linux-kernel - Re: [PATCH 02/12] mm: Convert pXd_devmap checks to vma_is

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Message-ID: <6840f4f65a981_2491100b3@dwillia2-xfh.jf.intel.com.notmuch>
Date: Wed, 4 Jun 2025 18:37:58 -0700
From: Dan Williams <dan.j.williams@...el.com>
To: Alistair Popple <apopple@...dia.com>, <linux-mm@...ck.org>
CC: Alistair Popple <apopple@...dia.com>, <gerald.schaefer@...ux.ibm.com>,
	<dan.j.williams@...el.com>, <jgg@...pe.ca>, <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 02/12] mm: Convert pXd_devmap checks to vma_is_dax

Alistair Popple wrote:
> Currently dax is the only user of pmd and pud mapped ZONE_DEVICE
> pages. Therefore page walkers that want to exclude DAX pages can check
> pmd_devmap or pud_devmap. However soon dax will no longer set PFN_DEV,
> meaning dax pages are mapped as normal pages.
> 
> Ensure page walkers that currently use pXd_devmap to skip DAX pages
> continue to do so by adding explicit checks of the VMA instead.

tl;dr:

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

So I went through all the p[mu]d_devmap() checks and indeed this is the
set I also come up with that are implicitly checking for "dax" instead
of checking for "is this a larger than base pte size mapping".

While I am a little uncomfortable with the generality of calling the
policy "dax" in these locations I think it is ok for now. I.e. the
fundamental detail in these paths is "huge pte, but not typical
page-allocator THP page"

Also I would have felt better if some of the leftover places that are
doing "dax" checks but not updated were noted in the changelog just for
review purposes. Like:

"Note paths like follow_huge_pud and follow_pmd_mask also have 'dax'
checks, but those paths are for maintaining dev_pagemap refcounts which
no longer (since v6.15) need to be managed for dax pages. A later patch
cleans those up."