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Message-ID: <CAMZfGtW7zpitVKD==cOArYPGC1uXrqibxKvPGqwHjndtEEJVyw@mail.gmail.com>
Date: Sat, 6 Mar 2021 14:02:16 +0800
From: Muchun Song <songmuchun@...edance.com>
To: Roman Gushchin <guro@...com>
Cc: Johannes Weiner <hannes@...xchg.org>,
Michal Hocko <mhocko@...nel.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Shakeel Butt <shakeelb@...gle.com>,
Vladimir Davydov <vdavydov.dev@...il.com>,
LKML <linux-kernel@...r.kernel.org>,
Linux Memory Management List <linux-mm@...ck.org>,
Xiongchun duan <duanxiongchun@...edance.com>
Subject: Re: [External] Re: [PATCH v2 2/5] mm: memcontrol: make
page_memcg{_rcu} only applicable for non-kmem page
On Sat, Mar 6, 2021 at 3:00 AM Roman Gushchin <guro@...com> wrote:
>
> On Wed, Mar 03, 2021 at 01:59:14PM +0800, Muchun Song wrote:
> > We want to reuse the obj_cgroup APIs to charge the kmem pages when
> > If we do that, we should store an object cgroup pointer to
> > page->memcg_data for the kmem pages.
> >
> > Finally, page->memcg_data can have 3 different meanings.
> >
> > 1) For the slab pages, page->memcg_data points to an object cgroups
> > vector.
> >
> > 2) For the kmem pages (exclude the slab pages), page->memcg_data
> > points to an object cgroup.
> >
> > 3) For the user pages (e.g. the LRU pages), page->memcg_data points
> > to a memory cgroup.
> >
> > Currently we always get the memory cgroup associated with a page via
> > page_memcg() or page_memcg_rcu(). page_memcg_check() is special, it
> > has to be used in cases when it's not known if a page has an
> > associated memory cgroup pointer or an object cgroups vector. Because
> > the page->memcg_data of the kmem page is not pointing to a memory
> > cgroup in the later patch, the page_memcg() and page_memcg_rcu()
> > cannot be applicable for the kmem pages. In this patch, make
> > page_memcg() and page_memcg_rcu() no longer apply to the kmem pages.
> > We do not change the behavior of the page_memcg_check(), it is also
> > applicable for the kmem pages.
> >
> > In the end, there are 3 helpers to get the memcg associated with a page.
> > Usage is as follows.
> >
> > 1) Get the memory cgroup associated with a non-kmem page (e.g. the LRU
> > pages).
> >
> > - page_memcg()
> > - page_memcg_rcu()
> >
> > 2) Get the memory cgroup associated with a page. It has to be used in
> > cases when it's not known if a page has an associated memory cgroup
> > pointer or an object cgroups vector. Returns NULL for slab pages or
> > uncharged pages. Otherwise, returns memory cgroup for charged pages
> > (e.g. the kmem pages, the LRU pages).
> >
> > - page_memcg_check()
> >
> > In some place, we use page_memcg() to check whether the page is charged.
> > Now introduce page_memcg_charged() helper to do that.
> >
> > This is a preparation for reparenting the kmem pages.
> >
> > Signed-off-by: Muchun Song <songmuchun@...edance.com>
>
> This patch also looks good to me, but, please, make it safe for adding
> new memcg_data flags. E.g. if someone will add a new flag with a completely
> new meaning, it shouldn't break the code.
I'm not thoughtful enough. Will do that in the next version.
Thanks for your suggestions.
>
> I'll ack it after another look at the final version, but overall it
> looks good.
>
> Thanks!
>
> > ---
> > include/linux/memcontrol.h | 36 ++++++++++++++++++++++++++++--------
> > mm/memcontrol.c | 23 +++++++++++++----------
> > mm/page_alloc.c | 4 ++--
> > 3 files changed, 43 insertions(+), 20 deletions(-)
> >
> > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > index e6dc793d587d..049b80246cbf 100644
> > --- a/include/linux/memcontrol.h
> > +++ b/include/linux/memcontrol.h
> > @@ -358,14 +358,26 @@ enum page_memcg_data_flags {
> >
> > #define MEMCG_DATA_FLAGS_MASK (__NR_MEMCG_DATA_FLAGS - 1)
> >
> > +/* Return true for charged page, otherwise false. */
> > +static inline bool page_memcg_charged(struct page *page)
> > +{
> > + unsigned long memcg_data = page->memcg_data;
> > +
> > + VM_BUG_ON_PAGE(PageSlab(page), page);
> > + VM_BUG_ON_PAGE(memcg_data & MEMCG_DATA_OBJCGS, page);
> > +
> > + return !!memcg_data;
> > +}
> > +
> > /*
> > - * page_memcg - get the memory cgroup associated with a page
> > + * page_memcg - get the memory cgroup associated with a non-kmem page
> > * @page: a pointer to the page struct
> > *
> > * Returns a pointer to the memory cgroup associated with the page,
> > * or NULL. This function assumes that the page is known to have a
> > * proper memory cgroup pointer. It's not safe to call this function
> > - * against some type of pages, e.g. slab pages or ex-slab pages.
> > + * against some type of pages, e.g. slab pages, kmem pages or ex-slab
> > + * pages.
