linux-kernel - Re: [PATCH v2 2/2] mm: memory_hotplug: introduce SECTION_CANNOT_OPTIMIZE

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Date:   Fri, 17 Jun 2022 17:10:23 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Oscar Salvador <osalvador@...e.de>, corbet@....net,
        akpm@...ux-foundation.org, paulmck@...nel.org,
        mike.kravetz@...cle.com, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        duanxiongchun@...edance.com, smuchun@...il.com
Subject: Re: [PATCH v2 2/2] mm: memory_hotplug: introduce
 SECTION_CANNOT_OPTIMIZE_VMEMMAP

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?

Thanks.