Re: [PATCH v9 3/8] hugetlb_cgroup: add reservation accounting for private mappings

[Mina Almasry]

On Mon, Jan 13, 2020 at 4:55 PM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
> > diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> > index dea6143aa0685..e6ab499ba2086 100644
> > --- a/include/linux/hugetlb.h
> > +++ b/include/linux/hugetlb.h
> > @@ -46,6 +46,15 @@ struct resv_map {
> >       long adds_in_progress;
> >       struct list_head region_cache;
> >       long region_cache_count;
> > +#ifdef CONFIG_CGROUP_HUGETLB
> > +     /*
> > +      * On private mappings, the counter to uncharge reservations is stored
> > +      * here. If these fields are 0, then the mapping is shared.
>
> Will *reservation_counter ALWAYS be non-NULL for private mappings?
>
> More on this below.
>
> > +      */
> > +     struct page_counter *reservation_counter;
> > +     unsigned long pages_per_hpage;
> > +     struct cgroup_subsys_state *css;
> > +#endif
> >  };
> >  extern struct resv_map *resv_map_alloc(void);
> >  void resv_map_release(struct kref *ref);
> > diff --git a/include/linux/hugetlb_cgroup.h b/include/linux/hugetlb_cgroup.h
> > index eab8a70d5bcb5..8c320accefe87 100644
> > --- a/include/linux/hugetlb_cgroup.h
> > +++ b/include/linux/hugetlb_cgroup.h
> > @@ -25,6 +25,33 @@ struct hugetlb_cgroup;
> >  #define HUGETLB_CGROUP_MIN_ORDER     2
> >
> >  #ifdef CONFIG_CGROUP_HUGETLB
> > +enum hugetlb_memory_event {
> > +     HUGETLB_MAX,
> > +     HUGETLB_NR_MEMORY_EVENTS,
> > +};
> > +
> > +struct hugetlb_cgroup {
> > +     struct cgroup_subsys_state css;
> > +
> > +     /*
> > +      * the counter to account for hugepages from hugetlb.
> > +      */
> > +     struct page_counter hugepage[HUGE_MAX_HSTATE];
> > +
> > +     /*
> > +      * the counter to account for hugepage reservations from hugetlb.
> > +      */
> > +     struct page_counter reserved_hugepage[HUGE_MAX_HSTATE];
> > +
> > +     atomic_long_t events[HUGE_MAX_HSTATE][HUGETLB_NR_MEMORY_EVENTS];
> > +     atomic_long_t events_local[HUGE_MAX_HSTATE][HUGETLB_NR_MEMORY_EVENTS];
> > +
> > +     /* Handle for "hugetlb.events" */
> > +     struct cgroup_file events_file[HUGE_MAX_HSTATE];
> > +
> > +     /* Handle for "hugetlb.events.local" */
> > +     struct cgroup_file events_local_file[HUGE_MAX_HSTATE];
> > +};
> >
> >  static inline struct hugetlb_cgroup *hugetlb_cgroup_from_page(struct page *page,
> >                                                             bool reserved)
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index e6e8240f1718c..7782977970301 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -665,6 +665,17 @@ struct resv_map *resv_map_alloc(void)
> >       INIT_LIST_HEAD(&resv_map->regions);
> >
> >       resv_map->adds_in_progress = 0;
> > +#ifdef CONFIG_CGROUP_HUGETLB
> > +     /*
> > +      * Initialize these to 0. On shared mappings, 0's here indicate these
> > +      * fields don't do cgroup accounting. On private mappings, these will be
> > +      * re-initialized to the proper values, to indicate that hugetlb cgroup
> > +      * reservations are to be un-charged from here.
> > +      */
> > +     resv_map->reservation_counter = NULL;
> > +     resv_map->pages_per_hpage = 0;
> > +     resv_map->css = NULL;
> > +#endif
> >
> >       INIT_LIST_HEAD(&resv_map->region_cache);
> >       list_add(&rg->link, &resv_map->region_cache);
> > @@ -3145,7 +3156,20 @@ static void hugetlb_vm_op_close(struct vm_area_struct *vma)
> >
> >       reserve = (end - start) - region_count(resv, start, end);
> >
> > -     kref_put(&resv->refs, resv_map_release);
> > +#ifdef CONFIG_CGROUP_HUGETLB
> > +     /*
> > +      * Since we check for HPAGE_RESV_OWNER above, this must a private
> > +      * mapping, and these values should be none-zero, and should point to
> > +      * the hugetlb_cgroup counter to uncharge for this reservation.
> > +      */
> > +     WARN_ON(!resv->reservation_counter);
> > +     WARN_ON(!resv->pages_per_hpage);
> > +     WARN_ON(!resv->css);
>
> I was once again wondering if these were always non-NULL for private mappings.
> It seems that reservation_counter (h_gc) would be NULL in these cases from
> these early checks in hugetlb_cgroup_charge_cgroup().
>

