linux-kernel - Re: [PATCH v3 05/21] mm/hugetlb: Introduce pgtable allocation/freeing helpers

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Date:   Mon, 9 Nov 2020 18:21:44 +0100
From:   Oscar Salvador <osalvador@...e.de>
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,
        mhocko@...e.com, duanxiongchun@...edance.com,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v3 05/21] mm/hugetlb: Introduce pgtable
 allocation/freeing helpers

On Sun, Nov 08, 2020 at 10:10:57PM +0800, Muchun Song wrote:
> +static inline unsigned int pgtable_pages_to_prealloc_per_hpage(struct hstate *h)
> +{
> +	unsigned long vmemmap_size = vmemmap_pages_size_per_hpage(h);
> +
> +	/*
> +	 * No need pre-allocate page tabels when there is no vmemmap pages
> +	 * to free.
 s /tabels/tables/

> +static int vmemmap_pgtable_prealloc(struct hstate *h, struct page *page)
> +{
> +	int i;
> +	pgtable_t pgtable;
> +	unsigned int nr = pgtable_pages_to_prealloc_per_hpage(h);
> +
> +	if (!nr)
> +		return 0;
> +
> +	vmemmap_pgtable_init(page);
> +
> +	for (i = 0; i < nr; i++) {
> +		pte_t *pte_p;
> +
> +		pte_p = pte_alloc_one_kernel(&init_mm);
> +		if (!pte_p)
> +			goto out;
> +		vmemmap_pgtable_deposit(page, virt_to_page(pte_p));
> +	}
> +
> +	return 0;
> +out:
> +	while (i-- && (pgtable = vmemmap_pgtable_withdraw(page)))
> +		pte_free_kernel(&init_mm, page_to_virt(pgtable));

	would not be enough to:

	while (pgtable = vmemmap_pgtable_withdrag(page))
		pte_free_kernel(&init_mm, page_to_virt(pgtable));

> +	return -ENOMEM;
> +}
> +
> +static void vmemmap_pgtable_free(struct hstate *h, struct page *page)
> +{
> +	pgtable_t pgtable;
> +	unsigned int nr = pgtable_pages_to_prealloc_per_hpage(h);
> +
> +	if (!nr)
> +		return;

We can get rid of "nr" and its check and keep only the check below, right?
AFAICS, they go together, e.g: if page_huge_pte does not return null,
it means that we preallocated a pagetable, and viceversa.


> +
> +	pgtable = page_huge_pte(page);
> +	if (!pgtable)
> +		return;
> +
> +	while (nr-- && (pgtable = vmemmap_pgtable_withdraw(page)))
> +		pte_free_kernel(&init_mm, page_to_virt(pgtable));

	Same as above, that "nr" can go?

-- 
Oscar Salvador
SUSE L3