Re: [RFC PATCH] mm, kvm: account kvm_vcpu_mmap to kmemcg

[Michal Hocko]

On Thu 28-03-19 18:28:36, Shakeel Butt wrote:
> A VCPU of a VM can allocate upto three pages which can be mmap'ed by the
> user space application. At the moment this memory is not charged. On a
> large machine running large number of VMs (or small number of VMs having
> large number of VCPUs), this unaccounted memory can be very significant.

Is this really the case. How many machines are we talking about? Say I
have a virtual machines with 1K cpus, this will result in 12MB. Is this
significant to the overal size of the virtual machine to even care?

> So, this memory should be charged to a kmemcg. However that is not
> possible as these pages are mmapped to the userspace and PageKmemcg()
> was designed with the assumption that such pages will never be mmapped
> to the userspace.
>
> One way to solve this problem is by introducing an additional memcg
> charging API similar to mem_cgroup_[un]charge_skmem(). However skmem
> charging API usage is contained and shared and no new users are
> expected but the pages which can be mmapped and should be charged to
> kmemcg can and will increase. So, requiring the usage for such API will
> increase the maintenance burden. The simplest solution is to remove the
> assumption of no mmapping PageKmemcg() pages to user space.

IIRC the only purpose of PageKmemcg is to keep accounting in the legacy
memcg right. Spending a page flag for that is just no-go. If PageKmemcg
cannot reuse mapping then we have to find a better place for it (e.g.
bottom bit in the page->memcg pointer or rethink the whole PageKmemcg.
 
