linux-kernel - Re: [PATCH RFC 10/35] mm/hugetlb: cleanup hugetlb_folio_init_tail

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Message-ID: <a72080b4-5156-4add-ac7c-1160b44e0dfe@redhat.com>
Date: Mon, 25 Aug 2025 16:38:03 +0200
From: David Hildenbrand <david@...hat.com>
To: Mike Rapoport <rppt@...nel.org>
Cc: Mika Penttilä <mpenttil@...hat.com>,
 linux-kernel@...r.kernel.org, Alexander Potapenko <glider@...gle.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
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 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,
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Subject: Re: [PATCH RFC 10/35] mm/hugetlb: cleanup
 hugetlb_folio_init_tail_vmemmap()

On 25.08.25 16:32, Mike Rapoport wrote:
> On Mon, Aug 25, 2025 at 02:48:58PM +0200, David Hildenbrand wrote:
>> On 23.08.25 10:59, Mike Rapoport wrote:
>>> On Fri, Aug 22, 2025 at 08:24:31AM +0200, David Hildenbrand wrote:
>>>> On 22.08.25 06:09, Mika Penttilä wrote:
>>>>>
>>>>> On 8/21/25 23:06, David Hildenbrand wrote:
>>>>>
>>>>>> All pages were already initialized and set to PageReserved() with a
>>>>>> refcount of 1 by MM init code.
>>>>>
>>>>> Just to be sure, how is this working with MEMBLOCK_RSRV_NOINIT, where MM is supposed not to
>>>>> initialize struct pages?
>>>>
>>>> Excellent point, I did not know about that one.
>>>>
>>>> Spotting that we don't do the same for the head page made me assume that
>>>> it's just a misuse of __init_single_page().
>>>>
>>>> But the nasty thing is that we use memblock_reserved_mark_noinit() to only
>>>> mark the tail pages ...
>>>
>>> And even nastier thing is that when CONFIG_DEFERRED_STRUCT_PAGE_INIT is
>>> disabled struct pages are initialized regardless of
>>> memblock_reserved_mark_noinit().
>>>
>>> I think this patch should go in before your updates:
>>
>> Shouldn't we fix this in memblock code?
>>
>> Hacking around that in the memblock_reserved_mark_noinit() user sound wrong
>> -- and nothing in the doc of memblock_reserved_mark_noinit() spells that
>> behavior out.
> 
> We can surely update the docs, but unfortunately I don't see how to avoid
> hacking around it in hugetlb.
> Since it's used to optimise HVO even further to the point hugetlb open
> codes memmap initialization, I think it's fair that it should deal with all
> possible configurations.

Remind me, why can't we support memblock_reserved_mark_noinit() when 
CONFIG_DEFERRED_STRUCT_PAGE_INIT is disabled?

-- 
Cheers

David / dhildenb