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SubjectRe: [PATCH v9 3/8] hugetlb_cgroup: add reservation accounting for private mappings
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On 12/17/19 3:16 PM, Mina Almasry wrote:
> Normally the pointer to the cgroup to uncharge hangs off the struct
> page, and gets queried when it's time to free the page. With
> hugetlb_cgroup reservations, this is not possible. Because it's possible
> for a page to be reserved by one task and actually faulted in by another
> task.
>
> The best place to put the hugetlb_cgroup pointer to uncharge for
> reservations is in the resv_map. But, because the resv_map has different
> semantics for private and shared mappings, the code patch to
> charge/uncharge shared and private mappings is different. This patch
> implements charging and uncharging for private mappings.
>
> For private mappings, the counter to uncharge is in
> resv_map->reservation_counter. On initializing the resv_map this is set
> to NULL. On reservation of a region in private mapping, the tasks
> hugetlb_cgroup is charged and the hugetlb_cgroup is placed is
> resv_map->reservation_counter.
>
> On hugetlb_vm_op_close, we uncharge resv_map->reservation_counter.
>
> Signed-off-by: Mina Almasry <almasrymina@google.com>
> Acked-by: Hillf Danton <hdanton@sina.com>
>
> ---
>
> Changes in V9:
> - Updated for reparenting of hugetlb reservation accounting.
>
> ---
> include/linux/hugetlb.h | 9 +++++++
> include/linux/hugetlb_cgroup.h | 27 +++++++++++++++++++
> mm/hugetlb.c | 47 +++++++++++++++++++++++++++++++++-
> mm/hugetlb_cgroup.c | 28 --------------------
> 4 files changed, 82 insertions(+), 29 deletions(-)
>
> 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().

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?
--
Mike Kravetz

> +#ifdef CONFIG_CGROUP_HUGETLB
> + /*
> + * Since this branch handles private mappings, we attach the
> + * counter to uncharge for this reservation off resv_map.
> + */
> + resv_map->reservation_counter =
> + &h_cg->reserved_hugepage[hstate_index(h)];
> + resv_map->pages_per_hpage = pages_per_huge_page(h);
> +#endif
> +
> set_vma_resv_map(vma, resv_map);
> set_vma_resv_flags(vma, HPAGE_RESV_OWNER);
> }

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