linux-ext4 - Re: [PATCH 07/12] mm: Remove redundant pXd

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Message-ID: <20250605120909.GA44681@ziepe.ca>
Date: Thu, 5 Jun 2025 09:09:09 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Dan Williams <dan.j.williams@...el.com>
Cc: Alistair Popple <apopple@...dia.com>, linux-mm@...ck.org,
	gerald.schaefer@...ux.ibm.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 07/12] mm: Remove redundant pXd_devmap calls

On Wed, Jun 04, 2025 at 07:35:24PM -0700, Dan Williams wrote:

> If all dax pages are special, then vm_normal_page() should never find
> them and gup should fail.
> 
> ...oh, but vm_normal_page_p[mu]d() is not used in the gup path, and
> 'special' is not set in the pte path.

That seems really suboptimal?? Why would pmd and pte be different?

> I think for any p[mu]d where p[mu]d_page() is ok to use should never set
> 'special', right?

There should be dedicated functions for installing pages and PFNs,
only the PFN one would set the special bit.

And certainly your tests *should* be failing as special entries should
never ever be converted to struct page.

Jason