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SubjectRe: [RFC PATCH v2 4/5] hugetlb_cgroup: Add accounting for shared mappings
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On 8/8/19 4:13 PM, Mina Almasry wrote:
> For shared mappings, the pointer to the hugetlb_cgroup to uncharge lives
> in the resv_map entries, in file_region->reservation_counter.
>
> When a file_region entry is added to the resv_map via region_add, we
> also charge the appropriate hugetlb_cgroup and put the pointer to that
> in file_region->reservation_counter. This is slightly delicate since we
> need to not modify the resv_map until we know that charging the
> reservation has succeeded. If charging doesn't succeed, we report the
> error to the caller, so that the kernel fails the reservation.

I wish we did not need to modify these region_() routines as they are
already difficult to understand. However, I see no other way with the
desired semantics.

> On region_del, which is when the hugetlb memory is unreserved, we delete
> the file_region entry in the resv_map, but also uncharge the
> file_region->reservation_counter.
>
> ---
> mm/hugetlb.c | 208 +++++++++++++++++++++++++++++++++++++++++----------
> 1 file changed, 170 insertions(+), 38 deletions(-)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 235996aef6618..d76e3137110ab 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -242,8 +242,72 @@ struct file_region {
> struct list_head link;
> long from;
> long to;
> +#ifdef CONFIG_CGROUP_HUGETLB
> + /*
> + * On shared mappings, each reserved region appears as a struct
> + * file_region in resv_map. These fields hold the info needed to
> + * uncharge each reservation.
> + */
> + struct page_counter *reservation_counter;
> + unsigned long pages_per_hpage;
> +#endif
> };
>
> +/* Must be called with resv->lock held. Calling this with dry_run == true will
> + * count the number of pages added but will not modify the linked list.
> + */
> +static long consume_regions_we_overlap_with(struct file_region *rg,
> + struct list_head *head, long f, long *t,
> + struct hugetlb_cgroup *h_cg,
> + struct hstate *h,
> + bool dry_run)
> +{
> + long add = 0;
> + struct file_region *trg = NULL, *nrg = NULL;
> +
> + /* Consume any regions we now overlap with. */
> + nrg = rg;
> + list_for_each_entry_safe(rg, trg, rg->link.prev, link) {
> + if (&rg->link == head)
> + break;
> + if (rg->from > *t)
> + break;
> +
> + /* If this area reaches higher then extend our area to
> + * include it completely. If this is not the first area
> + * which we intend to reuse, free it.
> + */
> + if (rg->to > *t)
> + *t = rg->to;
> + if (rg != nrg) {
> + /* Decrement return value by the deleted range.
> + * Another range will span this area so that by
> + * end of routine add will be >= zero
> + */
> + add -= (rg->to - rg->from);
> + if (!dry_run) {
> + list_del(&rg->link);
> + kfree(rg);

Is it possible that the region struct we are deleting pointed to
a reservation_counter? Perhaps even for another cgroup?
Just concerned with the way regions are coalesced that we may be
deleting counters.

--
Mike Kravetz

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