linux-ext4 - Re: [PATCH 03/12] mm/pagewalk: Skip dax pages in pagewalk

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Message-ID: <20250603133659.GD386142@ziepe.ca>
Date: Tue, 3 Jun 2025 10:36:59 -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 03/12] mm/pagewalk: Skip dax pages in pagewalk

On Thu, May 29, 2025 at 04:32:04PM +1000, Alistair Popple wrote:
> Previously dax pages were skipped by the pagewalk code as pud_special() or
> vm_normal_page{_pmd}() would be false for DAX pages. Now that dax pages are
> refcounted normally that is no longer the case, so add explicit checks to
> skip them.
> 
> Signed-off-by: Alistair Popple <apopple@...dia.com>
> ---
>  include/linux/memremap.h | 11 +++++++++++
>  mm/pagewalk.c            | 12 ++++++++++--
>  2 files changed, 21 insertions(+), 2 deletions(-)

But why do we want to skip them?

Like hmm uses pagewalk and it would like to see DAX pages?

I guess it makes sense from the perspective of not changing things,
but it seems like a comment should be left behind explaining that this
is just for legacy reasons until someone audits the callers.

Jason