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

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Message-ID: <20200910202659.1378404-1-guro@fb.com>
Date:   Thu, 10 Sep 2020 13:26:54 -0700
From:   Roman Gushchin <guro@...com>
To:     Andrew Morton <akpm@...ux-foundation.org>, <linux-mm@...ck.org>
CC:     =Shakeel Butt <shakeelb@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>, <kernel-team@...com>,
        <linux-kernel@...r.kernel.org>, Roman Gushchin <guro@...com>
Subject: [PATCH rfc 0/5] mm: allow mapping accounted kernel pages to userspace

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.

Roman Gushchin (5):
  mm: memcg/slab: fix racy access to page->mem_cgroup in
    mem_cgroup_from_obj()
  mm: memcontrol: use helpers to access page's memcg data
  mm: memcontrol/slab: use helpers to access slab page's memcg_data
  mm: introduce page memcg flags
  mm: convert page kmemcg type to a page memcg flag

 include/linux/memcontrol.h       | 161 +++++++++++++++++++++++++++++--
 include/linux/mm.h               |  22 -----
 include/linux/mm_types.h         |   5 +-
 include/linux/page-flags.h       |  11 +--
 include/trace/events/writeback.h |   2 +-
 mm/debug.c                       |   4 +-
 mm/huge_memory.c                 |   4 +-
 mm/memcontrol.c                  | 116 ++++++++++------------
 mm/migrate.c                     |   2 +-
 mm/page_alloc.c                  |   6 +-
 mm/page_io.c                     |   4 +-
 mm/slab.h                        |  28 +-----
 mm/workingset.c                  |   4 +-
 13 files changed, 221 insertions(+), 148 deletions(-)

-- 
2.26.2