Re: [PATCH 1/5] mm: uncharge kmem pages from generic free_page path

[Greg Thelen]

Vladimir Davydov wrote:

> Currently, to charge a page to kmemcg one should use alloc_kmem_pages
> helper. When the page is not needed anymore it must be freed with
> free_kmem_pages helper, which will uncharge the page before freeing it.
> Such a design is acceptable for thread info pages and kmalloc large
> allocations, which are currently the only users of alloc_kmem_pages, but
> it gets extremely inconvenient if one wants to make use of batched free
> (e.g. to charge page tables - see release_pages) or page reference
> counter (pipe buffers - see anon_pipe_buf_release).
>
> To overcome this limitation, this patch moves kmemcg uncharge code to
> the generic free path and zaps free_kmem_pages helper. To distinguish
> kmem pages from other page types, it makes alloc_kmem_pages initialize
> page->_mapcount to a special value and introduces a new PageKmem helper,
> which returns true if it sees this value.
>
> Signed-off-by: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>
> ---
>  include/linux/gfp.h        |  3 ---
>  include/linux/page-flags.h | 22 ++++++++++++++++++++++
>  kernel/fork.c              |  2 +-
>  mm/page_alloc.c            | 26 ++++++++------------------
>  mm/slub.c                  |  2 +-
>  mm/swap.c                  |  3 ++-
>  6 files changed, 34 insertions(+), 24 deletions(-)
>
> diff --git a/include/linux/gfp.h b/include/linux/gfp.h
> index f92cbd2f4450..b46147c45966 100644
> --- a/include/linux/gfp.h
> +++ b/include/linux/gfp.h
> @@ -384,9 +384,6 @@ extern void *__alloc_page_frag(struct page_frag_cache *nc,
>  			       unsigned int fragsz, gfp_t gfp_mask);
>  extern void __free_page_frag(void *addr);
>  
> -extern void __free_kmem_pages(struct page *page, unsigned int order);
> -extern void free_kmem_pages(unsigned long addr, unsigned int order);
> -
>  #define __free_page(page) __free_pages((page), 0)
>  #define free_page(addr) free_pages((addr), 0)
>  
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 416509e26d6d..a190719c2f46 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -594,6 +594,28 @@ static inline void __ClearPageBalloon(struct page *page)
>  }
>  
>  /*
> + * PageKmem() returns true if the page was allocated with alloc_kmem_pages().
> + */
> +#define PAGE_KMEM_MAPCOUNT_VALUE (-512)
> +
> +static inline int PageKmem(struct page *page)
> +{
> +	return atomic_read(&page->_mapcount) == PAGE_KMEM_MAPCOUNT_VALUE;
> +}
> +
> +static inline void __SetPageKmem(struct page *page)
> +{
> +	VM_BUG_ON_PAGE(atomic_read(&page->_mapcount) != -1, page);
> +	atomic_set(&page->_mapcount, PAGE_KMEM_MAPCOUNT_VALUE);
> +}

What do you think about several special mapcount values for various
types of kmem?

It's helps user and administrators break down memory usage.

A nice equation is:
  memory.usage_in_bytes = memory.stat[file + anon + unevictable + kmem]

Next, it's helpful to be able to breakdown kmem into:
  kmem = stack + pgtable + slab + ...

On one hand (and the kernel I use internally) we can use separate per
memcg counters for each kmem type.  Then reconstitute memory.kmem as
needed by adding them together.  But using keeping a single kernel kmem
counter is workable if there is a way to breakdown the memory charge to
a container (e.g. by walking /proc/kpageflags-ish or per memcg
memory.kpageflags-ish file).

> +static inline void __ClearPageKmem(struct page *page)
> +{
> +	VM_BUG_ON_PAGE(!PageKmem(page), page);
> +	atomic_set(&page->_mapcount, -1);
> +}
> +
> +/*
>   * If network-based swap is enabled, sl*b must keep track of whether pages
>   * were allocated from pfmemalloc reserves.
>   */
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 2845623fb582..c23f8a17e99e 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -169,7 +169,7 @@ static struct thread_info *alloc_thread_info_node(struct task_struct *tsk,
>  
>  static inline void free_thread_info(struct thread_info *ti)
>  {
> -	free_kmem_pages((unsigned long)ti, THREAD_SIZE_ORDER);
> +	free_pages((unsigned long)ti, THREAD_SIZE_ORDER);
>  }
>  # else
>  static struct kmem_cache *thread_info_cache;
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 48aaf7b9f253..88d85367c81e 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -942,6 +942,10 @@ static bool free_pages_prepare(struct page *page, unsigned int order)
>  
>  	if (PageAnon(page))
>  		page->mapping = NULL;
> +	if (PageKmem(page)) {
> +		memcg_kmem_uncharge_pages(page, order);
> +		__ClearPageKmem(page);
> +	}
>  	bad += free_pages_check(page);
>  	for (i = 1; i < (1 << order); i++) {
>  		if (compound)
> @@ -3434,6 +3438,8 @@ struct page *alloc_kmem_pages(gfp_t gfp_mask, unsigned int order)
>  		return NULL;
>  	page = alloc_pages(gfp_mask, order);
>  	memcg_kmem_commit_charge(page, memcg, order);
> +	if (page)
> +		__SetPageKmem(page);
>  	return page;
>  }
>  
> @@ -3446,27 +3452,11 @@ struct page *alloc_kmem_pages_node(int nid, gfp_t gfp_mask, unsigned int order)
>  		return NULL;
>  	page = alloc_pages_node(nid, gfp_mask, order);
>  	memcg_kmem_commit_charge(page, memcg, order);
> +	if (page)
> +		__SetPageKmem(page);
>  	return page;
>  }
>  
> -/*
> - * __free_kmem_pages and free_kmem_pages will free pages allocated with
> - * alloc_kmem_pages.
> - */
> -void __free_kmem_pages(struct page *page, unsigned int order)
> -{
> -	memcg_kmem_uncharge_pages(page, order);
> -	__free_pages(page, order);
> -}
> -
> -void free_kmem_pages(unsigned long addr, unsigned int order)
> -{
> -	if (addr != 0) {
> -		VM_BUG_ON(!virt_addr_valid((void *)addr));
> -		__free_kmem_pages(virt_to_page((void *)addr), order);
> -	}
> -}
> -
>  static void *make_alloc_exact(unsigned long addr, unsigned order, size_t size)
>  {
>  	if (addr) {
> diff --git a/mm/slub.c b/mm/slub.c
> index f614b5dc396b..f5248a7d9438 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -3516,7 +3516,7 @@ void kfree(const void *x)
>  	if (unlikely(!PageSlab(page))) {
>  		BUG_ON(!PageCompound(page));
>  		kfree_hook(x);
> -		__free_kmem_pages(page, compound_order(page));
> +		__free_pages(page, compound_order(page));
>  		return;
>  	}
>  	slab_free(page->slab_cache, page, object, _RET_IP_);
> diff --git a/mm/swap.c b/mm/swap.c
> index 983f692a47fd..8d8d03118a18 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -64,7 +64,8 @@ static void __page_cache_release(struct page *page)
>  		del_page_from_lru_list(page, lruvec, page_off_lru(page));
>  		spin_unlock_irqrestore(&zone->lru_lock, flags);
>  	}
> -	mem_cgroup_uncharge(page);
> +	if (!PageKmem(page))
> +		mem_cgroup_uncharge(page);
>  }
>  
>  static void __put_single_page(struct page *page)

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