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Date:   Fri, 17 Jun 2022 18:49:15 +0800
From:   Muchun Song <songmuchun@bytedance.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     Oscar Salvador <osalvador@suse.de>, corbet@lwn.net,
        akpm@linux-foundation.org, paulmck@kernel.org,
        mike.kravetz@oracle.com, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        duanxiongchun@bytedance.com, smuchun@gmail.com
Subject: Re: [PATCH v2 2/2] mm: memory_hotplug: introduce
 SECTION_CANNOT_OPTIMIZE_VMEMMAP
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On Fri, Jun 17, 2022 at 12:14:02PM +0200, David Hildenbrand wrote:
> On 17.06.22 11:54, Oscar Salvador wrote:
> > On Fri, Jun 17, 2022 at 09:43:33AM +0200, David Hildenbrand wrote:
> >> VmemmapSelfHosted, then the function names get nicer.
> > 
> > Definitely.
> > 
> >>
> >>> +#endif
> >>> +
> >>>  /*
> >>>   * On an anonymous page mapped into a user virtual memory area,
> >>>   * page->mapping points to its anon_vma, not to a struct address_space;
> >>> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
> >>> index 1089ea8a9c98..e2de7ed27e9e 100644
> >>> --- a/mm/hugetlb_vmemmap.c
> >>> +++ b/mm/hugetlb_vmemmap.c
> >>> @@ -101,6 +101,14 @@ void hugetlb_vmemmap_free(struct hstate *h, struct page *head)
> >>>  {
> >>>  	unsigned long vmemmap_addr = (unsigned long)head;
> >>>  	unsigned long vmemmap_end, vmemmap_reuse, vmemmap_pages;
> >>> +	struct mem_section *ms = __pfn_to_section(page_to_pfn(head));
> >>> +	struct page *memmap;
> >>> +
> >>> +	memmap = sparse_decode_mem_map(ms->section_mem_map,
> >>> +				       pfn_to_section_nr(page_to_pfn(head)));
> >>
> >> Why can't we check the head page directly? Either I need more coffee or
> >> this can be simplified.
> > 
> > Uhm, maybe I'm the one who needs coffe here but we have something like:
> > 
> > [    hot-plugges section   ]
> > [memmap pages][normal pages]
> 
> Oh, right, we need the memmap for our hugetlb page (which resides in the
> reserved section), but these are not marked. We need the memmap of that
> memmap.
> 
> I assume one could directly via the page address. Because we want the
> memmap of the memmap.
> 
> vmmemmap_page = virt_to_page(head);
>

I think this can works, more simple.

Thanks.
 
> Not sure, though, if that would work with that function.