> > *
> > * Any of the following ensures page and memcg binding stability:
> > * - the page lock
> > @@ -378,27 +390,30 @@ static inline struct mem_cgroup *page_memcg(struct page *page)
> > unsigned long memcg_data = page->memcg_data;
> >
> > VM_BUG_ON_PAGE(PageSlab(page), page);
> > - VM_BUG_ON_PAGE(memcg_data & MEMCG_DATA_OBJCGS, page);
> > + VM_BUG_ON_PAGE(memcg_data & MEMCG_DATA_FLAGS_MASK, page);
> >
> > - return (struct mem_cgroup *)(memcg_data & ~MEMCG_DATA_FLAGS_MASK);
> > + return (struct mem_cgroup *)memcg_data;
> > }
> >
> > /*
> > - * page_memcg_rcu - locklessly get the memory cgroup associated with a page
> > + * page_memcg_rcu - locklessly get the memory cgroup associated with a non-kmem page
> > * @page: a pointer to the page struct
> > *
> > * Returns a pointer to the memory cgroup associated with the page,
> > * or NULL. This function assumes that the page is known to have a
> > * proper memory cgroup pointer. It's not safe to call this function
> > - * against some type of pages, e.g. slab pages or ex-slab pages.
> > + * against some type of pages, e.g. slab pages, kmem pages or ex-slab
> > + * pages.
> > */
> > static inline struct mem_cgroup *page_memcg_rcu(struct page *page)
> > {
> > + unsigned long memcg_data = READ_ONCE(page->memcg_data);
> > +
> > VM_BUG_ON_PAGE(PageSlab(page), page);
> > + VM_BUG_ON_PAGE(memcg_data & MEMCG_DATA_FLAGS_MASK, page);
> > WARN_ON_ONCE(!rcu_read_lock_held());
> >
> > - return (struct mem_cgroup *)(READ_ONCE(page->memcg_data) &
> > - ~MEMCG_DATA_FLAGS_MASK);
> > + return (struct mem_cgroup *)memcg_data;
> > }
> >
> > /*
> > @@ -1072,6 +1087,11 @@ void mem_cgroup_split_huge_fixup(struct page *head);
> >
> > struct mem_cgroup;
> >
> > +static inline bool page_memcg_charged(struct page *page)
> > +{
> > + return false;
> > +}
> > +
> > static inline struct mem_cgroup *page_memcg(struct page *page)
> > {
> > return NULL;
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index faae16def127..86a8db937ec6 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -855,10 +855,11 @@ void __mod_lruvec_page_state(struct page *page, enum node_stat_item idx,
> > int val)
> > {
> > struct page *head = compound_head(page); /* rmap on tail pages */
> > - struct mem_cgroup *memcg = page_memcg(head);
> > + struct mem_cgroup *memcg;
> > pg_data_t *pgdat = page_pgdat(page);
> > struct lruvec *lruvec;
> >
> > + memcg = page_memcg_check(head);
> > /* Untracked pages have no memcg, no lruvec. Update only the node */
> > if (!memcg) {
> > __mod_node_page_state(pgdat, idx, val);
> > @@ -3166,12 +3167,13 @@ int __memcg_kmem_charge_page(struct page *page, gfp_t gfp, int order)
> > */
> > void __memcg_kmem_uncharge_page(struct page *page, int order)
> > {
> > - struct mem_cgroup *memcg = page_memcg(page);
> > + struct mem_cgroup *memcg;
> > unsigned int nr_pages = 1 << order;
> >
> > - if (!memcg)
> > + if (!page_memcg_charged(page))
> > return;
> >
> > + memcg = page_memcg_check(page);
> > VM_BUG_ON_PAGE(mem_cgroup_is_root(memcg), page);
> > __memcg_kmem_uncharge(memcg, nr_pages);
> > page->memcg_data = 0;
> > @@ -6827,24 +6829,25 @@ static void uncharge_batch(const struct uncharge_gather *ug)
> > static void uncharge_page(struct page *page, struct uncharge_gather *ug)
> > {
> > unsigned long nr_pages;
> > + struct mem_cgroup *memcg;
> >
> > VM_BUG_ON_PAGE(PageLRU(page), page);
> >
> > - if (!page_memcg(page))
> > + if (!page_memcg_charged(page))
> > return;
> >
> > /*
> > * Nobody should be changing or seriously looking at
> > - * page_memcg(page) at this point, we have fully
> > - * exclusive access to the page.
> > + * page memcg at this point, we have fully exclusive
> > + * access to the page.
> > */
> > -
> > - if (ug->memcg != page_memcg(page)) {
> > + memcg = page_memcg_check(page);
> > + if (ug->memcg != memcg) {
> > if (ug->memcg) {
> > uncharge_batch(ug);
> > uncharge_gather_clear(ug);
> > }
> > - ug->memcg = page_memcg(page);
> > + ug->memcg = memcg;
> >
> > /* pairs with css_put in uncharge_batch */
> > css_get(&ug->memcg->css);
> > @@ -6877,7 +6880,7 @@ void mem_cgroup_uncharge(struct page *page)
> > return;
> >
> > /* Don't touch page->lru of any random page, pre-check: */
> > - if (!page_memcg(page))
> > + if (!page_memcg_charged(page))
> > return;
> >
> > uncharge_gather_clear(&ug);
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index f10966e3b4a5..bcb58ae15e24 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -1124,7 +1124,7 @@ static inline bool page_expected_state(struct page *page,
> > if (unlikely((unsigned long)page->mapping |
> > page_ref_count(page) |
> > #ifdef CONFIG_MEMCG
> > - (unsigned long)page_memcg(page) |
> > + page_memcg_charged(page) |
> > #endif
> > (page->flags & check_flags)))
> > return false;
> > @@ -1149,7 +1149,7 @@ static const char *page_bad_reason(struct page *page, unsigned long flags)
> > bad_reason = "PAGE_FLAGS_CHECK_AT_FREE flag(s) set";
> > }
> > #ifdef CONFIG_MEMCG
> > - if (unlikely(page_memcg(page)))
> > + if (unlikely(page_memcg_charged(page)))
> > bad_reason = "page still charged to cgroup";
> > #endif
> > return bad_reason;
> > --
> > 2.11.0
> >
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