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: <684a5969ed654_24911000@dwillia2-xfh.jf.intel.com.notmuch>
Date: Wed, 11 Jun 2025 21:36:58 -0700
From: Dan Williams <dan.j.williams@...el.com>
To: David Hildenbrand <david@...hat.com>, <linux-kernel@...r.kernel.org>
CC: <linux-mm@...ck.org>, <nvdimm@...ts.linux.dev>,
	<linux-cxl@...r.kernel.org>, David Hildenbrand <david@...hat.com>, "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()

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.

Looks good, I like copying the sockptr_t approach for this, and agree that this
seems to not cause any problems in practice today, but definitely will be a
trip hazard going forward.

Reviewed-by: Dan Williams <dan.j.williams@...el.com>