linux-kernel - Re: [PATCH v2 7/9] mm/memory: factor out common code from vm_normal_page

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Message-ID: <b7457b96-2b78-4202-8380-4c7cd70767b9@redhat.com>
Date: Thu, 17 Jul 2025 22:03:44 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.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 7/9] mm/memory: factor out common code from
 vm_normal_page_*()

On 17.07.25 21:55, Lorenzo Stoakes wrote:
> On Thu, Jul 17, 2025 at 08:51:51PM +0100, Lorenzo Stoakes wrote:
>>> @@ -721,37 +772,21 @@ struct page *vm_normal_page_pmd(struct vm_area_struct *vma, unsigned long addr,
>>>   		print_bad_page_map(vma, addr, pmd_val(pmd), NULL);
>>>   		return NULL;
>>>   	}
>>> -
>>> -	if (unlikely(vma->vm_flags & (VM_PFNMAP|VM_MIXEDMAP))) {
>>> -		if (vma->vm_flags & VM_MIXEDMAP) {
>>> -			if (!pfn_valid(pfn))
>>> -				return NULL;
>>> -			goto out;
>>> -		} else {
>>> -			unsigned long off;
>>> -			off = (addr - vma->vm_start) >> PAGE_SHIFT;
>>> -			if (pfn == vma->vm_pgoff + off)
>>> -				return NULL;
>>> -			if (!is_cow_mapping(vma->vm_flags))
>>> -				return NULL;
>>> -		}
>>> -	}
>>> -
>>> -	if (is_huge_zero_pfn(pfn))
>>> -		return NULL;
>>> -	if (unlikely(pfn > highest_memmap_pfn)) {
>>> -		print_bad_page_map(vma, addr, pmd_val(pmd), NULL);
>>> -		return NULL;
>>> -	}
>>> -
>>> -	/*
>>> -	 * NOTE! We still have PageReserved() pages in the page tables.
>>> -	 * eg. VDSO mappings can cause them to exist.
>>> -	 */
>>> -out:
>>> -	return pfn_to_page(pfn);
>>> +	return vm_normal_page_pfn(vma, addr, pfn, pmd_val(pmd));
>>
>> Hmm this seems broken, because you're now making these special on arches with
>> pte_special() right? But then you're invoking the not-special function?
>>
>> Also for non-pte_special() arches you're kind of implying they _maybe_ could be
>> special.
> 
> OK sorry the diff caught me out here, you explicitly handle the pmd_special()
> case here, duplicatively (yuck).
> 
> Maybe you fix this up in a later patch :)

I had that, but the conditions depend on the level, meaning: unnecessary 
checks for pte/pmd/pud level.

I had a variant where I would pass "bool special" into 
vm_normal_page_pfn(), but I didn't like it.

To optimize out, I would have to provide a "level" argument, and did not 
convince myself yet that that is a good idea at this point.

-- 
Cheers,

David / dhildenb