linux-ext4 - Re: [PATCH v6 02/13] mm: remove extra ZONE

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Date:   Fri, 20 Aug 2021 06:40:12 +0200
From:   Christoph Hellwig <hch@....de>
To:     Felix Kuehling <felix.kuehling@....com>
Cc:     "Sierra Guiza, Alejandro (Alex)" <alex.sierra@....com>,
        Ralph Campbell <rcampbell@...dia.com>,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
        Theodore Ts'o <tytso@....edu>, amd-gfx@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org, hch@....de, jgg@...dia.com,
        jglisse@...hat.com
Subject: Re: [PATCH v6 02/13] mm: remove extra ZONE_DEVICE struct page
 refcount

On Thu, Aug 19, 2021 at 03:59:56PM -0400, Felix Kuehling wrote:
> I got lost trying to understand how DAX counts page references and how
> the PTE_SPECIAL option affects that. Theodore, can you help with this?
> Is there an easy way to test without CONFIG_ARCH_HAS_PTE_SPECIAL on x86,
> or do we need to test on a CPU architecture that doesn't support this
> feature?

I think the right answer is to simplify disallow ZONE_DEVICE pages
if ARCH_HAS_PTE_SPECIAL is not supported.  ARCH_HAS_PTE_SPECIAL is
supported by all modern architecture ports than can make use of
ZONE_DEVICE / dev_pagemap, so we can avoid this pocket of barely
testable code entirely:


diff --git a/mm/Kconfig b/mm/Kconfig
index 40a9bfcd5062e1..2823bbfd1c8c70 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -775,6 +775,7 @@ config ZONE_DMA32
 
 config ZONE_DEVICE
 	bool "Device memory (pmem, HMM, etc...) hotplug support"
+	depends on ARCH_HAS_PTE_SPECIAL
 	depends on MEMORY_HOTPLUG
 	depends on MEMORY_HOTREMOVE
 	depends on SPARSEMEM_VMEMMAP