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

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Message-ID: <aKx6SlYrj_hiPXBB@kernel.org>
Date: Mon, 25 Aug 2025 17:59:22 +0300
From: Mike Rapoport <rppt@...nel.org>
To: David Hildenbrand <david@...hat.com>
Cc: Mika Penttilä <mpenttil@...hat.com>,
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Subject: Re: [PATCH RFC 10/35] mm/hugetlb: cleanup
 hugetlb_folio_init_tail_vmemmap()

On Mon, Aug 25, 2025 at 04:38:03PM +0200, David Hildenbrand wrote:
> 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?

When CONFIG_DEFERRED_STRUCT_PAGE_INIT is disabled we initialize the entire
memmap early (setup_arch()->free_area_init()), and we may have a bunch of
memblock_reserved_mark_noinit() afterwards
 
> -- 
> Cheers
> 
> David / dhildenb
> 

-- 
Sincerely yours,
Mike.