Re: [PATCH v5 4/4] mm: convert page kmemcg type to a page memcg flag

From: Michal Hocko
Date: Wed Oct 14 2020 - 10:40:13 EST

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On Fri 02-10-20 10:25:59, 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.

Completely agreed. Also this concept is already used for anon/ksm
which set flags on the same pointer as the object. Having that
outsourced to elsewhere is really harder to follow.

> This commit replaces PageKmemcg() and __SetPageKmemcg() with
> PageMemcgKmem() and an open-coded OR operation setting the memcg
> pointer with the MEMCG_DATA_KMEM bit. __ClearPageKmemcg() can be
> simple deleted, as the whole memcg_data is zeroed at once.
>
> As a bonus, on !CONFIG_MEMCG build the PageMemcgKmem() check will
> be compiled out.
>
> Signed-off-by: Roman Gushchin <guro@xxxxxx>
> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

> ---
> cover.txt | 4 ++--
> include/linux/memcontrol.h | 37 +++++++++++++++++++++++++++++++++----
> include/linux/page-flags.h | 11 ++---------
> mm/memcontrol.c | 16 +++++-----------
> mm/page_alloc.c | 4 ++--
> 5 files changed, 44 insertions(+), 28 deletions(-)
>
> diff --git a/cover.txt b/cover.txt
> index 32460cd3316e..cf23753cad15 100644
> --- a/cover.txt
> +++ b/cover.txt
> @@ -15,10 +15,10 @@ their memory can't be accounted because of this implementation detail.
>
> This patchset removes this limitation by moving the PageKmemcg flag
> into one of the free bits of the page->mem_cgroup pointer. Also it
> -formalizes all accesses to the page->mem_cgroup and page->obj_cgroups
> +formalizes accesses to the page->mem_cgroup and page->obj_cgroups
> using new helpers, adds several checks and removes a couple of obsolete
> functions. As the result the code became more robust with fewer
> -open-coded bits tricks.
> +open-coded bit tricks.
>
> v5:
> - added ~MEMCG_DATA_FLAGS_MASK to page_memcg_rcu(), by Michal
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 99a4841d658b..7c9d43476166 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -346,8 +346,10 @@ extern struct mem_cgroup *root_mem_cgroup;
> enum page_memcg_data_flags {
> /* page->memcg_data is a pointer to an objcgs vector */
> MEMCG_DATA_OBJCGS = (1UL << 0),
> + /* page has been accounted as a non-slab kernel page */
> + MEMCG_DATA_KMEM = (1UL << 1),
> /* the next bit after the last actual flag */
> - __NR_MEMCG_DATA_FLAGS = (1UL << 1),
> + __NR_MEMCG_DATA_FLAGS = (1UL << 2),
> };
>
> #define MEMCG_DATA_FLAGS_MASK (__NR_MEMCG_DATA_FLAGS - 1)
> @@ -369,8 +371,12 @@ enum page_memcg_data_flags {
> */
> static inline struct mem_cgroup *page_memcg(struct page *page)
> {
> + unsigned long memcg_data = page->memcg_data;
> +
> VM_BUG_ON_PAGE(PageSlab(page), page);
> - return (struct mem_cgroup *)page->memcg_data;
> + VM_BUG_ON_PAGE(memcg_data & MEMCG_DATA_OBJCGS, page);
> +
> + return (struct mem_cgroup *)(memcg_data & ~MEMCG_DATA_FLAGS_MASK);
> }
>
> /*
> @@ -387,7 +393,8 @@ static inline struct mem_cgroup *page_memcg_rcu(struct page *page)
> VM_BUG_ON_PAGE(PageSlab(page), page);
> WARN_ON_ONCE(!rcu_read_lock_held());
>
> - return (struct mem_cgroup *)READ_ONCE(page->memcg_data);
> + return (struct mem_cgroup *)(READ_ONCE(page->memcg_data) &
> + ~MEMCG_DATA_FLAGS_MASK);
> }
>
> /*
> @@ -416,7 +423,21 @@ static inline struct mem_cgroup *page_memcg_check(struct page *page)
> if (memcg_data & MEMCG_DATA_OBJCGS)
> return NULL;
>
> - return (struct mem_cgroup *)memcg_data;
> + return (struct mem_cgroup *)(memcg_data & ~MEMCG_DATA_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(page->memcg_data & MEMCG_DATA_OBJCGS, page);
> + return page->memcg_data & MEMCG_DATA_KMEM;
> }
>
> #ifdef CONFIG_MEMCG_KMEM
