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

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;

+}