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

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Message-ID: <6880f125-803d-4eea-88ac-b67fdcc5995d@redhat.com>
Date: Thu, 28 Aug 2025 10:18:23 +0200
From: David Hildenbrand <david@...hat.com>
To: Mike Rapoport <rppt@...nel.org>
Cc: linux-kernel@...r.kernel.org, Alexander Potapenko <glider@...gle.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Brendan Jackman <jackmanb@...gle.com>, Christoph Lameter <cl@...two.org>,
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 Johannes Weiner <hannes@...xchg.org>, John Hubbard <jhubbard@...dia.com>,
 kasan-dev@...glegroups.com, kvm@...r.kernel.org,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>, linux-arm-kernel@...s.com,
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Subject: Re: [PATCH v1 13/36] mm/hugetlb: cleanup
 hugetlb_folio_init_tail_vmemmap()

On 28.08.25 10:06, Mike Rapoport wrote:
> 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.
> 	 */

I prefer what I currently have, but thanks for the review.

-- 
Cheers

David / dhildenb