> @@ -435,6 +456,7 @@ static inline struct obj_cgroup **page_objcgs(struct page *page)
> unsigned long memcg_data = READ_ONCE(page->memcg_data);
>
> VM_BUG_ON_PAGE(memcg_data && !(memcg_data & MEMCG_DATA_OBJCGS), page);
> + VM_BUG_ON_PAGE(memcg_data & MEMCG_DATA_KMEM, page);
>
> return (struct obj_cgroup **)(memcg_data & ~MEMCG_DATA_FLAGS_MASK);
> }
> @@ -454,6 +476,8 @@ static inline struct obj_cgroup **page_objcgs_check(struct page *page)
> if (!memcg_data || !(memcg_data & MEMCG_DATA_OBJCGS))
> return NULL;
>
> + VM_BUG_ON_PAGE(memcg_data & MEMCG_DATA_KMEM, page);
> +
> return (struct obj_cgroup **)(memcg_data & ~MEMCG_DATA_FLAGS_MASK);
> }
>
> @@ -1109,6 +1133,11 @@ static inline struct mem_cgroup *page_memcg_check(struct page *page)
> return NULL;
> }
>
> +static inline bool PageMemcgKmem(struct page *page)
> +{
> + return false;
> +}
> +
> static inline bool mem_cgroup_is_root(struct mem_cgroup *memcg)
> {
> return true;
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 4f6ba9379112..fc0e1bd48e73 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -715,9 +715,8 @@ 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_guard 0x00000800
> +#define PG_table 0x00000200
> +#define PG_guard 0x00000400
>
> #define PageType(page, flag) \
> ((page->page_type & (PAGE_TYPE_BASE | flag)) == PAGE_TYPE_BASE)
> @@ -768,12 +767,6 @@ 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/mm/memcontrol.c b/mm/memcontrol.c
> index a8eb20283dcd..e3fc240101ff 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -3090,8 +3090,8 @@ int __memcg_kmem_charge_page(struct page *page, gfp_t gfp, int order)
> if (memcg && !mem_cgroup_is_root(memcg)) {
> ret = __memcg_kmem_charge(memcg, gfp, 1 << order);
> if (!ret) {
> - page->memcg_data = (unsigned long)memcg;
> - __SetPageKmemcg(page);
> + page->memcg_data = (unsigned long)memcg |
> + MEMCG_DATA_KMEM;
> return 0;
> }
> css_put(&memcg->css);
> @@ -3116,10 +3116,6 @@ void __memcg_kmem_uncharge_page(struct page *page, int order)
> __memcg_kmem_uncharge(memcg, nr_pages);
> page->memcg_data = 0;
> css_put(&memcg->css);
> -
> - /* slab pages do not have PageKmemcg flag set */
> - if (PageKmemcg(page))
> - __ClearPageKmemcg(page);
> }
>
> static bool consume_obj_stock(struct obj_cgroup *objcg, unsigned int nr_bytes)
> @@ -6863,12 +6859,10 @@ static void uncharge_page(struct page *page, struct uncharge_gather *ug)
> nr_pages = compound_nr(page);
> ug->nr_pages += nr_pages;
>
> - if (!PageKmemcg(page)) {
> - ug->pgpgout++;
> - } else {
> + if (PageMemcgKmem(page))
> ug->nr_kmem += nr_pages;
> - __ClearPageKmemcg(page);
> - }
> + else
> + ug->pgpgout++;
>
> ug->dummy_page = page;
> page->memcg_data = 0;
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index e9f0fe4a143e..5ebd50183d93 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1179,7 +1179,7 @@ static __always_inline bool free_pages_prepare(struct page *page,
> * Do not let hwpoison pages hit pcplists/buddy
> * Untie memcg state and reset page's owner
> */
> - if (memcg_kmem_enabled() && PageKmemcg(page))
> + if (memcg_kmem_enabled() && PageMemcgKmem(page))
> __memcg_kmem_uncharge_page(page, order);
> reset_page_owner(page, order);
> return false;
> @@ -1209,7 +1209,7 @@ static __always_inline bool free_pages_prepare(struct page *page,
> }
> if (PageMappingFlags(page))
> page->mapping = NULL;
> - if (memcg_kmem_enabled() && PageKmemcg(page))
> + if (memcg_kmem_enabled() && PageMemcgKmem(page))
> __memcg_kmem_uncharge_page(page, order);
> if (check_free)
> bad += check_free_page(page);
> --
> 2.26.2

--
Michal Hocko
SUSE Labs

Next message: Lukasz Stelmach: "Re: [PATCH] net: phy: Prevent reporting advertised modes when autoneg is off"
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