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Date: Wed, 5 Jun 2024 15:54:00 -0700
From: Dan Williams <dan.j.williams@intel.com>
To: Peter Xu <peterx@redhat.com>, <linux-kernel@vger.kernel.org>,
	<linux-mm@kvack.org>
CC: Andrew Morton <akpm@linux-foundation.org>, <peterx@redhat.com>, "Dan
 Williams" <dan.j.williams@intel.com>, Pasha Tatashin
	<pasha.tatashin@soleen.com>, Alistair Popple <apopple@nvidia.com>
Subject: Re: [PATCH] mm/page_table_check: Fix crash on ZONE_DEVICE
Message-ID: <6660ec883d31c_1ad63294f4@dwillia2-mobl3.amr.corp.intel.com.notmuch>
References: <20240605212146.994486-1-peterx@redhat.com>
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[ add Alistair ]

Peter Xu wrote:
> Not all pages may apply to pgtable check.  One example is ZONE_DEVICE
> pages: they map PFNs directly, and they don't allocate page_ext at all even
> if there's struct page around.  One may reference devm_memremap_pages().
> 
> When both ZONE_DEVICE and page-table-check enabled, then try to map some
> dax memories, one can trigger kernel bug constantly now when the kernel was
> trying to inject some pfn maps on the dax device:
> 
>  kernel BUG at mm/page_table_check.c:55!
> 
> While it's pretty legal to use set_pxx_at() for ZONE_DEVICE pages for page
> fault resolutions, skip all the checks if page_ext doesn't even exist in
> pgtable checker, which applies to ZONE_DEVICE but maybe more.

This looks correct to me, and needed in the near term. You can add:

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

In the long term, the page_ext check may not be needed. I.e. the reason
I added Alistair was in case his work to make DAX pages behave like
typical pages for reference counting would also make them behave the
same for the presence of page_ext.