linux-kernel - Re: [PATCH 5.10.y 01/11] mm: memcontrol: Use helpers to read page's memcg data

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Date:   Tue, 17 Aug 2021 19:02:17 -0700
From:   Roman Gushchin <guro@...com>
To:     Chen Huang <chenhuang5@...wei.com>
CC:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Muchun Song <songmuchun@...edance.com>,
        Wang Hai <wanghai38@...wei.com>,
        <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
        <stable@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexei Starovoitov <ast@...nel.org>
Subject: Re: [PATCH 5.10.y 01/11] mm: memcontrol: Use helpers to read page's
 memcg data

On Mon, Aug 16, 2021 at 09:21:11PM +0800, Chen Huang wrote:
> 
> 
> 在 2021/8/16 16:34, Greg Kroah-Hartman 写道:
> > On Mon, Aug 16, 2021 at 07:21:37AM +0000, Chen Huang wrote:
> >> From: Roman Gushchin <guro@...com>
> > 
> > What is the git commit id of this patch in Linus's tree?
> > 
> >>
> >> Patch series "mm: allow mapping accounted kernel pages to userspace", v6.
> >>
> >> 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
> >> 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 bit tricks.
> >>
> >> This patch (of 4):
> >>
> >> Currently there are many open-coded reads of the page->mem_cgroup pointer,
> >> as well as a couple of read helpers, which are barely used.
> >>
> >> It creates an obstacle on a way to reuse some bits of the pointer for
> >> storing additional bits of information.  In fact, we already do this for
> >> slab pages, where the last bit indicates that a pointer has an attached
> >> vector of objcg pointers instead of a regular memcg pointer.
> >>
> >> This commits uses 2 existing helpers and introduces a new helper to
> >> converts all read sides to calls of these helpers:
> >>   struct mem_cgroup *page_memcg(struct page *page);
> >>   struct mem_cgroup *page_memcg_rcu(struct page *page);
> >>   struct mem_cgroup *page_memcg_check(struct page *page);
> >>
> >> page_memcg_check() is intended to be used in cases when the page can be a
> >> slab page and have a memcg pointer pointing at objcg vector.  It does
> >> check the lowest bit, and if set, returns NULL.  page_memcg() contains a
> >> VM_BUG_ON_PAGE() check for the page not being a slab page.
> >>
> >> To make sure nobody uses a direct access, struct page's
> >> mem_cgroup/obj_cgroups is converted to unsigned long memcg_data.
> >>
> >> Signed-off-by: Roman Gushchin <guro@...com>
> >> Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
> >> Signed-off-by: Alexei Starovoitov <ast@...nel.org>
> >> Reviewed-by: Shakeel Butt <shakeelb@...gle.com>
> >> Acked-by: Johannes Weiner <hannes@...xchg.org>
> >> Acked-by: Michal Hocko <mhocko@...e.com>
> >> Link: https://lkml.kernel.org/r/20201027001657.3398190-1-guro@fb.com
> >> Link: https://lkml.kernel.org/r/20201027001657.3398190-2-guro@fb.com
> >> Link: https://lore.kernel.org/bpf/20201201215900.3569844-2-guro@fb.com
> >>
> >> Conflicts:
> >> 	mm/memcontrol.c
> > 
> > The "Conflicts:" lines should be removed.
> > 
> > Please fix up the patch series and resubmit.  But note, this seems
> > really intrusive, are you sure these are all needed?
> >

Sorry for jumping in late.

I agree that the patchset is quite intrusive and I really doubt we
need to backport it. The main goal of my patchset was to enable
memory accounting for bpf maps (which can be mmaped to userspace).
I don't see why we need it otherwise.
Muchun's patchset unifies the treatment of non-slab kernel objects
(e.g. large kmallocs) with slab objects and prevents them to pin
dying memory cgroups. However the problem existed for years and
I doubt we need it in 5.10 so badly.

> 
> OK，I will resend the patchset.
> Roman Gushchin's patchset formalize accesses to the page->mem_cgroup and
> page->obj_cgroups. But for LRU pages and most other raw memcg, they may
> pin to a memcg cgroup pointer, which should always point to an object cgroup
> pointer. That's the problem I met. And Muchun Song's patchset fix this.
> So I think these are all needed.

Can you, please, be more specific here?

Thanks!