linux-kernel - Re: [PATCH v2 06/14] mm/huge_memory: Remove pXd_devmap usage from insert_pXd

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Message-ID: <6bb233ef-5e56-4546-b571-6a5f052d8b45@redhat.com>
Date: Tue, 17 Jun 2025 11:49:20 +0200
From: David Hildenbrand <david@...hat.com>
To: Alistair Popple <apopple@...dia.com>, akpm@...ux-foundation.org
Cc: linux-mm@...ck.org, gerald.schaefer@...ux.ibm.com,
 dan.j.williams@...el.com, jgg@...pe.ca, willy@...radead.org,
 linux-kernel@...r.kernel.org, nvdimm@...ts.linux.dev,
 linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
 linux-xfs@...r.kernel.org, jhubbard@...dia.com, hch@....de,
 zhang.lyra@...il.com, debug@...osinc.com, bjorn@...nel.org,
 balbirs@...dia.com, lorenzo.stoakes@...cle.com,
 linux-arm-kernel@...ts.infradead.org, loongarch@...ts.linux.dev,
 linuxppc-dev@...ts.ozlabs.org, linux-riscv@...ts.infradead.org,
 linux-cxl@...r.kernel.org, dri-devel@...ts.freedesktop.org, John@...ves.net,
 m.szyprowski@...sung.com
Subject: Re: [PATCH v2 06/14] mm/huge_memory: Remove pXd_devmap usage from
 insert_pXd_pfn()

On 16.06.25 13:58, Alistair Popple wrote:
> Nothing uses PFN_DEV anymore so no need to create devmap pXd's when
> mapping a PFN. Instead special mappings will be created which ensures
> vm_normal_page_pXd() will not return pages which don't have an
> associated page. This could change behaviour slightly on architectures
> where pXd_devmap() does not imply pXd_special() as the normal page
> checks would have fallen through to checking VM_PFNMAP/MIXEDMAP instead,
> which in theory at least could have returned a page.
> 
> However vm_normal_page_pXd() should never have been returning pages for
> pXd_devmap() entries anyway, so anything relying on that would have been
> a bug.
> 
> Signed-off-by: Alistair Popple <apopple@...dia.com>
> 
> ---
> 
> Changes since v1:
> 
>   - New for v2
> ---
>   mm/huge_memory.c | 12 ++----------
>   1 file changed, 2 insertions(+), 10 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index b096240..6514e25 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1415,11 +1415,7 @@ static int insert_pmd(struct vm_area_struct *vma, unsigned long addr,
>   		add_mm_counter(mm, mm_counter_file(fop.folio), HPAGE_PMD_NR);
>   	} else {
>   		entry = pmd_mkhuge(pfn_t_pmd(fop.pfn, prot));
> -
> -		if (pfn_t_devmap(fop.pfn))
> -			entry = pmd_mkdevmap(entry);
> -		else
> -			entry = pmd_mkspecial(entry);
> +		entry = pmd_mkspecial(entry);
>   	}
>   	if (write) {
>   		entry = pmd_mkyoung(pmd_mkdirty(entry));
> @@ -1565,11 +1561,7 @@ static void insert_pud(struct vm_area_struct *vma, unsigned long addr,
>   		add_mm_counter(mm, mm_counter_file(fop.folio), HPAGE_PUD_NR);
>   	} else {
>   		entry = pud_mkhuge(pfn_t_pud(fop.pfn, prot));
> -
> -		if (pfn_t_devmap(fop.pfn))
> -			entry = pud_mkdevmap(entry);
> -		else
> -			entry = pud_mkspecial(entry);
> +		entry = pud_mkspecial(entry);
>   	}
>   	if (write) {
>   		entry = pud_mkyoung(pud_mkdirty(entry));


Why not squash this patch into #3, and remove the pmd_special() check 
from vm_normal_page_pmd() in the same go? Seems wrong to handle the 
PMD/PUD case separately.

But now I am confused why some pte_devmap() checks are removed in patch 
#3, while others are removed in #7.

Why not split it up into (a) stop setting p*_devmap() and (b) remove 
p*_devmap().

Logically makes more sense to me ... :)

-- 
Cheers,

David / dhildenb