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Date: Thu, 8 Oct 2015 16:40:06 +0200
From: Michal Hocko <mhocko@...nel.org>
To: Vladimir Davydov <vdavydov@...tuozzo.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Johannes Weiner <hannes@...xchg.org>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] memcg: simplify charging kmem pages
On Mon 05-10-15 01:21:41, Vladimir Davydov wrote:
> Charging kmem pages proceeds in two steps. First, we try to charge the
> allocation size to the memcg the current task belongs to, then we
> allocate a page and "commit" the charge storing the pointer to the memcg
> in the page struct.
>
> Such a design looks overcomplicated, because there is no much sense in
> trying charging the allocation before actually allocating a page: we
> won't be able to consume much memory over the limit even if we charge
> after doing the actual allocation, besides we already charge user pages
> post factum, so being pedantic with kmem pages just looks pointless.
>
> So this patch simplifies the design by merging the "charge" and the
> "commit" steps into the same function, which takes the allocated page.
Yes this makes sense.
> Also, rename the charge and uncharge methods to memcg_kmem_charge and
> memcg_kmem_uncharge and make the charge method return error code instead
> of bool to conform to mem_cgroup_try_charge.
OK
> Signed-off-by: Vladimir Davydov <vdavydov@...tuozzo.com>
Nice cleanup!
Acked-by: Michal Hocko <mhocko@...e.com>
> ---
> include/linux/memcontrol.h | 69 +++++++++++++---------------------------------
> mm/memcontrol.c | 39 +++-----------------------
> mm/page_alloc.c | 18 ++++++------
> 3 files changed, 32 insertions(+), 94 deletions(-)
>
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index a3e0296eb063..9e1f4d5efc56 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -752,11 +752,8 @@ static inline bool memcg_kmem_is_active(struct mem_cgroup *memcg)
> * conditions, but because they are pretty simple, they are expected to be
> * fast.
> */
> -bool __memcg_kmem_newpage_charge(gfp_t gfp, struct mem_cgroup **memcg,
> - int order);
> -void __memcg_kmem_commit_charge(struct page *page,
> - struct mem_cgroup *memcg, int order);
> -void __memcg_kmem_uncharge_pages(struct page *page, int order);
> +int __memcg_kmem_charge(struct page *page, gfp_t gfp, int order);
> +void __memcg_kmem_uncharge(struct page *page, int order);
>
> /*
> * helper for acessing a memcg's index. It will be used as an index in the
> @@ -789,52 +786,30 @@ static inline bool __memcg_kmem_bypass(gfp_t gfp)
> }
>
> /**
> - * memcg_kmem_newpage_charge: verify if a new kmem allocation is allowed.
> - * @gfp: the gfp allocation flags.
> - * @memcg: a pointer to the memcg this was charged against.
> - * @order: allocation order.
> + * memcg_kmem_charge: charge a kmem page
> + * @page: page to charge
> + * @gfp: reclaim mode
> + * @order: allocation order
> *
> - * returns true if the memcg where the current task belongs can hold this
> - * allocation.
> - *
> - * We return true automatically if this allocation is not to be accounted to
> - * any memcg.
> + * Returns 0 on success, an error code on failure.
> */
> -static inline bool
> -memcg_kmem_newpage_charge(gfp_t gfp, struct mem_cgroup **memcg, int order)
> +static __always_inline int memcg_kmem_charge(struct page *page,
> + gfp_t gfp, int order)
> {
> if (__memcg_kmem_bypass(gfp))
> - return true;
> - return __memcg_kmem_newpage_charge(gfp, memcg, order);
> + return 0;
> + return __memcg_kmem_charge(page, gfp, order);
> }
>
> /**
> - * memcg_kmem_uncharge_pages: uncharge pages from memcg
> - * @page: pointer to struct page being freed
> - * @order: allocation order.
> + * memcg_kmem_uncharge: uncharge a kmem page
> + * @page: page to uncharge
> + * @order: allocation order
> */
> -static inline void
> -memcg_kmem_uncharge_pages(struct page *page, int order)
> +static __always_inline void memcg_kmem_uncharge(struct page *page, int order)
> {
> if (memcg_kmem_enabled())
> - __memcg_kmem_uncharge_pages(page, order);
> -}
> -
> -/**
> - * memcg_kmem_commit_charge: embeds correct memcg in a page
> - * @page: pointer to struct page recently allocated
> - * @memcg: the memcg structure we charged against
> - * @order: allocation order.
> - *
> - * Needs to be called after memcg_kmem_newpage_charge, regardless of success or
> - * failure of the allocation. if @page is NULL, this function will revert the
> - * charges. Otherwise, it will commit @page to @memcg.
