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

[Mike Kravetz <mike.kravetz@oracle.com>]

On 09/08/23 10:39, Muchun Song wrote:
> 
> 
> > On Sep 8, 2023, at 02:37, Mike Kravetz <mike.kravetz@oracle.com> 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@bytedance.com>
> >> ---
> >> 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?

They were calls in the same routine: gather_bootmem_prealloc().

> > 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.

Agree that speculative refs should not happen this early.  How about making
the following changes?
- Instead of set_page_count() in hugetlb_folio_init_tail_vmemmap, do a
  page_ref_freeze and VM_BUG_ON if not ref_count != 1.
- In the commit message, mention 'The WARN_ON for increased ref count in
  gather_bootmem_prealloc was changed to a VM_BUG_ON.  This is OK as
  there should be no speculative references this early in boot process.
  The VM_BUG_ON's are there just in case such code is introduced.'
-- 
Mike Kravetz