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: <20250612180232.GB1130869@ziepe.ca>
Date: Thu, 12 Jun 2025 15:02:32 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: David Hildenbrand <david@...hat.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>,
	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 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(-)

Reviewed-by: Jason Gunthorpe <jgg@...dia.com>

Jason