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References: <20240605212146.994486-1-peterx@redhat.com>
 <6660ec883d31c_1ad63294f4@dwillia2-mobl3.amr.corp.intel.com.notmuch>
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From: Alistair Popple <apopple@nvidia.com>
To: Dan Williams <dan.j.williams@intel.com>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrew Morton
 <akpm@linux-foundation.org>, peterx@redhat.com, Pasha Tatashin
 <pasha.tatashin@soleen.com>
Subject: Re: [PATCH] mm/page_table_check: Fix crash on ZONE_DEVICE
Date: Thu, 06 Jun 2024 08:58:08 +1000
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Dan Williams <dan.j.williams@intel.com> writes:

> [ 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.

It doesn't currently. However I did run into this bug while I was
developing those so please add:

Reviewed-by: Alistair Popple <apopple@nvidia.com>