linux-kernel - Re: [PATCH v11 6/9] hugetlb_cgroup: support noreserve mappings

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Date:   Thu, 6 Feb 2020 14:31:46 -0800
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Mina Almasry <almasrymina@...gle.com>
Cc:     shuah@...nel.org, rientjes@...gle.com, shakeelb@...gle.com,
        gthelen@...gle.com, akpm@...ux-foundation.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-kselftest@...r.kernel.org, cgroups@...r.kernel.org
Subject: Re: [PATCH v11 6/9] hugetlb_cgroup: support noreserve mappings

On 2/3/20 3:22 PM, Mina Almasry wrote:
> Support MAP_NORESERVE accounting as part of the new counter.
> 
> For each hugepage allocation, at allocation time we check if there is
> a reservation for this allocation or not. If there is a reservation for
> this allocation, then this allocation was charged at reservation time,
> and we don't re-account it. If there is no reserevation for this
> allocation, we charge the appropriate hugetlb_cgroup.
> 
> The hugetlb_cgroup to uncharge for this allocation is stored in
> page[3].private. We use new APIs added in an earlier patch to set this
> pointer.

Ah!  That reminded me to look at the migration code.  Turns out that none
of the existing cgroup information (page[2]) is being migrated today.  That
is a bug. :(  I'll confirm and fix in a patch separate from this series.
We will need to make sure that new information added by this series in page[3]
is also migrated.  That would be in an earlier patch where the use of the
field is introduced.

> 
> Signed-off-by: Mina Almasry <almasrymina@...gle.com>
> 
> ---
> 
> Changes in v10:
> - Refactored deferred_reserve check.
> 
> ---
>  mm/hugetlb.c | 28 +++++++++++++++++++++++++++-
>  1 file changed, 27 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 33818ccaf7e89..ec0b55ea1506e 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1339,6 +1339,9 @@ static void __free_huge_page(struct page *page)
>  	clear_page_huge_active(page);
>  	hugetlb_cgroup_uncharge_page(hstate_index(h), pages_per_huge_page(h),
>  				     page, false);
> +	hugetlb_cgroup_uncharge_page(hstate_index(h), pages_per_huge_page(h),
> +				     page, true);
> +

When looking at the code without change markings, the two above lines
look so similar my first thought is there must be a mistake.

A suggestion for better code readability:
- hugetlb_cgroup_uncharge_page could just take "struct hstate *h" and
  get both hstate_index(h) and pages_per_huge_page(h).
- Perhaps make hugetlb_cgroup_uncharge_page and
  hugetlb_cgroup_uncharge_page_rsvd be wrappers around a common routine.
  Then the above would look like:

  hugetlb_cgroup_uncharge_page(h, page);
  hugetlb_cgroup_uncharge_page_rsvd(h, page);
  

>  	if (restore_reserve)
>  		h->resv_huge_pages++;
> 
> @@ -2172,6 +2175,7 @@ struct page *alloc_huge_page(struct vm_area_struct *vma,
>  	long gbl_chg;
>  	int ret, idx;
>  	struct hugetlb_cgroup *h_cg;
> +	bool deferred_reserve;
> 
>  	idx = hstate_index(h);
>  	/*
> @@ -2209,10 +2213,20 @@ struct page *alloc_huge_page(struct vm_area_struct *vma,
>  			gbl_chg = 1;
>  	}
> 
> +	/* If this allocation is not consuming a reservation, charge it now.
> +	 */
> +	deferred_reserve = map_chg || avoid_reserve || !vma_resv_map(vma);
> +	if (deferred_reserve) {
> +		ret = hugetlb_cgroup_charge_cgroup(idx, pages_per_huge_page(h),
> +						   &h_cg, true);
> +		if (ret)
> +			goto out_subpool_put;
> +	}
> +
>  	ret = hugetlb_cgroup_charge_cgroup(idx, pages_per_huge_page(h), &h_cg,
>  					   false);

Hmmm?  I'm starting to like the wrapper idea more as a way to help with
readability of the bool rsvd argument.

hugetlb_cgroup_charge_cgroup_rsvd()
hugetlb_cgroup_charge_cgroup()

At least to me it makes it easier to read.
-- 
Mike Kravetz

>  	if (ret)
> -		goto out_subpool_put;
> +		goto out_uncharge_cgroup_reservation;
> 
>  	spin_lock(&hugetlb_lock);
>  	/*
> @@ -2236,6 +2250,14 @@ struct page *alloc_huge_page(struct vm_area_struct *vma,
>  	}
>  	hugetlb_cgroup_commit_charge(idx, pages_per_huge_page(h), h_cg, page,
>  				     false);
> +	/* If allocation is not consuming a reservation, also store the
> +	 * hugetlb_cgroup pointer on the page.
> +	 */
> +	if (deferred_reserve) {
> +		hugetlb_cgroup_commit_charge(idx, pages_per_huge_page(h), h_cg,
> +					     page, true);
> +	}
> +
>  	spin_unlock(&hugetlb_lock);
> 
>  	set_page_private(page, (unsigned long)spool);
> @@ -2261,6 +2283,10 @@ struct page *alloc_huge_page(struct vm_area_struct *vma,
>  out_uncharge_cgroup:
>  	hugetlb_cgroup_uncharge_cgroup(idx, pages_per_huge_page(h), h_cg,
>  				       false);
> +out_uncharge_cgroup_reservation:
> +	if (deferred_reserve)
> +		hugetlb_cgroup_uncharge_cgroup(idx, pages_per_huge_page(h),
> +					       h_cg, true);
>  out_subpool_put:
>  	if (map_chg || avoid_reserve)
>  		hugepage_subpool_put_pages(spool, 1);
> --
> 2.25.0.341.g760bfbb309-goog