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

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Message-ID: <377449bd-3c06-4a09-8647-e41354e64b30@redhat.com>
Date: Thu, 28 Aug 2025 09:44:27 +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>,
 Dennis Zhou <dennis@...nel.org>, Dmitry Vyukov <dvyukov@...gle.com>,
 dri-devel@...ts.freedesktop.org, intel-gfx@...ts.freedesktop.org,
 iommu@...ts.linux.dev, io-uring@...r.kernel.org,
 Jason Gunthorpe <jgg@...dia.com>, Jens Axboe <axboe@...nel.dk>,
 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,
 linux-arm-kernel@...ts.infradead.org, linux-crypto@...r.kernel.org,
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 linux-riscv@...ts.infradead.org, linux-s390@...r.kernel.org,
 linux-scsi@...r.kernel.org, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
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 wireguard@...ts.zx2c4.com, x86@...nel.org, Zi Yan <ziy@...dia.com>
Subject: Re: [PATCH v1 13/36] mm/hugetlb: cleanup
 hugetlb_folio_init_tail_vmemmap()

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?

-- 
Cheers

David / dhildenb