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Date:   Thu, 24 Sep 2020 13:42:57 -0700
From:   Roman Gushchin <guro@fb.com>
To:     Johannes Weiner <hannes@cmpxchg.org>
CC:     Andrew Morton <akpm@linux-foundation.org>,
        Shakeel Butt <shakeelb@google.com>,
        Michal Hocko <mhocko@kernel.org>,
        <linux-kernel@vger.kernel.org>, <linux-mm@kvack.org>,
        <kernel-team@fb.com>
Subject: Re: [PATCH v1 4/4] mm: convert page kmemcg type to a page memcg flag
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On Thu, Sep 24, 2020 at 04:14:17PM -0400, Johannes Weiner wrote:
> On Tue, Sep 22, 2020 at 01:37:00PM -0700, Roman Gushchin wrote:
> > PageKmemcg flag is currently defined as a page type (like buddy,
> > offline, table and guard). Semantically it means that the page
> > was accounted as a kernel memory by the page allocator and has
> > to be uncharged on the release.
> > 
> > As a side effect of defining the flag as a page type, the accounted
> > page can't be mapped to userspace (look at page_has_type() and
> > comments above). In particular, this blocks the accounting of
> > vmalloc-backed memory used by some bpf maps, because these maps
> > do map the memory to userspace.
> > 
> > One option is to fix it by complicating the access to page->mapcount,
> > which provides some free bits for page->page_type.
> > 
> > But it's way better to move this flag into page->memcg_data flags.
> > Indeed, the flag makes no sense without enabled memory cgroups
> > and memory cgroup pointer set in particular.
> > 
> > This commit replaces PageKmemcg() and __SetPageKmemcg() with
> > PageMemcgKmem() and SetPageMemcgKmem(). __ClearPageKmemcg()
> > can be simple deleted because clear_page_mem_cgroup() already
> > does the job.
> > 
> > As a bonus, on !CONFIG_MEMCG build the PageMemcgKmem() check will
> > be compiled out.
> > 
> > Signed-off-by: Roman Gushchin <guro@fb.com>
> 
> That sounds good to me!

Great!

> 
> > ---
> >  include/linux/memcontrol.h | 58 ++++++++++++++++++++++++++++++++++++--
> >  include/linux/page-flags.h | 11 ++------
> >  mm/memcontrol.c            | 14 +++------
> >  mm/page_alloc.c            |  2 +-
> >  4 files changed, 62 insertions(+), 23 deletions(-)
> > 
> > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > index 9a49f1e1c0c7..390db58500d5 100644
> > --- a/include/linux/memcontrol.h
> > +++ b/include/linux/memcontrol.h
> > @@ -346,8 +346,14 @@ extern struct mem_cgroup *root_mem_cgroup;
> >  enum page_memcg_flags {
> >  	/* page->memcg_data is a pointer to an objcgs vector */
> >  	PG_MEMCG_OBJ_CGROUPS,
> > +	/* page has been accounted as a non-slab kernel page */
> > +	PG_MEMCG_KMEM,
> > +	/* the next bit after the last actual flag */
> > +	PG_MEMCG_LAST_FLAG,
> 
> *_NR_FLAGS would be customary.

Ok, __NR_PAGE_MEMCG_FLAGS ? Similar to __NR_PAGE_FLAGS.

> 
> >  };
> >  
> > +#define MEMCG_FLAGS_MASK ((1UL << PG_MEMCG_LAST_FLAG) - 1)
> 
> Probably best to stick to the same prefix as the enum items.

You mean PG_MEMCG_FLAGS_MASK?

> 
> > + * PageMemcgKmem - check if the page has MemcgKmem flag set
> > + * @page: a pointer to the page struct
> > + *
> > + * Checks if the page has MemcgKmem flag set. The caller must ensure that
> > + * the page has an associated memory cgroup. It's not safe to call this function
> > + * against some types of pages, e.g. slab pages.
> > + */
> > +static inline bool PageMemcgKmem(struct page *page)
> > +{
> > +	VM_BUG_ON_PAGE(test_bit(PG_MEMCG_OBJ_CGROUPS, &page->memcg_data), page);
> > +	return test_bit(PG_MEMCG_KMEM, &page->memcg_data);
> > +}
> > +
> > +/*
> > + * SetPageMemcgKmem - set the page's MemcgKmem flag
> > + * @page: a pointer to the page struct
> > + *
> > + * Set the page's MemcgKmem flag. The caller must ensure that the page has
> > + * an associated memory cgroup. It's not safe to call this function
> > + * against some types of pages, e.g. slab pages.
> > + */
> > +static inline void SetPageMemcgKmem(struct page *page)
> > +{
> > +	VM_BUG_ON_PAGE(!page->memcg_data, page);
> > +	VM_BUG_ON_PAGE(test_bit(PG_MEMCG_OBJ_CGROUPS, &page->memcg_data), page);
> > +	__set_bit(PG_MEMCG_KMEM, &page->memcg_data);
> 
> It may be good to keep the __ prefix from __SetPageMemcg as long as
> this uses __set_bit, in case we later add atomic bit futzing.

Yeah, I agree. I though about it. Maybe not so useful now, but more future-proof.

Thanks!