> Signed-off-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
> ---
>  arch/s390/kvm/kvm-s390.c       |  2 +-
>  arch/x86/kvm/x86.c             |  2 +-
>  include/linux/page-flags.h     | 26 ++++++++++++++++++--------
>  include/trace/events/mmflags.h |  9 ++++++++-
>  virt/kvm/coalesced_mmio.c      |  2 +-
>  virt/kvm/kvm_main.c            |  2 +-
>  6 files changed, 30 insertions(+), 13 deletions(-)
> 
> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> index 4638303ba6a8..1a9e337ed5da 100644
> --- a/arch/s390/kvm/kvm-s390.c
> +++ b/arch/s390/kvm/kvm-s390.c
> @@ -2953,7 +2953,7 @@ struct kvm_vcpu *kvm_arch_vcpu_create(struct kvm *kvm,
>  		goto out;
>  
>  	BUILD_BUG_ON(sizeof(struct sie_page) != 4096);
> -	sie_page = (struct sie_page *) get_zeroed_page(GFP_KERNEL);
> +	sie_page = (struct sie_page *) get_zeroed_page(GFP_KERNEL_ACCOUNT);
>  	if (!sie_page)
>  		goto out_free_cpu;
>  
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 65e4559eef2f..05c0c7eaa5c6 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -9019,7 +9019,7 @@ int kvm_arch_vcpu_init(struct kvm_vcpu *vcpu)
>  	else
>  		vcpu->arch.mp_state = KVM_MP_STATE_UNINITIALIZED;
>  
> -	page = alloc_page(GFP_KERNEL | __GFP_ZERO);
> +	page = alloc_page(GFP_KERNEL_ACCOUNT | __GFP_ZERO);
>  	if (!page) {
>  		r = -ENOMEM;
>  		goto fail;
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 9f8712a4b1a5..b47a6a327d6a 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -78,6 +78,10 @@
>   * PG_hwpoison indicates that a page got corrupted in hardware and contains
>   * data with incorrect ECC bits that triggered a machine check. Accessing is
>   * not safe since it may cause another machine check. Don't touch!
> + *
> + * PG_kmemcg indicates that a kmem page is charged to a memcg. If kmemcg is
> + * enabled, the page allocator will set PageKmemcg() on  pages allocated with
> + * __GFP_ACCOUNT. It gets cleared on page free.
>   */
>  
>  /*
> @@ -130,6 +134,9 @@ enum pageflags {
>  #if defined(CONFIG_IDLE_PAGE_TRACKING) && defined(CONFIG_64BIT)
>  	PG_young,
>  	PG_idle,
> +#endif
> +#ifdef CONFIG_MEMCG_KMEM
> +	PG_kmemcg,
>  #endif
>  	__NR_PAGEFLAGS,
>  
> @@ -289,6 +296,9 @@ static inline int Page##uname(const struct page *page) { return 0; }
>  #define SETPAGEFLAG_NOOP(uname)						\
>  static inline void SetPage##uname(struct page *page) {  }
>  
> +#define __SETPAGEFLAG_NOOP(uname)					\
> +static inline void __SetPage##uname(struct page *page) {  }
> +
>  #define CLEARPAGEFLAG_NOOP(uname)					\
>  static inline void ClearPage##uname(struct page *page) {  }
>  
> @@ -427,6 +437,13 @@ TESTCLEARFLAG(Young, young, PF_ANY)
>  PAGEFLAG(Idle, idle, PF_ANY)
>  #endif
>  
> +#ifdef CONFIG_MEMCG_KMEM
> +__PAGEFLAG(Kmemcg, kmemcg, PF_NO_TAIL)
> +#else
> +TESTPAGEFLAG_FALSE(kmemcg)
> +__SETPAGEFLAG_NOOP(kmemcg)
> +__CLEARPAGEFLAG_NOOP(kmemcg)
> +#endif
>  /*
>   * On an anonymous page mapped into a user virtual memory area,
>   * page->mapping points to its anon_vma, not to a struct address_space;
> @@ -701,8 +718,7 @@ PAGEFLAG_FALSE(DoubleMap)
>  #define PAGE_MAPCOUNT_RESERVE	-128
>  #define PG_buddy	0x00000080
>  #define PG_offline	0x00000100
> -#define PG_kmemcg	0x00000200
> -#define PG_table	0x00000400
> +#define PG_table	0x00000200
>  
>  #define PageType(page, flag)						\
>  	((page->page_type & (PAGE_TYPE_BASE | flag)) == PAGE_TYPE_BASE)
> @@ -743,12 +759,6 @@ PAGE_TYPE_OPS(Buddy, buddy)
>   */
>  PAGE_TYPE_OPS(Offline, offline)
>  
> -/*
> - * If kmemcg is enabled, the buddy allocator will set PageKmemcg() on
> - * pages allocated with __GFP_ACCOUNT. It gets cleared on page free.
> - */
> -PAGE_TYPE_OPS(Kmemcg, kmemcg)
> -
>  /*
>   * Marks pages in use as page tables.
>   */
> diff --git a/include/trace/events/mmflags.h b/include/trace/events/mmflags.h
> index a1675d43777e..d93b78eac5b9 100644
> --- a/include/trace/events/mmflags.h
> +++ b/include/trace/events/mmflags.h
> @@ -79,6 +79,12 @@
>  #define IF_HAVE_PG_IDLE(flag,string)
>  #endif
>  
> +#ifdef CONFIG_MEMCG_KMEM
> +#define IF_HAVE_PG_KMEMCG(flag,string) ,{1UL << flag, string}
> +#else
> +#define IF_HAVE_PG_KMEMCG(flag,string)
> +#endif
> +
>  #define __def_pageflag_names						\
>  	{1UL << PG_locked,		"locked"	},		\
>  	{1UL << PG_waiters,		"waiters"	},		\
> @@ -105,7 +111,8 @@ IF_HAVE_PG_MLOCK(PG_mlocked,		"mlocked"	)		\
>  IF_HAVE_PG_UNCACHED(PG_uncached,	"uncached"	)		\
>  IF_HAVE_PG_HWPOISON(PG_hwpoison,	"hwpoison"	)		\
>  IF_HAVE_PG_IDLE(PG_young,		"young"		)		\
> -IF_HAVE_PG_IDLE(PG_idle,		"idle"		)
> +IF_HAVE_PG_IDLE(PG_idle,		"idle"		)		\
> +IF_HAVE_PG_KMEMCG(PG_kmemcg,		"kmemcg"	)
>  
>  #define show_page_flags(flags)						\
>  	(flags) ? __print_flags(flags, "|",				\
> diff --git a/virt/kvm/coalesced_mmio.c b/virt/kvm/coalesced_mmio.c
> index 5294abb3f178..ebf1601de2a5 100644
> --- a/virt/kvm/coalesced_mmio.c
> +++ b/virt/kvm/coalesced_mmio.c
> @@ -110,7 +110,7 @@ int kvm_coalesced_mmio_init(struct kvm *kvm)
>  	int ret;
>  
>  	ret = -ENOMEM;
> -	page = alloc_page(GFP_KERNEL | __GFP_ZERO);
> +	page = alloc_page(GFP_KERNEL_ACCOUNT | __GFP_ZERO);
>  	if (!page)
>  		goto out_err;
>  
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index f25aa98a94df..de6328dff251 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -306,7 +306,7 @@ int kvm_vcpu_init(struct kvm_vcpu *vcpu, struct kvm *kvm, unsigned id)
>  	vcpu->pre_pcpu = -1;
>  	INIT_LIST_HEAD(&vcpu->blocked_vcpu_list);
>  
> -	page = alloc_page(GFP_KERNEL | __GFP_ZERO);
> +	page = alloc_page(GFP_KERNEL_ACCOUNT | __GFP_ZERO);
>  	if (!page) {
>  		r = -ENOMEM;
>  		goto fail;
> -- 
> 2.21.0.392.gf8f6787159e-goog
> 

-- 
Michal Hocko
SUSE Labs