linux-kernel - Re: [PATCH v13 09/12] mm: hugetlb: add a kernel parameter hugetlb_free

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Date:   Mon, 25 Jan 2021 13:08:32 +0100
From:   Oscar Salvador <osalvador@...e.de>
To:     David Hildenbrand <david@...hat.com>
Cc:     Muchun Song <songmuchun@...edance.com>, 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,
        naoya.horiguchi@....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 v13 09/12] mm: hugetlb: add a kernel parameter
 hugetlb_free_vmemmap

On Mon, Jan 25, 2021 at 12:43:23PM +0100, David Hildenbrand wrote:
> > -	if (end - start < PAGES_PER_SECTION * sizeof(struct page))
> > +	if (is_hugetlb_free_vmemmap_enabled() ||
> > +	    end - start < PAGES_PER_SECTION * sizeof(struct page))
> 
> This looks irresponsible. You ignore any altmap, even though current
> altmap users (ZONE_DEVICE) will not actually result in applicable
> vmemmaps that huge pages could ever use.
> 
> Why do you ignore the altmap completely? This has to be properly
> documented, but IMHO it's not even the right approach to mess with
> altmap here.

The goal was not to ignore altmap but to disable PMD mapping sections
when the feature was enabled.
Shame on me I did not notice that with this, altmap will be ignored.

Something like below maybe:

int __meminit vmemmap_populate(unsigned long start, unsigned long end, int node,
                struct vmem_altmap *altmap)
{
        int err;
        bool populate_base_pages = false;

        if ((end - start < PAGES_PER_SECTION * sizeof(struct page)) ||
            (is_hugetlb_free_vmemmap_enabled() && !altmap))
                populate_base_pages = true;

        if (populate_base_pages) {
                err = vmemmap_populate_basepages(start, end, node, NULL);
        } else if (boot_cpu_has(X86_FEATURE_PSE)) {
	....


> 
> -- 
> Thanks,
> 
> David / dhildenb
> 
> 

-- 
Oscar Salvador
SUSE L3