You are right. I'm fixing in v10 the code and comments to account for
h_cg potentially being NULL, but I'm having trouble testing. Looking
at the code, I'm a bit confused by the checks. Seems to me
hugetlb_cgroup_disabled() is the same as #ifdef CONFIG_CGROUP_HUGETLB;
I can't find a way to enable the Kconfig but have that return false
unless I hack the code. Also seems to me checking huge_page_order is
just super definsive; I skimmed the hugepage sizes allowed code and I
can't find an arch that allows you to configure hugetlb page size to <
2^HUGETLB_CGROUP_MIN_ORDER pages. So in reality these will never fire,
IIUC.

> int hugetlb_cgroup_charge_cgroup(int idx, unsigned long nr_pages,
>                                  struct hugetlb_cgroup **ptr, bool reserved)
> {
>         int ret = 0;
>         struct page_counter *counter;
>         struct hugetlb_cgroup *h_cg = NULL;
>
>         if (hugetlb_cgroup_disabled())
>                 goto done;
>         /*
>          * We don't charge any cgroup if the compound page have less
>          * than 3 pages.
>          */
>         if (huge_page_order(&hstates[idx]) < HUGETLB_CGROUP_MIN_ORDER)
>                 goto done;
>         ...
>
> It seems like the following hugetlb_cgroup_uncharge_counter() guards
> against reservation_counter being NULL (for some of the same reasons).
>
> > +
> > +     hugetlb_cgroup_uncharge_counter(resv->reservation_counter,
> > +                                     (end - start) * resv->pages_per_hpage,
> > +                                     resv->css);
> > +#endif
> >
> >       if (reserve) {
> >               /*
> > @@ -3155,6 +3179,8 @@ static void hugetlb_vm_op_close(struct vm_area_struct *vma)
> >               gbl_reserve = hugepage_subpool_put_pages(spool, reserve);
> >               hugetlb_acct_memory(h, -gbl_reserve);
> >       }
> > +
> > +     kref_put(&resv->refs, resv_map_release);
> >  }
> >
> >  static int hugetlb_vm_op_split(struct vm_area_struct *vma, unsigned long addr)
> > @@ -4501,6 +4527,7 @@ int hugetlb_reserve_pages(struct inode *inode,
> >       struct hstate *h = hstate_inode(inode);
> >       struct hugepage_subpool *spool = subpool_inode(inode);
> >       struct resv_map *resv_map;
> > +     struct hugetlb_cgroup *h_cg;
> >       long gbl_reserve;
> >
> >       /* This should never happen */
> > @@ -4534,12 +4561,30 @@ int hugetlb_reserve_pages(struct inode *inode,
> >               chg = region_chg(resv_map, from, to);
> >
> >       } else {
> > +             /* Private mapping. */
> >               resv_map = resv_map_alloc();
> >               if (!resv_map)
> >                       return -ENOMEM;
> >
> >               chg = to - from;
> >
> > +             if (hugetlb_cgroup_charge_cgroup(hstate_index(h),
> > +                                              chg * pages_per_huge_page(h),
> > +                                              &h_cg, true)) {
> > +                     kref_put(&resv_map->refs, resv_map_release);
> > +                     return -ENOMEM;
> > +             }
> > +
>
> Shouldn't this code be in the #ifdef CONFIG_CGROUP_HUGETLB block?

Not necessary AFAICT, hugetlb_cgroup_charge_cgroup stub returns 0 (no-op).