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: <684a5a32ee1dd_249110080@dwillia2-xfh.jf.intel.com.notmuch>
Date: Wed, 11 Jun 2025 21:40:19 -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 3/3] mm/huge_memory: don't mark refcounted folios
 special in vmf_insert_folio_pud()

David Hildenbrand wrote:
> Marking PUDs that map a "normal" refcounted folios as special is
> against our rules documented for vm_normal_page().
> 
> Fortunately, there are not that many pud_special() check that can be
> mislead and are right now 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 introduce
> folio_normal_page_pud() and start using it in more place where we
> currently special-case based on other VMA flags.
> 
> Fix it just like we fixed vmf_insert_folio_pmd().
> 
> Add folio_mk_pud() to mimic what we do with folio_mk_pmd().
> 
> Fixes: dbe54153296d ("mm/huge_memory: add vmf_insert_folio_pud()")
> Signed-off-by: David Hildenbrand <david@...hat.com>

Looks good to me.

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