linux-kernel - Re: [PATCH v2 3/9] mm/huge_memory: support huge zero folio in vmf_insert_folio

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Message-ID: <8d832915-152d-4776-9b8e-b5846e6ec47e@lucifer.local>
Date: Thu, 17 Jul 2025 16:47:50 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        xen-devel@...ts.xenproject.org, linux-fsdevel@...r.kernel.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>,
        Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>,
        Zi Yan <ziy@...dia.com>, Baolin Wang <baolin.wang@...ux.alibaba.com>,
        Nico Pache <npache@...hat.com>, Ryan Roberts <ryan.roberts@....com>,
        Dev Jain <dev.jain@....com>, Barry Song <baohua@...nel.org>,
        Jann Horn <jannh@...gle.com>, Pedro Falcato <pfalcato@...e.de>,
        Hugh Dickins <hughd@...gle.com>, Oscar Salvador <osalvador@...e.de>,
        Lance Yang <lance.yang@...ux.dev>
Subject: Re: [PATCH v2 3/9] mm/huge_memory: support huge zero folio in
 vmf_insert_folio_pmd()

On Thu, Jul 17, 2025 at 01:52:06PM +0200, David Hildenbrand wrote:
> Just like we do for vmf_insert_page_mkwrite() -> ... ->
> insert_page_into_pte_locked() with the shared zeropage, support the
> huge zero folio in vmf_insert_folio_pmd().
>
> When (un)mapping the huge zero folio in page tables, we neither
> adjust the refcount nor the mapcount, just like for the shared zeropage.
>
> For now, the huge zero folio is not marked as special yet, although
> vm_normal_page_pmd() really wants to treat it as special. We'll change
> that next.
>
> Reviewed-by: Oscar Salvador <osalvador@...e.de>
> Signed-off-by: David Hildenbrand <david@...hat.com>

LGTM, so:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>

> ---
>  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 849feacaf8064..db08c37b87077 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1429,9 +1429,11 @@ 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);
> --
> 2.50.1
>