> - */
> -static inline void
> -memcg_kmem_commit_charge(struct page *page, struct mem_cgroup *memcg, int order)
> -{
> - if (memcg_kmem_enabled() && memcg)
> - __memcg_kmem_commit_charge(page, memcg, order);
> + __memcg_kmem_uncharge(page, order);
> }
>
> /**
> @@ -878,18 +853,12 @@ static inline bool memcg_kmem_is_active(struct mem_cgroup *memcg)
> return false;
> }
>
> -static inline bool
> -memcg_kmem_newpage_charge(gfp_t gfp, struct mem_cgroup **memcg, int order)
> -{
> - return true;
> -}
> -
> -static inline void memcg_kmem_uncharge_pages(struct page *page, int order)
> +static inline int memcg_kmem_charge(struct page *page, gfp_t gfp, int order)
> {
> + return 0;
> }
>
> -static inline void
> -memcg_kmem_commit_charge(struct page *page, struct mem_cgroup *memcg, int order)
> +static inline void memcg_kmem_uncharge(struct page *page, int order)
> {
> }
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 44706a17cddc..7c0af36fc8d0 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2404,57 +2404,26 @@ void __memcg_kmem_put_cache(struct kmem_cache *cachep)
> css_put(&cachep->memcg_params.memcg->css);
> }
>
> -/*
> - * We need to verify if the allocation against current->mm->owner's memcg is
> - * possible for the given order. But the page is not allocated yet, so we'll
> - * need a further commit step to do the final arrangements.
> - *
> - * It is possible for the task to switch cgroups in this mean time, so at
> - * commit time, we can't rely on task conversion any longer. We'll then use
> - * the handle argument to return to the caller which cgroup we should commit
> - * against. We could also return the memcg directly and avoid the pointer
> - * passing, but a boolean return value gives better semantics considering
> - * the compiled-out case as well.
> - *
> - * Returning true means the allocation is possible.
> - */
> -bool
> -__memcg_kmem_newpage_charge(gfp_t gfp, struct mem_cgroup **_memcg, int order)
> +int __memcg_kmem_charge(struct page *page, gfp_t gfp, int order)
> {
> struct mem_cgroup *memcg;
> int ret;
>
> - *_memcg = NULL;
> -
> memcg = get_mem_cgroup_from_mm(current->mm);
>
> if (!memcg_kmem_is_active(memcg)) {
> css_put(&memcg->css);
> - return true;
> + return 0;
> }
>
> ret = memcg_charge_kmem(memcg, gfp, 1 << order);
> - if (!ret)
> - *_memcg = memcg;
>
> css_put(&memcg->css);
> - return (ret == 0);
> -}
> -
> -void __memcg_kmem_commit_charge(struct page *page, struct mem_cgroup *memcg,
> - int order)
> -{
> - VM_BUG_ON(mem_cgroup_is_root(memcg));
> -
> - /* The page allocation failed. Revert */
> - if (!page) {
> - memcg_uncharge_kmem(memcg, 1 << order);
> - return;
> - }
> page->mem_cgroup = memcg;
> + return ret;
> }
>
> -void __memcg_kmem_uncharge_pages(struct page *page, int order)
> +void __memcg_kmem_uncharge(struct page *page, int order)
> {
> struct mem_cgroup *memcg = page->mem_cgroup;
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index e19132074404..91d1a1923eb8 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3414,24 +3414,24 @@ EXPORT_SYMBOL(__free_page_frag);
> struct page *alloc_kmem_pages(gfp_t gfp_mask, unsigned int order)
> {
> struct page *page;
> - struct mem_cgroup *memcg = NULL;
>
> - if (!memcg_kmem_newpage_charge(gfp_mask, &memcg, order))
> - return NULL;
> page = alloc_pages(gfp_mask, order);
> - memcg_kmem_commit_charge(page, memcg, order);
> + if (page && memcg_kmem_charge(page, gfp_mask, order) != 0) {
> + __free_pages(page, order);
> + page = NULL;
> + }
> return page;
> }
>
> struct page *alloc_kmem_pages_node(int nid, gfp_t gfp_mask, unsigned int order)
> {
> struct page *page;
> - struct mem_cgroup *memcg = NULL;
>
> - if (!memcg_kmem_newpage_charge(gfp_mask, &memcg, order))
> - return NULL;
> page = alloc_pages_node(nid, gfp_mask, order);
> - memcg_kmem_commit_charge(page, memcg, order);
> + if (page && memcg_kmem_charge(page, gfp_mask, order) != 0) {
> + __free_pages(page, order);
> + page = NULL;
> + }
> return page;
> }
>
> @@ -3441,7 +3441,7 @@ struct page *alloc_kmem_pages_node(int nid, gfp_t gfp_mask, unsigned int order)
> */
> void __free_kmem_pages(struct page *page, unsigned int order)
> {
> - memcg_kmem_uncharge_pages(page, order);
> + memcg_kmem_uncharge(page, order);
> __free_pages(page, order);
> }
>
> --
> 2.1.4
--
Michal Hocko
SUSE Labs
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