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SubjectRe: [PATCH] mm/memcg: set memcg when split pages
On Tue, 2 Mar 2021, Michal Hocko wrote:
> [Cc Johannes for awareness and fixup Nick's email]
>
> On Tue 02-03-21 01:34:51, Zhou Guanghui wrote:
> > When split page, the memory cgroup info recorded in first page is
> > not copied to tail pages. In this case, when the tail pages are
> > freed, the uncharge operation is not performed. As a result, the
> > usage of this memcg keeps increasing, and the OOM may occur.
> >
> > So, the copying of first page's memory cgroup info to tail pages
> > is needed when split page.
>
> I was not aware that alloc_pages_exact is used for accounted allocations
> but git grep told me otherwise so this is not a theoretical one. Both
> users (arm64 and s390 kvm) are quite recent AFAICS. split_page is also
> used in dma allocator but I got lost in indirection so I have no idea
> whether there are any users there.

Yes, it's a bit worrying that such a low-level thing as split_page()
can now get caught up in memcg accounting, but I suppose that's okay.

I feel rather strongly that whichever way it is done, THP splitting
and split_page() should use the same interface to memcg.

And a look at mem_cgroup_split_huge_fixup() suggests that nowadays
there need to be css_get()s too - or better, a css_get_many().

Its #ifdef CONFIG_TRANSPARENT_HUGEPAGE should be removed, rename
it mem_cgroup_split_page_fixup(), and take order from caller.

Though I've never much liked that separate pass: would it be
better page by page, like this copy_page_memcg() does? Though
mem_cgroup_disabled() and css_getting make that less appealing.

Hugh

>
> The page itself looks reasonable to me.
>
> > Signed-off-by: Zhou Guanghui <zhouguanghui1@huawei.com>
>
> Acked-by: Michal Hocko <mhocko@suse.com>
>
> Minor nit
>
> > ---
> > include/linux/memcontrol.h | 10 ++++++++++
> > mm/page_alloc.c | 4 +++-
> > 2 files changed, 13 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > index e6dc793d587d..c7e2b4421dc1 100644
> > --- a/include/linux/memcontrol.h
> > +++ b/include/linux/memcontrol.h
> > @@ -867,6 +867,12 @@ void mem_cgroup_print_oom_group(struct mem_cgroup *memcg);
> > extern bool cgroup_memory_noswap;
> > #endif
> >
> > +static inline void copy_page_memcg(struct page *dst, struct page *src)
> > +{
> > + if (src->memcg_data)
> > + dst->memcg_data = src->memcg_data;
>
> I would just drop the test. The struct page is a single cache line which
> is dirty by the reference count so another store will unlikely be
> noticeable even when NULL is stored here and you safe a conditional.
>
> > +}
> > +
> > struct mem_cgroup *lock_page_memcg(struct page *page);
> > void __unlock_page_memcg(struct mem_cgroup *memcg);
> > void unlock_page_memcg(struct page *page);
> > @@ -1291,6 +1297,10 @@ mem_cgroup_print_oom_meminfo(struct mem_cgroup *memcg)
> > {
> > }
> >
> > +static inline void copy_page_memcg(struct page *dst, struct page *src)
> > +{
> > +}
> > +
> > static inline struct mem_cgroup *lock_page_memcg(struct page *page)
> > {
> > return NULL;
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 3e4b29ee2b1e..ee0a63dc1c9b 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -3307,8 +3307,10 @@ void split_page(struct page *page, unsigned int order)
> > VM_BUG_ON_PAGE(PageCompound(page), page);
> > VM_BUG_ON_PAGE(!page_count(page), page);
> >
> > - for (i = 1; i < (1 << order); i++)
> > + for (i = 1; i < (1 << order); i++) {
> > set_page_refcounted(page + i);
> > + copy_page_memcg(page + i, page);
> > + }
> > split_page_owner(page, 1 << order);
> > }
> > EXPORT_SYMBOL_GPL(split_page);
> > --
> > 2.25.0
> >
>
> --
> Michal Hocko
> SUSE Labs

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