On Fri, Sep 29, 2023 at 08:11:54AM -0700, Yosry Ahmed wrote: > On Fri, Sep 29, 2023 at 8:08 AM Johannes Weiner <hannes@xxxxxxxxxxx> wrote: > > > > On Thu, Sep 28, 2023 at 06:18:19PM -0700, Yosry Ahmed wrote: > > > My concern is the scenario where the memory controller is mounted in > > > cgroup v1, and cgroup v2 is mounted with memory_hugetlb_accounting. > > > > > > In this case it seems like the current code will only check whether > > > memory_hugetlb_accounting was set on cgroup v2 or not, disregarding > > > the fact that cgroup v1 did not enable hugetlb accounting. > > > > > > I obviously prefer that any features are also added to cgroup v1, > > > because we still didn't make it to cgroup v2, especially when the > > > infrastructure is shared. On the other hand, I am pretty sure the > > > maintainers will not like what I am saying :) > > > > I have a weak preference. > > > > It's definitely a little weird that the v1 controller's behavior > > changes based on the v2 mount flag. And that if you want it as an > > otherwise exclusive v1 user, you'd have to mount a dummy v2. > > > > But I also don't see a scenario where it would hurt, or where there > > would be an unresolvable conflict between v1 and v2 in expressing > > desired behavior, since the memory controller is exclusive to one. > > > > While we could eliminate this quirk with a simple > > !cgroup_subsys_on_dfl(memory_cgrp_subsys) inside the charge function, > > it would seem almost punitive to add extra code just to take something > > away that isn't really a problem and could be useful to some people. > > > > If Tejun doesn't object, I'd say let's just keep implied v1 behavior. > > I agree that adding extra code to take a feature away from v1 is > probably too much, but I also think relying on a v2 mount option is > weird. Would it be too much to just have a v1-specific flag as well > and use cgroup_subsys_on_dfl(memory_cgrp_subsys) to decide which flag > to read? Yeah, let's not preemptively add explicit new features to cgroup1. Since we agree the incidental support is weird, let's filter hugetlb charging on cgroup_subsys_on_dfl(memory_cgrp_subsys) after all. If somebody wants this for v1 - and it doesn't sound like Google is even in that category according to Frank - they should send a separate patch and we can go through all the reasons why switching to v2 is not an option for them.
