linux-kernel - Re: [PATCH v9 09/11] mm/hugetlb: Introduce nr_free_vmemmap

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Date:   Wed, 16 Dec 2020 14:43:54 +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, song.bao.hua@...ilicon.com, david@...hat.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 v9 09/11] mm/hugetlb: Introduce nr_free_vmemmap_pages in
 the struct hstate

On Sun, Dec 13, 2020 at 11:45:32PM +0800, Muchun Song wrote:
> All the infrastructure is ready, so we introduce nr_free_vmemmap_pages
> field in the hstate to indicate how many vmemmap pages associated with
> a HugeTLB page that we can free to buddy allocator. And initialize it
"can be freed to buddy allocator"

> in the hugetlb_vmemmap_init(). This patch is actual enablement of the
> feature.
> 
> Signed-off-by: Muchun Song <songmuchun@...edance.com>
> Acked-by: Mike Kravetz <mike.kravetz@...cle.com>

With below nits addressed you can add:

Reviewed-by: Oscar Salvador <osalvador@...e.de>

>  static int __init early_hugetlb_free_vmemmap_param(char *buf)
>  {
> +	/* We cannot optimize if a "struct page" crosses page boundaries. */
> +	if (!is_power_of_2(sizeof(struct page)))
> +		return 0;
> +

I wonder if we should report a warning in case someone wants to enable this
feature and stuct page size it not power of 2.
In case someone wonders why it does not work for him/her.

> +void __init hugetlb_vmemmap_init(struct hstate *h)
> +{
> +	unsigned int nr_pages = pages_per_huge_page(h);
> +	unsigned int vmemmap_pages;
> +
> +	if (!hugetlb_free_vmemmap_enabled)
> +		return;
> +
> +	vmemmap_pages = (nr_pages * sizeof(struct page)) >> PAGE_SHIFT;
> +	/*
> +	 * The head page and the first tail page are not to be freed to buddy
> +	 * system, the others page will map to the first tail page. So there
> +	 * are the remaining pages that can be freed.
"the other pages will map to the first tail page, so they can be freed."
> +	 *
> +	 * Could RESERVE_VMEMMAP_NR be greater than @vmemmap_pages? It is true
> +	 * on some architectures (e.g. aarch64). See Documentation/arm64/
> +	 * hugetlbpage.rst for more details.
> +	 */
> +	if (likely(vmemmap_pages > RESERVE_VMEMMAP_NR))
> +		h->nr_free_vmemmap_pages = vmemmap_pages - RESERVE_VMEMMAP_NR;
> +
> +	pr_info("can free %d vmemmap pages for %s\n", h->nr_free_vmemmap_pages,
> +		h->name);

Maybe specify this is hugetlb code:

pr_info("%s: blabla", __func__, ...)
or
pr_info("hugetlb: blalala", ...);

although I am not sure whether we need that at all, or maybe just use
pr_debug().

-- 
Oscar Salvador
SUSE L3