linux-kernel - Re: [PATCH 6/6] mm: hugetlb_vmemmap: improve hugetlb

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Date:   Mon, 13 Jun 2022 17:01:43 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Oscar Salvador <osalvador@...e.de>
Cc:     mike.kravetz@...cle.com, david@...hat.com,
        akpm@...ux-foundation.org, corbet@....net, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH 6/6] mm: hugetlb_vmemmap: improve hugetlb_vmemmap code
 readability

On Mon, Jun 13, 2022 at 10:33:48AM +0200, Oscar Salvador wrote:
> On Mon, Jun 13, 2022 at 02:35:12PM +0800, Muchun Song wrote:
> > -static __init int hugetlb_vmemmap_sysctls_init(void)
> > +static int __init hugetlb_vmemmap_init(void)
> >  {
> > +	const struct hstate *h;
> > +	bool optimizable = false;
> > +
> >  	/*
> > -	 * If "struct page" crosses page boundaries, the vmemmap pages cannot
> > -	 * be optimized.
> > +	 * There are only (RESERVE_VMEMMAP_SIZE / sizeof(struct page)) struct
> > +	 * page structs that can be used when HVO is enabled.
> >  	 */
> > -	if (is_power_of_2(sizeof(struct page)))
> > -		register_sysctl_init("vm", hugetlb_vmemmap_sysctls);
> > +	BUILD_BUG_ON(__NR_USED_SUBPAGE >= RESERVE_VMEMMAP_SIZE / sizeof(struct page));
> 
> I need to take another look, but from the first glance there is something
> here that caught my eye.
>

Thanks for taking a look. This is introduced in commit f41f2ed43ca5.
 
> > +
> > +	for_each_hstate(h) {
> > +		char buf[16];
> > +		unsigned int size = 0;
> > +
> > +		if (hugetlb_vmemmap_optimizable(h))
> > +			size = hugetlb_vmemmap_size(h) - RESERVE_VMEMMAP_SIZE;
> > +		optimizable = size ? true : optimizable;
> 
> This feels weird, just use false instead of optimizable.
>

This is a loop, we shoud keep "optimizable" as "true" as long as there is one
hstate is optimizable. How about:

  if (size)
	optimizable = true;

> > +		string_get_size(huge_page_size(h), 1, STRING_UNITS_2, buf,
> > +				sizeof(buf));
> > +		pr_info("%d KiB vmemmap can be optimized for a %s page\n",
> > +			size / SZ_1K, buf);
> 
> I do not have a strong opinion but I wonder whether this brings a lot.
>

I thought the users can know what size HugeTLB is optimizable via
this log.  E.g. On aarch64, 64KB HugeTLB cannot be optimizable.
I do not have a strong opinion as well, if anyone think it is
unnecessary, I'll drop it in next version.

Thanks.