Re: [v4 4/4] mm: hugetlb: Skip initialization of gigantic tail struct pages if freed by HVO

[Muchun Song]

> On Sep 8, 2023, at 02:37, Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
> 
> On 09/06/23 12:26, Usama Arif wrote:
>> The new boot flow when it comes to initialization of gigantic pages
>> is as follows:
>> - At boot time, for a gigantic page during __alloc_bootmem_hugepage,
>> the region after the first struct page is marked as noinit.
>> - This results in only the first struct page to be
>> initialized in reserve_bootmem_region. As the tail struct pages are
>> not initialized at this point, there can be a significant saving
>> in boot time if HVO succeeds later on.
>> - Later on in the boot, the head page is prepped and the first
>> HUGETLB_VMEMMAP_RESERVE_SIZE / sizeof(struct page) - 1 tail struct pages
>> are initialized.
>> - HVO is attempted. If it is not successful, then the rest of the
>> tail struct pages are initialized. If it is successful, no more
>> tail struct pages need to be initialized saving significant boot time.
>> 
>> Signed-off-by: Usama Arif <usama.arif@xxxxxxxxxxxxx>
>> ---
>> mm/hugetlb.c         | 61 +++++++++++++++++++++++++++++++++++++-------
>> mm/hugetlb_vmemmap.c |  2 +-
>> mm/hugetlb_vmemmap.h |  9 ++++---
>> mm/internal.h        |  3 +++
>> mm/mm_init.c         |  2 +-
>> 5 files changed, 62 insertions(+), 15 deletions(-)
> 
> As mentioned, in general this looks good.  One small point below.
> 
>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>> index c32ca241df4b..540e0386514e 100644
>> --- a/mm/hugetlb.c
>> +++ b/mm/hugetlb.c
>> @@ -3169,6 +3169,15 @@ int __alloc_bootmem_huge_page(struct hstate *h, int nid)
>> }
>> 
>> found:
>> +
>> + 	/*
>> + 	 * Only initialize the head struct page in memmap_init_reserved_pages,
>> + 	 * rest of the struct pages will be initialized by the HugeTLB subsystem itself.
>> + 	 * The head struct page is used to get folio information by the HugeTLB
>> + 	 * subsystem like zone id and node id.
>> + 	 */
>> + 	memblock_reserved_mark_noinit(virt_to_phys((void *)m + PAGE_SIZE),
>> + 	huge_page_size(h) - PAGE_SIZE);
>> 	/* Put them into a private list first because mem_map is not up yet */
>> 	INIT_LIST_HEAD(&m->list);
>> 	list_add(&m->list, &huge_boot_pages);
>> @@ -3176,6 +3185,40 @@ int __alloc_bootmem_huge_page(struct hstate *h, int nid)
>> 	return 1;
>> }
>> 
>> +/* Initialize [start_page:end_page_number] tail struct pages of a hugepage */
>> +static void __init hugetlb_folio_init_tail_vmemmap(struct folio *folio,
>> +     		unsigned long start_page_number,
>> +     		unsigned long end_page_number)
>> +{
>> + 	enum zone_type zone = zone_idx(folio_zone(folio));
>> + 	int nid = folio_nid(folio);
>> + 	unsigned long head_pfn = folio_pfn(folio);
>> + 	unsigned long pfn, end_pfn = head_pfn + end_page_number;
>> +
>> + 	for (pfn = head_pfn + start_page_number; pfn < end_pfn; pfn++) {
>> + 	struct page *page = pfn_to_page(pfn);
>> +
>> + 		__init_single_page(page, pfn, zone, nid);
>> + 		prep_compound_tail((struct page *)folio, pfn - head_pfn);
>> + 		set_page_count(page, 0);
>> + 	}
>> +}
>> +
>> +static void __init hugetlb_folio_init_vmemmap(struct folio *folio, struct hstate *h,
>> +        unsigned long nr_pages)
>> +{
>> + 	int ret;
>> +
>> + 	/* Prepare folio head */
>> +	 __folio_clear_reserved(folio);
>> + 	__folio_set_head(folio);
>> + 	ret = page_ref_freeze(&folio->page, 1);
>> + 	VM_BUG_ON(!ret);
> 
> In the current code, we print a warning and free the associated pages to
> buddy if we ever experience an increased ref count.  The routine
> hugetlb_folio_init_tail_vmemmap does not check for this.
> 
> I do not believe speculative/temporary ref counts this early in the boot
> process are possible.  It would be great to get input from someone else.

Yes, it is a very early stage and other tail struct pages haven't been
initialized yet, anyone should not reference them. It it the same case
as CONFIG_DEFERRED_STRUCT_PAGE_INIT enabled.

> 
> When I wrote the existing code, it was fairly easy to WARN and continue
> if we encountered an increased ref count.  Things would be bit more

In your case, I think it is not in the boot process, right?

> complicated here.  So, it may not be worth the effort.

Agree. Note that tail struct pages are not initialized here, if we want to
handle head page, how to handle tail pages? It really cannot resolved.
We should make the same assumption as CONFIG_DEFERRED_STRUCT_PAGE_INIT
that anyone should not reference those pages.

Thanks.