On 10/26/22 2:49 PM, Michal Hocko wrote: > On Wed 26-10-22 16:00:13, Feng Tang wrote: >> On Wed, Oct 26, 2022 at 03:49:48PM +0800, Aneesh Kumar K V wrote: >>> On 10/26/22 1:13 PM, Feng Tang wrote: >>>> In page reclaim path, memory could be demoted from faster memory tier >>>> to slower memory tier. Currently, there is no check about cpuset's >>>> memory policy, that even if the target demotion node is not allowd >>>> by cpuset, the demotion will still happen, which breaks the cpuset >>>> semantics. >>>> >>>> So add cpuset policy check in the demotion path and skip demotion >>>> if the demotion targets are not allowed by cpuset. >>>> >>> >>> What about the vma policy or the task memory policy? Shouldn't we respect >>> those memory policy restrictions while demoting the page? >> >> Good question! We have some basic patches to consider memory policy >> in demotion path too, which are still under test, and will be posted >> soon. And the basic idea is similar to this patch. > > For that you need to consult each vma and it's owning task(s) and that > to me sounds like something to be done in folio_check_references. > Relying on memcg to get a cpuset cgroup is really ugly and not really > 100% correct. Memory controller might be disabled and then you do not > have your association anymore. > I was looking at this recently and I am wondering whether we should worry about VM_SHARE vmas. ie, page_to_policy() can just reverse lookup just one VMA and fetch the policy right? if it VM_SHARE it will be a shared policy we can find using vma->vm_file? For non anonymous and anon vma not having any policy set it is owning task vma->vm_mm->owner task policy ? We don't worry about multiple tasks that can be possibly sharing that page right? > This all can get quite expensive so the primary question is, does the > existing behavior generates any real issues or is this more of an > correctness exercise? I mean it certainly is not great to demote to an > incompatible numa node but are there any reasonable configurations when > the demotion target node is explicitly excluded from memory > policy/cpuset? I guess vma policy is important. Applications want to make sure that they don't have variable performance and they go to lengths to avoid that by using MEM_BIND. So if they access the memory they always know access is satisfied from a specific set of NUMA nodes. Swapin can cause performance impact but then all continued access will be from a specific NUMA nodes. With slow memory demotion that is not going to be the case. Large in-memory database applications are observed to be sensitive to these access latencies. -aneesh
