linux-kernel - Re: [PATCH v18 4/9] mm: hugetlb: alloc the vmemmap pages associated with each HugeTLB page

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Date:   Wed, 10 Mar 2021 16:19:39 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     corbet@....net, mike.kravetz@...cle.com, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, x86@...nel.org, hpa@...or.com,
        dave.hansen@...ux.intel.com, luto@...nel.org, peterz@...radead.org,
        viro@...iv.linux.org.uk, akpm@...ux-foundation.org,
        paulmck@...nel.org, mchehab+huawei@...nel.org,
        pawan.kumar.gupta@...ux.intel.com, rdunlap@...radead.org,
        oneukum@...e.com, anshuman.khandual@....com, jroedel@...e.de,
        almasrymina@...gle.com, rientjes@...gle.com, willy@...radead.org,
        osalvador@...e.de, song.bao.hua@...ilicon.com, david@...hat.com,
        naoya.horiguchi@....com, joao.m.martins@...cle.com,
        duanxiongchun@...edance.com, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, Chen Huang <chenhuang5@...wei.com>,
        Bodeddula Balasubramaniam <bodeddub@...zon.com>
Subject: Re: [PATCH v18 4/9] mm: hugetlb: alloc the vmemmap pages associated
 with each HugeTLB page

On Mon 08-03-21 18:28:02, Muchun Song wrote:
[...]
> -static void update_and_free_page(struct hstate *h, struct page *page)
> +static int update_and_free_page(struct hstate *h, struct page *page)
> +	__releases(&hugetlb_lock) __acquires(&hugetlb_lock)
>  {
>  	int i;
>  	struct page *subpage = page;
> +	int nid = page_to_nid(page);
>  
>  	if (hstate_is_gigantic(h) && !gigantic_page_runtime_supported())
> -		return;
> +		return 0;
>  
>  	h->nr_huge_pages--;
> -	h->nr_huge_pages_node[page_to_nid(page)]--;
> +	h->nr_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);

> +	set_page_refcounted(page);
> +	set_compound_page_dtor(page, NULL_COMPOUND_DTOR);
> +
> +	/*
> +	 * If the vmemmap pages associated with the HugeTLB page can be
> +	 * optimized or the page is gigantic, we might block in
> +	 * alloc_huge_page_vmemmap() or free_gigantic_page(). In both
> +	 * cases, drop the hugetlb_lock.
> +	 */
> +	if (free_vmemmap_pages_per_hpage(h) || hstate_is_gigantic(h))
> +		spin_unlock(&hugetlb_lock);
> +
> +	if (alloc_huge_page_vmemmap(h, page)) {
> +		spin_lock(&hugetlb_lock);
> +		INIT_LIST_HEAD(&page->lru);
> +		set_compound_page_dtor(page, HUGETLB_PAGE_DTOR);
> +		h->nr_huge_pages++;
> +		h->nr_huge_pages_node[nid]++;
> +
> +		/*
> +		 * If we cannot allocate vmemmap pages, just refuse to free the
> +		 * page and put the page back on the hugetlb free list and treat
> +		 * as a surplus page.
> +		 */
> +		h->surplus_huge_pages++;
> +		h->surplus_huge_pages_node[nid]++;
> +
> +		/*
> +		 * The refcount can possibly be increased by memory-failure or
> +		 * soft_offline handlers.

This comment could be more helpful. I believe you want to say this
		/*
		 * HWpoisoning code can increment the reference
		 * count here. If there is a race then bail out
		 * the holder of the additional reference count will
		 * free up the page with put_page.
> +		 */
> +		if (likely(put_page_testzero(page))) {
> +			arch_clear_hugepage_flags(page);
> +			enqueue_huge_page(h, page);
> +		}
> +
> +		return -ENOMEM;
> +	}
> +
>  	for (i = 0; i < pages_per_huge_page(h);
>  	     i++, subpage = mem_map_next(subpage, page, i)) {
>  		subpage->flags &= ~(1 << PG_locked | 1 << PG_error |
[...]
> @@ -1447,7 +1486,7 @@ void free_huge_page(struct page *page)
>  	/*
>  	 * Defer freeing if in non-task context to avoid hugetlb_lock deadlock.
>  	 */
> -	if (!in_task()) {
> +	if (in_atomic()) {

As I've said elsewhere in_atomic doesn't work for CONFIG_PREEMPT_COUNT=n.
We need this change for other reasons and so it would be better to pull
it out into a separate patch which also makes HUGETLB depend on
PREEMPT_COUNT.

[...]
> @@ -1771,8 +1813,12 @@ int dissolve_free_huge_page(struct page *page)
>  		h->free_huge_pages--;
>  		h->free_huge_pages_node[nid]--;
>  		h->max_huge_pages--;
> -		update_and_free_page(h, head);
> -		rc = 0;
> +		rc = update_and_free_page(h, head);
> +		if (rc) {
> +			h->surplus_huge_pages--;
> +			h->surplus_huge_pages_node[nid]--;
> +			h->max_huge_pages++;

This is quite ugly and confusing. update_and_free_page is careful to do
the proper counters accounting and now you just override it partially.
Why cannot we rely on update_and_free_page do the right thing?

-- 
Michal Hocko
SUSE Labs