linux-kernel - Re: [PATCH v2 3/3] mm/huge_memory: don't mark refcounted folios special in vmf_insert_folio

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Message-ID: <11d1ff4d-3f75-42a5-968e-8f4bad84ab78@redhat.com>
Date: Thu, 12 Jun 2025 19:00:01 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, nvdimm@...ts.linux.dev,
 linux-cxl@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>,
 Alistair Popple <apopple@...dia.com>,
 "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>, Dan Williams <dan.j.williams@...el.com>,
 Oscar Salvador <osalvador@...e.de>
Subject: Re: [PATCH v2 3/3] mm/huge_memory: don't mark refcounted folios
 special in vmf_insert_folio_pud()

On 12.06.25 18:49, Lorenzo Stoakes wrote:
> On Wed, Jun 11, 2025 at 02:06:54PM +0200, David Hildenbrand wrote:
>> Marking PUDs that map a "normal" refcounted folios as special is
>> against our rules documented for vm_normal_page().
> 
> Might be worth referring to specifically which rule. I'm guessing it's the
> general one of special == don't touch (from vm_normal_page() comment):
> 
> /*
>   * vm_normal_page -- This function gets the "struct page" associated with a pte.
>   *
>   * "Special" mappings do not wish to be associated with a "struct page" (either
>   * it doesn't exist, or it exists but they don't want to touch it). In this
>   * case, NULL is returned here. "Normal" mappings do have a struct page.
>   *
>   * ...
>   *
>   */

Well, yes, the one vm_normal_page() is all about ... ? :)

> 
> But don't we already violate this E.g.:
> 
> 		if (vma->vm_ops && vma->vm_ops->find_special_page)
> 			return vma->vm_ops->find_special_page(vma, addr);
 > > I mean this in itself perhaps means we should update this comment 
to say 'except
> when file-backed and there is a find_special_page() hook'.

I rather hope we severely break this case such that we can remove that hack.

Read as in: I couldn't care less about this XEN hack, in particular, not 
documenting it.

I was already wondering about hiding it behind a XEN config so not each 
and every sane user of this function has to perform this crappy-hack check.

[...]

>>   	}
>>
>> -	entry = pud_mkhuge(pfn_t_pud(pfn, prot));
>> -	if (pfn_t_devmap(pfn))
>> -		entry = pud_mkdevmap(entry);
>> -	else
>> -		entry = pud_mkspecial(entry);
>> +	if (fop.is_folio) {
>> +		entry = folio_mk_pud(fop.folio, vma->vm_page_prot);
>> +
>> +		folio_get(fop.folio);
>> +		folio_add_file_rmap_pud(fop.folio, &fop.folio->page, vma);
>> +		add_mm_counter(mm, mm_counter_file(fop.folio), HPAGE_PUD_NR);
> 
> Nit, but might be nice to abstract for PMD/PUD.

Which part exactly? Likely a follow-up if it should be abstracted.

> 
>> +	} else {
>> +		entry = pud_mkhuge(pfn_t_pud(fop.pfn, prot));
> 
> Same incredibly pedantic whitespace comment from previous patch :)

;)

-- 
Cheers,

David / dhildenb