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: <20250612180250.GC1130869@ziepe.ca>
Date: Thu, 12 Jun 2025 15:02:50 -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 3/3] mm/huge_memory: don't mark refcounted folios
 special in vmf_insert_folio_pud()

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().
> 
> 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>
> ---
>  include/linux/mm.h | 19 ++++++++++++++++-
>  mm/huge_memory.c   | 51 +++++++++++++++++++++++++---------------------
>  2 files changed, 46 insertions(+), 24 deletions(-)

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

Jason