netdev - Re: [PATCH RFC 09/35] mm/mm_init: make memmap_init_compound() look more like prep_compound

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Message-ID: <1a3ca0c5-0720-4882-b425-031297c1abb7@redhat.com>
Date: Fri, 22 Aug 2025 20:09:54 +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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Subject: Re: [PATCH RFC 09/35] mm/mm_init: make memmap_init_compound() look
 more like prep_compound_page()

On 22.08.25 17:27, Mike Rapoport wrote:
> On Thu, Aug 21, 2025 at 10:06:35PM +0200, David Hildenbrand wrote:
>> Grepping for "prep_compound_page" leaves on clueless how devdax gets its
>> compound pages initialized.
>>
>> Let's add a comment that might help finding this open-coded
>> prep_compound_page() initialization more easily.
>>
>> Further, let's be less smart about the ordering of initialization and just
>> perform the prep_compound_head() call after all tail pages were
>> initialized: just like prep_compound_page() does.
>>
>> No need for a lengthy comment then: again, just like prep_compound_page().
>>
>> Note that prep_compound_head() already does initialize stuff in page[2]
>> through prep_compound_head() that successive tail page initialization
>> will overwrite: _deferred_list, and on 32bit _entire_mapcount and
>> _pincount. Very likely 32bit does not apply, and likely nobody ever ends
>> up testing whether the _deferred_list is empty.
>>
>> So it shouldn't be a fix at this point, but certainly something to clean
>> up.
>>
>> Signed-off-by: David Hildenbrand <david@...hat.com>
>> ---
>>   mm/mm_init.c | 13 +++++--------
>>   1 file changed, 5 insertions(+), 8 deletions(-)
>>
>> diff --git a/mm/mm_init.c b/mm/mm_init.c
>> index 5c21b3af216b2..708466c5b2cc9 100644
>> --- a/mm/mm_init.c
>> +++ b/mm/mm_init.c
>> @@ -1091,6 +1091,10 @@ static void __ref memmap_init_compound(struct page *head,
>>   	unsigned long pfn, end_pfn = head_pfn + nr_pages;
>>   	unsigned int order = pgmap->vmemmap_shift;
>>   
>> +	/*
>> +	 * This is an open-coded prep_compound_page() whereby we avoid
>> +	 * walking pages twice by initializing them in the same go.
>> +	 */
> 
> While on it, can you also mention that prep_compound_page() is not used to
> properly set page zone link?

Sure, thanks!

-- 
Cheers

David / dhildenb