netdev - Re: [PATCH v1 13/36] mm/hugetlb: cleanup hugetlb_folio_init_tail

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Message-ID: <aLAN7xS4WQsN6Hpm@kernel.org>
Date: Thu, 28 Aug 2025 11:06:07 +0300
From: Mike Rapoport <rppt@...nel.org>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, Alexander Potapenko <glider@...gle.com>,
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Subject: Re: [PATCH v1 13/36] mm/hugetlb: cleanup
 hugetlb_folio_init_tail_vmemmap()

On Thu, Aug 28, 2025 at 09:44:27AM +0200, David Hildenbrand wrote:
> On 28.08.25 09:21, Mike Rapoport wrote:
> > On Thu, Aug 28, 2025 at 12:01:17AM +0200, David Hildenbrand wrote:
> > > We can now safely iterate over all pages in a folio, so no need for the
> > > pfn_to_page().
> > > 
> > > Also, as we already force the refcount in __init_single_page() to 1,
> > > we can just set the refcount to 0 and avoid page_ref_freeze() +
> > > VM_BUG_ON. Likely, in the future, we would just want to tell
> > > __init_single_page() to which value to initialize the refcount.
> > > 
> > > Further, adjust the comments to highlight that we are dealing with an
> > > open-coded prep_compound_page() variant, and add another comment explaining
> > > why we really need the __init_single_page() only on the tail pages.
> > > 
> > > Note that the current code was likely problematic, but we never ran into
> > > it: prep_compound_tail() would have been called with an offset that might
> > > exceed a memory section, and prep_compound_tail() would have simply
> > > added that offset to the page pointer -- which would not have done the
> > > right thing on sparsemem without vmemmap.
> > > 
> > > Signed-off-by: David Hildenbrand <david@...hat.com>
> > > ---
> > >   mm/hugetlb.c | 20 ++++++++++++--------
> > >   1 file changed, 12 insertions(+), 8 deletions(-)
> > > 
> > > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > > index 4a97e4f14c0dc..1f42186a85ea4 100644
> > > --- a/mm/hugetlb.c
> > > +++ b/mm/hugetlb.c
> > > @@ -3237,17 +3237,18 @@ static void __init hugetlb_folio_init_tail_vmemmap(struct folio *folio,
> > >   {
> > >   	enum zone_type zone = zone_idx(folio_zone(folio));
> > >   	int nid = folio_nid(folio);
> > > +	struct page *page = folio_page(folio, start_page_number);
> > >   	unsigned long head_pfn = folio_pfn(folio);
> > >   	unsigned long pfn, end_pfn = head_pfn + end_page_number;
> > > -	int ret;
> > > -
> > > -	for (pfn = head_pfn + start_page_number; pfn < end_pfn; pfn++) {
> > > -		struct page *page = pfn_to_page(pfn);
> > > +	/*
> > > +	 * We mark all tail pages with memblock_reserved_mark_noinit(),
> > > +	 * so these pages are completely uninitialized.
> > 
> >                               ^ not? ;-)
> 
> Can you elaborate?

Oh, sorry, I misread "uninitialized".
Still, I'd phrase it as 

	/*
	 * We marked all tail pages with memblock_reserved_mark_noinit(),
	 * so we must initialize them here.
	 */

> -- 
> Cheers
> 
> David / dhildenb
> 

-- 
Sincerely yours,
Mike.