On Fri, Sep 11, 2020 at 10:34:57AM -0700, Shakeel Butt wrote: > On Fri, Sep 11, 2020 at 10:34 AM Shakeel Butt <shakeelb@xxxxxxxxxx> wrote: > > > > On Thu, Sep 10, 2020 at 1:27 PM Roman Gushchin <guro@xxxxxx> wrote: > > > > > > Currently a non-slab kernel page which has been charged to a memory > > > cgroup can't be mapped to userspace. The underlying reason is simple: > > > PageKmemcg flag is defined as a page type (like buddy, offline, etc), > > > so it takes a bit from a page->mapped counter. Pages with a type set > > > can't be mapped to userspace. > > > > > > But in general the kmemcg flag has nothing to do with mapping to > > > userspace. It only means that the page has been accounted by the page > > > allocator, so it has to be properly uncharged on release. > > > > > > Some bpf maps are mapping the vmalloc-based memory to userspace, and > > > their memory can't be accounted because of this implementation detail. > > > > > > This patchset removes this limitation by moving the PageKmemcg flag > > > into one of the free bits of the page->mem_cgroup pointer. Also it > > > formalizes all accesses to the page->mem_cgroup and page->obj_cgroups > > > using new helpers, adds several checks and removes a couple of obsolete > > > functions. As the result the code became more robust with fewer > > > open-coded bits tricks. > > > > > > The first patch in the series is a bugfix, which I already sent separately. > > > Including it in rfc to make the whole series compile. > > > > > > > > > > This would be a really beneficial feature. I tried to fix the similar > > issue for kvm_vcpu_mmap [1] but using the actual page flag bit but > > your solution would be non controversial. > > > > I think this might also help the accounting of TCP zerocopy receive > > mmapped memory. The memory is charged in skbs but once it is mmapped, > > the skbs get uncharged and we can have a very large amount of > > uncharged memory. > > > > I will take a look at the series. > > [1] https://lore.kernel.org/kvm/20190329012836.47013-1-shakeelb@xxxxxxxxxx/ Cool, thank you for the link! It's very nice that this feature is useful behind the bpf case. Thanks!
