linux-kernel - Re: [PATCH v2 2/3] mm/huge_memory: don't mark refcounted folios special in vmf_insert_folio

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Message-ID: <0302ec30-856d-4e4b-be7b-1105966733e8@redhat.com>
Date: Thu, 12 Jun 2025 09:06:04 +0200
From: David Hildenbrand <david@...hat.com>
To: Alistair Popple <apopple@...dia.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>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.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 2/3] mm/huge_memory: don't mark refcounted folios
 special in vmf_insert_folio_pmd()

On 12.06.25 04:17, Alistair Popple wrote:
> On Wed, Jun 11, 2025 at 02:06:53PM +0200, David Hildenbrand wrote:
>> Marking PMDs that map a "normal" refcounted folios as special is
>> against our rules documented for vm_normal_page().
>>
>> Fortunately, there are not that many pmd_special() check that can be
>> mislead, and most vm_normal_page_pmd()/vm_normal_folio_pmd() users that
>> would get this wrong right now are rather harmless: e.g., none so far
>> bases decisions whether to grab a folio reference on that decision.
>>
>> Well, and GUP-fast will fallback to GUP-slow. All in all, so far no big
>> implications as it seems.
>>
>> Getting this right will get more important as we use
>> folio_normal_page_pmd() in more places.
>>
>> Fix it by teaching insert_pfn_pmd() to properly handle folios and
>> pfns -- moving refcount/mapcount/etc handling in there, renaming it to
>> insert_pmd(), and distinguishing between both cases using a new simple
>> "struct folio_or_pfn" structure.
>>
>> Use folio_mk_pmd() to create a pmd for a folio cleanly.
>>
>> Fixes: 6c88f72691f8 ("mm/huge_memory: add vmf_insert_folio_pmd()")
>> Signed-off-by: David Hildenbrand <david@...hat.com>
>> ---
>>   mm/huge_memory.c | 58 ++++++++++++++++++++++++++++++++----------------
>>   1 file changed, 39 insertions(+), 19 deletions(-)
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 49b98082c5401..7e3e9028873e5 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -1372,9 +1372,17 @@ vm_fault_t do_huge_pmd_anonymous_page(struct vm_fault *vmf)
>>   	return __do_huge_pmd_anonymous_page(vmf);
>>   }
>>   
>> -static int insert_pfn_pmd(struct vm_area_struct *vma, unsigned long addr,
>> -		pmd_t *pmd, pfn_t pfn, pgprot_t prot, bool write,
>> -		pgtable_t pgtable)
>> +struct folio_or_pfn {
>> +	union {
>> +		struct folio *folio;
>> +		pfn_t pfn;
>> +	};
>> +	bool is_folio;
>> +};
> 
> I know it's simple, but I'm still not a fan particularly as these types of
> patterns tend to proliferate once introduced. See below for a suggestion.

It's much better than abusing pfn_t for folios -- and I don't 
particularly see a problem with this pattern here as long as it stays in 
this file.

> 
>> +static int insert_pmd(struct vm_area_struct *vma, unsigned long addr,
>> +		pmd_t *pmd, struct folio_or_pfn fop, pgprot_t prot,
>> +		bool write, pgtable_t pgtable)
>>   {
>>   	struct mm_struct *mm = vma->vm_mm;
>>   	pmd_t entry;
>> @@ -1382,8 +1390,11 @@ static int insert_pfn_pmd(struct vm_area_struct *vma, unsigned long addr,
>>   	lockdep_assert_held(pmd_lockptr(mm, pmd));
>>   
>>   	if (!pmd_none(*pmd)) {
>> +		const unsigned long pfn = fop.is_folio ? folio_pfn(fop.folio) :
>> +					  pfn_t_to_pfn(fop.pfn);
>> +
>>   		if (write) {
>> -			if (pmd_pfn(*pmd) != pfn_t_to_pfn(pfn)) {
>> +			if (pmd_pfn(*pmd) != pfn) {
>>   				WARN_ON_ONCE(!is_huge_zero_pmd(*pmd));
>>   				return -EEXIST;
>>   			}
>> @@ -1396,11 +1407,19 @@ static int insert_pfn_pmd(struct vm_area_struct *vma, unsigned long addr,
>>   		return -EEXIST;
>>   	}
>>   
>> -	entry = pmd_mkhuge(pfn_t_pmd(pfn, prot));
>> -	if (pfn_t_devmap(pfn))
>> -		entry = pmd_mkdevmap(entry);
>> -	else
>> -		entry = pmd_mkspecial(entry);
>> +	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);
>> +	} 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);
>> +	}
> 
> Could we change insert_pfn_pmd() to insert_pmd_entry() and have callers call
> something like pfn_to_pmd_entry() or folio_to_pmd_entry() to create the pmd_t
> entry as appropriate, which is then passed to insert_pmd_entry() to do the bits
> common to both?

Yeah, I had that idea as well but discarded it, because the 
refcounting+mapcounting handling is better placed where we are actually 
inserting the pmd (not possibly only upgrading permissions of an 
existing mapping). Avoid 4-line comments as the one we are removing in 
patch #3 ...

-- 
Cheers,

David / dhildenb