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From: Dan Williams <dan.j.williams@intel.com>
Date: Thu, 11 Nov 2021 16:51:35 -0800
Message-ID: <CAPcyv4imjWNuwsQKhWinq+vtuSgXAznhLXVfsy69Dq7q7eiXbA@mail.gmail.com>
Subject: Re: [RFT PATCH] x86/pat: Fix set_mce_nospec() for pmem
To: Jane Chu <jane.chu@oracle.com>
Cc: Borislav Petkov <bp@alien8.de>, "Luck, Tony" <tony.luck@intel.com>, 
	Linux NVDIMM <nvdimm@lists.linux.dev>, Luis Chamberlain <mcgrof@suse.com>
Content-Type: text/plain; charset="UTF-8"

On Thu, Nov 11, 2021 at 4:30 PM Jane Chu <jane.chu@oracle.com> wrote:
>
> Just a quick update -
>
> I managed to test the 'NP' and 'UC' effect on a pmem dax file.
> The result is, as expected, both setting 'NP' and 'UC' works
> well in preventing the prefetcher from accessing the poisoned
> pmem page.
>
> I injected back-to-back poisons to the 3rd block(512B) of
> the 3rd page in my dax file.  With 'NP', the 'mc_safe read'
> stops  after reading the 1st and 2nd pages, with 'UC',
> the 'mc_safe read' was able to read [2 pages + 2 blocks] on
> my test machine.

My expectation is that dax_direct_access() / dax_recovery_read() has
installed a temporary UC alias for the pfn, or has temporarily flipped
NP to UC. Outside of dax_recovery_read() the page will always be NP.