linux-kernel - Re: [PATCH v2 04/19] mm/hugetlb: Introduce nr_free_vmemmap

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Date:   Thu, 29 Oct 2020 14:26:27 +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,
        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 v2 04/19] mm/hugetlb: Introduce nr_free_vmemmap_pages in
 the struct hstate

On Mon, Oct 26, 2020 at 10:50:59PM +0800, Muchun Song wrote:
> If the size of hugetlb page is 2MB, we need 512 struct page structures
> (8 pages) to be associated with it. As far as I know, we only use the
> first 4 struct page structures.

As Mike pointed out, better describe what those "4" mean.
 
> For tail pages, the value of compound_dtor is the same. So we can reuse

I might be missing something, but HUGETLB_PAGE_DTOR is only set on the
first tail, right?

> +#ifdef CONFIG_HUGETLB_PAGE_FREE_VMEMMAP
> +#define RESERVE_VMEMMAP_NR	2U

Although you can get that from the changelog, maybe a brief comment explaining
why RESERVE_VMEMMAP_NR == 2.
> +
> +static inline unsigned int nr_free_vmemmap(struct hstate *h)
> +{
> +	return h->nr_free_vmemmap_pages;
> +}

Better add this in the patch that is used?

> +	if (vmemmap_pages > RESERVE_VMEMMAP_NR)
> +		h->nr_free_vmemmap_pages = vmemmap_pages - RESERVE_VMEMMAP_NR;
> +	else
> +		h->nr_free_vmemmap_pages = 0;

Can we really have an scenario where we end up with vmemmap_pages < RESERVE_VMEMMAP_NR?

> +
> +	pr_info("HugeTLB: can free %d vmemmap pages for %s\n",
> +		h->nr_free_vmemmap_pages, h->name);

I do not think this is useful unless debugging situations, so I would either
scratch that or make it pr_debug.


-- 
Oscar Salvador
SUSE L3