linux-kernel - Re: [PATCH RFC 06/14] mm/huge_memory: support huge zero folio in vmf_insert_folio

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Message-ID: <aFuwGmM0Ebl1IRGl@localhost.localdomain>
Date: Wed, 25 Jun 2025 10:15:22 +0200
From: Oscar Salvador <osalvador@...e.de>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, nvdimm@...ts.linux.dev,
	Andrew Morton <akpm@...ux-foundation.org>,
	Juergen Gross <jgross@...e.com>,
	Stefano Stabellini <sstabellini@...nel.org>,
	Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
	Dan Williams <dan.j.williams@...el.com>,
	Alistair Popple <apopple@...dia.com>,
	Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>, Zi Yan <ziy@...dia.com>,
	Baolin Wang <baolin.wang@...ux.alibaba.com>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	"Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Nico Pache <npache@...hat.com>, Ryan Roberts <ryan.roberts@....com>,
	Dev Jain <dev.jain@....com>, Barry Song <baohua@...nel.org>,
	Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
	Suren Baghdasaryan <surenb@...gle.com>,
	Michal Hocko <mhocko@...e.com>, Jann Horn <jannh@...gle.com>,
	Pedro Falcato <pfalcato@...e.de>
Subject: Re: [PATCH RFC 06/14] mm/huge_memory: support huge zero folio in
 vmf_insert_folio_pmd()

On Tue, Jun 17, 2025 at 05:43:37PM +0200, David Hildenbrand wrote:
> Just like we do for vmf_insert_page_mkwrite() -> ... ->
> insert_page_into_pte_locked(), support the huge zero folio.
> 
> Signed-off-by: David Hildenbrand <david@...hat.com>

insert_page_into_pte_locked() creates a special pte in case it finds the
zero folio while insert_pmd() doesn't.
I know that we didn't want to create special mappings for normal refcount folios
but this seems inconsistent? I'm pretty sure there's a reason but could you
elaborate on that?

> ---
>  mm/huge_memory.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 1ea23900b5adb..92400f3baa9ff 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1418,9 +1418,11 @@ static vm_fault_t insert_pmd(struct vm_area_struct *vma, unsigned long addr,
>  	if (fop.is_folio) {
>  		entry = folio_mk_pmd(fop.folio, vma->vm_page_prot);
>  
> -		folio_get(fop.folio);
> -		folio_add_file_rmap_pmd(fop.folio, &fop.folio->page, vma);
> -		add_mm_counter(mm, mm_counter_file(fop.folio), HPAGE_PMD_NR);
> +		if (!is_huge_zero_folio(fop.folio)) {
> +			folio_get(fop.folio);
> +			folio_add_file_rmap_pmd(fop.folio, &fop.folio->page, vma);
> +			add_mm_counter(mm, mm_counter_file(fop.folio), HPAGE_PMD_NR);
> +		}
>  	} else {
>  		entry = pmd_mkhuge(pfn_pmd(fop.pfn, prot));
>  		entry = pmd_mkspecial(entry);
> -- 
> 2.49.0
> 
> 

-- 
Oscar Salvador
SUSE Labs