linux-kernel - Re: [PATCH 04/12] mm: Convert vmf_insert_mixed() from using pte_devmap to pte

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Message-ID: <20250603133740.GE386142@ziepe.ca>
Date: Tue, 3 Jun 2025 10:37:40 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Alistair Popple <apopple@...dia.com>
Cc: linux-mm@...ck.org, gerald.schaefer@...ux.ibm.com,
	dan.j.williams@...el.com, willy@...radead.org, david@...hat.com,
	linux-kernel@...r.kernel.org, nvdimm@...ts.linux.dev,
	linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
	linux-xfs@...r.kernel.org, jhubbard@...dia.com, hch@....de,
	zhang.lyra@...il.com, debug@...osinc.com, bjorn@...nel.org,
	balbirs@...dia.com, lorenzo.stoakes@...cle.com,
	linux-arm-kernel@...ts.infradead.org, loongarch@...ts.linux.dev,
	linuxppc-dev@...ts.ozlabs.org, linux-riscv@...ts.infradead.org,
	linux-cxl@...r.kernel.org, dri-devel@...ts.freedesktop.org,
	John@...ves.net
Subject: Re: [PATCH 04/12] mm: Convert vmf_insert_mixed() from using
 pte_devmap to pte_special

On Thu, May 29, 2025 at 04:32:05PM +1000, Alistair Popple wrote:
> DAX no longer requires device PTEs as it always has a ZONE_DEVICE page
> associated with the PTE that can be reference counted normally. Other users
> of pte_devmap are drivers that set PFN_DEV when calling vmf_insert_mixed()
> which ensures vm_normal_page() returns NULL for these entries.
> 
> There is no reason to distinguish these pte_devmap users so in order to
> free up a PTE bit use pte_special instead for entries created with
> vmf_insert_mixed(). This will ensure vm_normal_page() will continue to
> return NULL for these pages.
> 
> Architectures that don't support pte_special also don't support pte_devmap
> so those will continue to rely on pfn_valid() to determine if the page can
> be mapped.
> 
> Signed-off-by: Alistair Popple <apopple@...dia.com>
> ---
>  mm/hmm.c    |  3 ---
>  mm/memory.c | 20 ++------------------
>  mm/vmscan.c |  2 +-
>  3 files changed, 3 insertions(+), 22 deletions(-)

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

Jason