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Date:   Fri, 17 Jun 2022 17:40:28 +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 11:25:20AM +0200, David Hildenbrand wrote:
> On 17.06.22 11:10, Muchun Song wrote:
> > On Fri, Jun 17, 2022 at 09:39:27AM +0200, David Hildenbrand wrote:
> >> On 17.06.22 09:28, Muchun Song wrote:
> >>> On Fri, Jun 17, 2022 at 07:46:53AM +0200, Oscar Salvador wrote:
> >>>> On Thu, Jun 16, 2022 at 09:30:33AM +0200, David Hildenbrand wrote:
> >>>>> IIRC, that was used to skip these patches on the offlining path before
> >>>>> we provided the ranges to offline_pages().
> >>>>
> >>>> Yeah, it was designed for that purpose back then.
> >>>>
> >>>>> I'd not mess with PG_reserved, and give them a clearer name, to not
> >>>>> confuse them with other, ordinary, vmemmap pages that are not
> >>>>> self-hosted (maybe in the future we might want to flag all vmemmap pages
> >>>>> with a new type?).
> >>>>
> >>>> Not sure whether a new type is really needed, or to put it another way, I
> >>>> cannot see the benefit.
> >>>>
> >>>>>
> >>>>> I'd just try reusing the flag PG_owner_priv_1. And eventually, flag all
> >>>>> (v)memmap pages with a type PG_memmap. However, the latter would be
> >>>>> optional and might not be strictly required
> >>>>>
> >>>>>
> >>>>> So what think could make sense is
> >>>>>
> >>>>> /* vmemmap pages that are self-hosted and cannot be optimized/freed. */
> >>>>> PG_vmemmap_self_hosted = PG_owner_priv_1,
> >>>>
> >>>> Sure, I just lightly tested the below, and seems to work, but not sure
> >>>> whether that is what you are referring to.
> >>>> @Munchun: thoughts?
> >>>>
> >>>
> >>> I think it works and fits my requirement.
> >>>
> >>>> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> >>>> index e66f7aa3191d..a4556afd7bda 100644
> >>>> --- a/include/linux/page-flags.h
> >>>> +++ b/include/linux/page-flags.h
> >>>> @@ -193,6 +193,11 @@ enum pageflags {
> >>>>  
> >>>>  	/* Only valid for buddy pages. Used to track pages that are reported */
> >>>>  	PG_reported = PG_uptodate,
> >>>> +
> >>>> +#ifdef CONFIG_MEMORY_HOTPLUG
> >>>> +	/* For self-hosted memmap pages */
> >>>> +	PG_vmemmap_self_hosted = PG_owner_priv_1,
> >>>> +#endif
> >>>>  };
> >>>>  
> >>>>  #define PAGEFLAGS_MASK		((1UL << NR_PAGEFLAGS) - 1)
> >>>> @@ -628,6 +633,10 @@ PAGEFLAG_FALSE(SkipKASanPoison, skip_kasan_poison)
> >>>>   */
> >>>>  __PAGEFLAG(Reported, reported, PF_NO_COMPOUND)
> >>>>  
> >>>> +#ifdef CONFIG_MEMORY_HOTPLUG
> >>>> +PAGEFLAG(Vmemmap_self_hosted, vmemmap_self_hosted, PF_ANY)
> >>>> +#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)));
> >>>> +
> >>>> +	if (PageVmemmap_self_hosted(memmap))
> >>>> +		return;
> >>>
> >>> I think here needs a loop if it is a 1GB page (spans multiple sections).
> >>> Right?  Here is an implementation based on another approach. But I think
> >>> your implementation is more simpler and efficient.  Would you mind me
> >>> squash your diff into my patch and with your "Co-developed-by"?
> >>
> >> Due to hugtlb alignment requirements, and the vmemmap pages being at the
> >> start of the hotplugged memory region, I think that cannot currently
> >> happen. Checking the first vmemmap page might be good enough for now,
> >> and probably for the future.
> >>
> > 
> > If the memory block size is 128MB, then a 1GB huge page spans 8 blocks.
> > Is it possible that some blocks of them are vmemmap-hosted?
> 
> No, don't think so. If you think about it, a huge/gigantic page can only
> start in a memmap-on-memory region but never end in on (or overlap one)
> -- because the reserved memmap part of the memory block always precedes
> actually usable data.
> 
> So even with variable-size memory blocks and weird address alignment,
> checking the first memmap of a huge page for vmemmp-on-memory should be
> sufficient.
> 
> Unless I am missing something.
>

Got it. You are awesome. I ignored the fact that we have reserved
some memory as vmemmap pages in memmap-on-memory case, the whole
memory block cannot be used as a gigantic page. Thanks for your
nice explanation.