problems on its own because this could be just a DoS vector by > > > > > other unrelated tasks sharing the shmem mount point without a gracefull > > > > > fallback. Retrying the page fault is hard to detect. SIGBUS might be > > > > > something that helps with the latest. The question is how to communicate > > > > > this requerement down to the memcg code to know that the memory reclaim > > > > > should happen (Should it? How hard we should try?) but do not invoke the > > > > > oom killer. The more I think about this the nastier this is. > > > > > > > > So actually I thought the ENOSPC suggestion was interesting so I took > > > > the liberty to prototype it. The changes required: > > > > > > > > 1. In out_of_memory() we return false if !oc->chosen && > > > > is_remote_oom(). This gets bubbled up to try_charge_memcg() as > > > > mem_cgroup_oom() returning OOM_FAILED. > > > > 2. In try_charge_memcg(), if we get an OOM_FAILED we again check > > > > is_remote_oom(), if it is a remote oom, return ENOSPC. > > > > 3. The calling code would return ENOSPC to the user in the no-fault > > > > path, and SIGBUS the user in the fault path with no changes. > > > > > > I think this should be implemented at the caller side rather than > > > somehow hacked into the memcg core. It is the caller to know what to do. > > > The caller can use gfp flags to control the reclaim behavior. > > > > > > > Hmm I'm a bit struggling to envision this. So would it be acceptable > > at the call sites where we doing a remote charge, such as > > shmem_add_to_page_cache(), if we get ENOMEM from the > > mem_cgroup_charge(), and we know we're doing a remote charge (because > > current's memcg != the super block memcg), then we return ENOSPC from > > shmem_add_to_page_cache()? I believe that will return ENOSPC to the > > userspace in the non-pagefault path and SIGBUS in the pagefault path. > > Or you had something else in mind? > > Yes, exactly. I meant that all this special casing would be done at the > shmem layer as it knows how to communicate this usecase. > Awesome. The more I think of it I think the ENOSPC handling is perfect for this use case, because it gives all users of the shared memory and remote chargers a chance to gracefully handle the ENOSPC or the SIGBUS when we hit the nothing to kill case. The only issue is finding a clean implementation, and if the implementation I just proposed sounds good to you then I see no issues and I'm happy to submit this in the next version. Shakeel and others I would love to know what you think either now or when I post the next version. > [...] > > > > And just a small clarification. Tmpfs is fundamentally problematic from > > > the OOM handling POV. The nuance here is that the OOM happens in a > > > different memcg and thus a different resource domain. If you kill a task > > > in the target memcg then you effectively DoS that workload. If you kill > > > the allocating task then it is DoSed by anybody allowed to write to that > > > shmem. All that without a graceful fallback. > > > > I don't know if this addresses your concern, but I'm limiting the > > memcg= use to processes that can enter that memcg. Therefore they > > would be able to allocate memory in that memcg anyway by entering it. > > So if they wanted to intentionally DoS that memcg they can already do > > it without this feature. > > Can you elaborate some more? How do you enforce that the mount point > cannot be accessed by anybody outside of that constraint? So if I'm a bad actor that wants to intentionally DoS random memcgs on the system I can: mount -t tmpfs -o memcg=/sys/fs/cgroup/unified/memcg-to-dos tmpfs /mnt/tmpfs cat /dev/random > /mnt/tmpfs That will reliably DoS the container. But we only allow you to mount with memcg=/sys/fs/cgroup/unified/memcg-to-dos if you're able to enter that memcg, so I can just do: echo $$ > /sys/fs/cgroup/unified/memcg-to-dos/cgroup.procs allocate_infinited_memory() So we haven't added an attack vector really. More reasonably a sys admin will set up a tmpfs mount with memcg=/sys/fs/cgroup/unified/shared-memory-owner, and set the limit of the shared-memory-owner to be big enough to handle the tasks running in that memcg _and_ all the shared memory. The sys admin can also limit the tmpfs with the size= option to limit the total size of the shared memory. I think the sys admin could also set permissions on the mount so only the users that share the memory can read/write, etc. I'm sorry if this wasn't clear before and I'll take a good look at the commit messages I'm writing and put as much info as possible in each. As always thank you very much for your review and feedback. > -- > Michal Hocko > SUSE Labs
