Re: [PATCH rfc 0/5] mm: allow mapping accounted kernel pages to userspace

[Shakeel Butt <shakeelb@google.com>]

On Thu, Sep 10, 2020 at 1:27 PM Roman Gushchin <guro@fb.com> 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.