linux-kernel - Re: [PATCH RFC 10/14] mm/memory: factor out common code from vm_normal_page

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Message-ID: <1ea2de52-7684-4e27-a8e9-233390f63eeb@redhat.com>
Date: Wed, 25 Jun 2025 10:57:39 +0200
From: David Hildenbrand <david@...hat.com>
To: Oscar Salvador <osalvador@...e.de>
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 10/14] mm/memory: factor out common code from
 vm_normal_page_*()

On 25.06.25 10:53, Oscar Salvador wrote:
> On Tue, Jun 17, 2025 at 05:43:41PM +0200, David Hildenbrand wrote:
>> Let's reduce the code duplication and factor out the non-pte/pmd related
>> magic into vm_normal_page_pfn().
>>
>> To keep it simpler, check the pfn against both zero folios. We could
>> optimize this, but as it's only for the !CONFIG_ARCH_HAS_PTE_SPECIAL
>> case, it's not a compelling micro-optimization.
>>
>> With CONFIG_ARCH_HAS_PTE_SPECIAL we don't have to check anything else,
>> really.
>>
>> It's a good question if we can even hit the !CONFIG_ARCH_HAS_PTE_SPECIAL
>> scenario in the PMD case in practice: but doesn't really matter, as
>> it's now all unified in vm_normal_page_pfn().
>>
>> While at it, add a check that pmd_special() is really only set where we
>> would expect it.
>>
>> No functional change intended.
>>
>> Signed-off-by: David Hildenbrand <david@...hat.com>
> 
> Reviewed-by: Oscar Salvador <osalvador@...e.de>
> 
> Comment below
> 
>>   struct folio *vm_normal_folio(struct vm_area_struct *vma, unsigned long addr,
>> @@ -650,35 +661,12 @@ struct page *vm_normal_page_pmd(struct vm_area_struct *vma, unsigned long addr,
>>   {
>>   	unsigned long pfn = pmd_pfn(pmd);
>>   
>> -	/* Currently it's only used for huge pfnmaps */
> 
> Although the check kind of spells it out, we could leave this one and also add
> that huge_zero_pfn, to make it more explicit.

I don't think that comment is required anymore -- we do exactly what 
vm_normal_page() does + documents,

What the current users are is not particularly important anymore.

Or why do you think it would still be important?

-- 
Cheers,

David / dhildenb