linux-kernel - Re: [PATCH 06/11] mm/hugetlb: Remove fake head pages

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Message-ID: <s7uxtfbfta45d26g5e5iznkkbj7ccpsar7dppacqmbwms5bcdb@7cr4bo4jqnd2>
Date: Mon, 8 Dec 2025 10:40:40 +0000
From: Kiryl Shutsemau <kas@...nel.org>
To: Usama Arif <usamaarif642@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 
	Muchun Song <muchun.song@...ux.dev>, David Hildenbrand <david@...nel.org>, 
	Oscar Salvador <osalvador@...e.de>, Mike Rapoport <rppt@...nel.org>, 
	Vlastimil Babka <vbabka@...e.cz>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, 
	Matthew Wilcox <willy@...radead.org>, Zi Yan <ziy@...dia.com>, Baoquan He <bhe@...hat.com>, 
	Michal Hocko <mhocko@...e.com>, Johannes Weiner <hannes@...xchg.org>, 
	Jonathan Corbet <corbet@....net>, kernel-team@...a.com, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH 06/11] mm/hugetlb: Remove fake head pages

On Sat, Dec 06, 2025 at 05:03:25PM +0000, Usama Arif wrote:
> > @@ -518,7 +519,24 @@ static bool vmemmap_should_optimize_folio(const struct hstate *h, struct folio *
> >  	return true;
> >  }
> >  
> > -static int __hugetlb_vmemmap_optimize_folio(const struct hstate *h,
> > +static void hugetlb_vmemmap_tail_alloc(struct hstate *h)
> > +{
> > +	struct page *p;
> > +
> > +	if (h->vmemmap_tail)
> > +		return;
> > +
> 
> The above check is unnecessary as we already check for
> !h->vmemmap_tail in __hugetlb_vmemmap_optimize_folio?

Right. I will streamline this codepath.

> Is it possible that we could have a race here? Where 2 threads both
> trying to allocate a hugetlb page when none exist in the system, both
> see h->vmemmap_tail == NULL, both call alloc_page and set
> h->vmemmap_tail?

Good catch. Will fix. I guess, serializing h-vmemmap_tail with
hugetlb_lock should be good enough.

> Also, is there a good point where we can see that the number of
> hstate->nr_huge_pages has gone down to 0 and free
> h->vmemmap_tail? Its a single page per hstate so not a big deal, but
> would be nice to have cleanup for it?

I didn't want to go to this complexity, but if you folks think that it
is needed, sure, can do.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov