linux-kernel - Re: [External] Re: [PATCH v12 04/13] mm/hugetlb: Free the vmemmap pages associated with each HugeTLB page

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Message-ID: <20210114115248.GA24592@localhost.localdomain>
Date:   Thu, 14 Jan 2021 12:52:48 +0100
From:   Oscar Salvador <osalvador@...e.de>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        Jonathan Corbet <corbet@....net>,
        Thomas Gleixner <tglx@...utronix.de>, mingo@...hat.com,
        bp@...en8.de, x86@...nel.org, hpa@...or.com,
        dave.hansen@...ux.intel.com, luto@...nel.org,
        Peter Zijlstra <peterz@...radead.org>, viro@...iv.linux.org.uk,
        Andrew Morton <akpm@...ux-foundation.org>, paulmck@...nel.org,
        mchehab+huawei@...nel.org, pawan.kumar.gupta@...ux.intel.com,
        Randy Dunlap <rdunlap@...radead.org>, oneukum@...e.com,
        anshuman.khandual@....com, jroedel@...e.de,
        Mina Almasry <almasrymina@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Michal Hocko <mhocko@...e.com>,
        "Song Bao Hua (Barry Song)" <song.bao.hua@...ilicon.com>,
        David Hildenbrand <david@...hat.com>,
        HORIGUCHI NAOYA(堀口 直也) 
        <naoya.horiguchi@....com>,
        Xiongchun duan <duanxiongchun@...edance.com>,
        linux-doc@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [External] Re: [PATCH v12 04/13] mm/hugetlb: Free the vmemmap
 pages associated with each HugeTLB page

On Thu, Jan 14, 2021 at 06:54:30PM +0800, Muchun Song wrote:
> I think this approach may be only suitable for generic huge page only.
> So we can implement it only for huge page.
> 
> Hi Oscar,
> 
> What's your opinion about this?

I tried something like:

static void vmemmap_pte_range(pmd_t *pmd, unsigned long addr,
			      unsigned long end,
			      struct vmemmap_remap_walk *walk)
{
	pte_t *pte;

	pte = pte_offset_kernel(pmd, addr);

	if (!walk->reuse_page) {
		BUG_ON(pte_none(*pte));

		walk->reuse_page = pte_page(*pte++);
		addr = walk->remap_start;
	}

	for (; addr != end; addr += PAGE_SIZE, pte++) {
		BUG_ON(pte_none(*pte));

		walk->remap_pte(pte, addr, walk);
	}
}

void vmemmap_remap_free(unsigned long start, unsigned long end,
			unsigned long reuse)
{
	LIST_HEAD(vmemmap_pages);
	struct vmemmap_remap_walk walk = {
		.remap_pte	= vmemmap_remap_pte,
		.reuse_addr	= reuse,
		.remap_start = start,
		.vmemmap_pages	= &vmemmap_pages,
	};

	BUG_ON(start != reuse + PAGE_SIZE);

	vmemmap_remap_range(reuse, end, &walk);
	free_vmemmap_page_list(&vmemmap_pages);
}

but it might overcomplicate things and I am not sure it is any better.
So I am fine with keeping it as is.
Should another user come in the future, we can always revisit.
Maybe just add a little comment in vmemmap_pte_range(), explaining while we
are "+= PAGE_SIZE" for address and I would like to see a comment in 
vmemmap_remap_free why the BUG_ON and more important what it is checking.

-- 
Oscar Salvador
SUSE L3