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Subject: Re: uninitialized pmem struct pages
To:     Michal Hocko <mhocko@suse.com>,
        Dan Williams <dan.j.williams@intel.com>
Cc:     Linux MM <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>
References: <20210104100323.GC13207@dhcp22.suse.cz>
 <033e1cd6-9762-5de6-3e88-47d3038fda7f@redhat.com>
 <CAPcyv4h6mdKrwpqXfO0e_=sKjB-pY5KbP9ii+tQyFsK5bPkb=A@mail.gmail.com>
 <20210105075028.GS13207@dhcp22.suse.cz>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Message-ID: <5fa1ff35-71f3-2526-6819-34c1ee959295@redhat.com>
Date:   Tue, 5 Jan 2021 10:16:33 +0100
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On 05.01.21 08:50, Michal Hocko wrote:
> On Mon 04-01-21 21:17:43, Dan Williams wrote:
>> On Mon, Jan 4, 2021 at 2:45 AM David Hildenbrand <david@redhat.com> wrote:
> [...]
>>> I believe Dan mentioned somewhere that he wants to see a real instance
>>> of this producing a BUG before actually moving forward with a fix. I
>>> might be wrong.
>>
>> I think I'm missing an argument for the user-visible effects of the
>> "Bad." statements above. I think soft_offline_page() is a candidate
>> for a local fix because mm/memory-failure.c already has a significant
>> amount of page-type specific knowledge. So teaching it "yes" for
>> MEMORY_DEVICE_PRIVATE-ZONE_DEVICE and "no" for other ZONE_DEVICE seems
>> ok to me.
> 
> I believe we do not want to teach _every_ pfn walker about zone device
> pages. This would be quite error prone. Especially when a missig check
> could lead to a silently broken data or BUG_ON with debugging enabled
> (which is not the case for many production users). Or are we talking
> about different bugs here?

I'd like us to stick to the documentation, e.g., include/linux/mmzone.h


"
pfn_valid() is meant to be able to tell if a given PFN has valid memmap
associated with it or not. This means that a struct page exists for this
pfn. The caller cannot assume the page is fully initialized in general.
Hotplugable pages might not have been onlined yet. pfn_to_online_page()
will ensure the struct page is fully online and initialized. Special
pages (e.g. ZONE_DEVICE) are never onlined and should be treated
accordingly.
"

-- 
Thanks,

David / dhildenb