linux-kernel - Re: [PATCH v4 4/8] hugetlb: create remove_hugetlb

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Message-ID: <YGxk85nXJD+NxzYX@localhost.localdomain>
Date:   Tue, 6 Apr 2021 15:41:07 +0200
From:   Oscar Salvador <osalvador@...e.de>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Roman Gushchin <guro@...com>, Michal Hocko <mhocko@...e.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        David Hildenbrand <david@...hat.com>,
        Muchun Song <songmuchun@...edance.com>,
        David Rientjes <rientjes@...gle.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        HORIGUCHI NAOYA <naoya.horiguchi@....com>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>,
        Waiman Long <longman@...hat.com>, Peter Xu <peterx@...hat.com>,
        Mina Almasry <almasrymina@...gle.com>,
        Hillf Danton <hdanton@...a.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Barry Song <song.bao.hua@...ilicon.com>,
        Will Deacon <will@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v4 4/8] hugetlb: create remove_hugetlb_page() to separate
 functionality

On Mon, Apr 05, 2021 at 04:00:39PM -0700, Mike Kravetz wrote:
> +static void remove_hugetlb_page(struct hstate *h, struct page *page,
> +							bool adjust_surplus)
> +{
> +	int nid = page_to_nid(page);
> +
> +	if (hstate_is_gigantic(h) && !gigantic_page_runtime_supported())
> +		return;
> +
> +	list_del(&page->lru);
> +
> +	if (HPageFreed(page)) {
> +		h->free_huge_pages--;
> +		h->free_huge_pages_node[nid]--;
> +		ClearHPageFreed(page);
> +	}
> +	if (adjust_surplus) {
> +		h->surplus_huge_pages--;
> +		h->surplus_huge_pages_node[nid]--;
> +	}
> +
> +	VM_BUG_ON_PAGE(hugetlb_cgroup_from_page(page), page);
> +	VM_BUG_ON_PAGE(hugetlb_cgroup_from_page_rsvd(page), page);

These checks feel a bit odd here.
I would move them above, before we start messing with the counters?

> +
> +	ClearHPageTemporary(page);

Why clearing it unconditionally? I guess we do not really care, but
someone might wonder why when reading the core.
So I would either do as we used to do and only clear it in case of
HPageTemporary(), or drop a comment.

> +	set_page_refcounted(page);
> +	set_compound_page_dtor(page, NULL_COMPOUND_DTOR);
> +
> +	h->nr_huge_pages--;
> +	h->nr_huge_pages_node[nid]--;
> +}

As Michal pointed out, remove_hugetlb_page() from
alloc_and_dissolve_huge_page() might be problematic in some cases because
the new_page has not been enqueued yet.

Now, I guess this might be easily fixed with checking list_empty()
before going ahead with list_del() call, or with another bool parameter
'delete', with a fat comment explaining why we can get to call remove_huge_page()
on a page that is not in any list.

Another solution that comes to my mind is to split remove_huge_page() functionality in
1) delete from list + unaccount free and surplus pages and 2) reset page's state + unaccount
nr_huge_pages

But that might be just biased as would fit for my usecase.
So maybe a list_empty() or the bool parameter might just do.
 

-- 
Oscar Salvador
SUSE L3