Re: [RFC PATCH 15/39] KVM: guest_memfd: hugetlb: allocate and truncate from hugetlb

Peter Xu
2024-12-01T17:55:44Z

On Tue, Sep 10, 2024 at 11:43:46PM +0000, Ackerley Tng wrote:
> +static struct folio *kvm_gmem_hugetlb_alloc_folio(struct hstate *h,
> + struct hugepage_subpool *spool)
> +{
> + bool memcg_charge_was_prepared;
> + struct mem_cgroup *memcg;
> + struct mempolicy *mpol;
> + nodemask_t *nodemask;
> + struct folio *folio;
> + gfp_t gfp_mask;
> + int ret;
> + int nid;
> +
> + gfp_mask = htlb_alloc_mask(h);
> +
> + memcg = get_mem_cgroup_from_current();
> + ret = mem_cgroup_hugetlb_try_charge(memcg,
> + gfp_mask | __GFP_RETRY_MAYFAIL,
> + pages_per_huge_page(h));
> + if (ret == -ENOMEM)
> + goto err;
> +
> + memcg_charge_was_prepared = ret != -EOPNOTSUPP;
> +
> + /* Pages are only to be taken from guest_memfd subpool and nowhere else. */
> + if (hugepage_subpool_get_pages(spool, 1))
> + goto err_cancel_charge;
> +
> + nid = kvm_gmem_get_mpol_node_nodemask(htlb_alloc_mask(h), &mpol,
> + &nodemask);
> + /*
> + * charge_cgroup_reservation is false because we didn't make any cgroup
> + * reservations when creating the guest_memfd subpool.

Hmm.. isn't this the exact reason to set charge_cgroup_reservation==true
instead?

IIUC gmem hugetlb pages should participate in the hugetlb cgroup resv
charge as well. It is already involved in the rest cgroup charges, and I
wonder whether it's intended that the patch treated the resv accounting
specially.

Thanks,

> + *
> + * use_hstate_resv is true because we reserved from global hstate when
> + * creating the guest_memfd subpool.
> + */
> + folio = hugetlb_alloc_folio(h, mpol, nid, nodemask, false, true);
> + mpol_cond_put(mpol);
> +
> + if (!folio)
> + goto err_put_pages;
> +
> + hugetlb_set_folio_subpool(folio, spool);
> +
> + if (memcg_charge_was_prepared)
> + mem_cgroup_commit_charge(folio, memcg);
> +
> +out:
> + mem_cgroup_put(memcg);
> +
> + return folio;
> +
> +err_put_pages:
> + hugepage_subpool_put_pages(spool, 1);
> +
> +err_cancel_charge:
> + if (memcg_charge_was_prepared)
> + mem_cgroup_cancel_charge(memcg, pages_per_huge_page(h));
> +
> +err:
> + folio = ERR_PTR(-ENOMEM);
> + goto out;
> +}

--
Peter Xu