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Subject: Re: uninitialized pmem struct pages
To:     Michal Hocko <mhocko@suse.com>
Cc:     Dan Williams <dan.j.williams@intel.com>,
        Linux MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Oscar Salvador <osalvador@suse.de>
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From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Message-ID: <239b166d-c215-f5e3-132e-a1b7703f9504@redhat.com>
Date:   Tue, 5 Jan 2021 10:27:06 +0100
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On 05.01.21 10:25, Michal Hocko wrote:
> On Tue 05-01-21 10:13:49, David Hildenbrand wrote:
>> On 05.01.21 10:05, Michal Hocko wrote:
>>> On Tue 05-01-21 00:57:43, Dan Williams wrote:
>>>> On Tue, Jan 5, 2021 at 12:42 AM Michal Hocko <mhocko@suse.com> wrote:
>>>>>
>>>>> On Tue 05-01-21 00:27:34, Dan Williams wrote:
>>>>>> On Tue, Jan 5, 2021 at 12:17 AM Michal Hocko <mhocko@suse.com> wrote:
>>>>>>>
>>>>>>> On Tue 05-01-21 09:01:00, Michal Hocko wrote:
>>>>>>>> On Mon 04-01-21 16:44:52, David Hildenbrand wrote:
>>>>>>>>> On 04.01.21 16:43, David Hildenbrand wrote:
>>>>>>>>>> On 04.01.21 16:33, Michal Hocko wrote:
>>>>>>>>>>> On Mon 04-01-21 16:15:23, David Hildenbrand wrote:
>>>>>>>>>>>> On 04.01.21 16:10, Michal Hocko wrote:
>>>>>>>>>>> [...]
>>>>>>>>>>>> Do the physical addresses you see fall into the same section as boot
>>>>>>>>>>>> memory? Or what's around these addresses?
>>>>>>>>>>>
>>>>>>>>>>> Yes I am getting a garbage for the first struct page belonging to the
>>>>>>>>>>> pmem section [1]
>>>>>>>>>>> [    0.020161] ACPI: SRAT: Node 0 PXM 0 [mem 0x100000000-0x603fffffff]
>>>>>>>>>>> [    0.020163] ACPI: SRAT: Node 4 PXM 4 [mem 0x6060000000-0x11d5fffffff] non-volatile
>>>>>>>>>>>
>>>>>>>>>>> The pfn without the initialized struct page is 0x6060000. This is a
>>>>>>>>>>> first pfn in a section.
>>>>>>>>>>
>>>>>>>>>> Okay, so we're not dealing with the "early section" mess I described,
>>>>>>>>>> different story.
>>>>>>>>>>
>>>>>>>>>> Due to [1], is_mem_section_removable() called
>>>>>>>>>> pfn_to_page(PHYS_PFN(0x6060000)). page_zone(page) made it crash, as not
>>>>>>>>>> initialized.
>>>>>>>>>>
>>>>>>>>>> Let's assume this is indeed a reserved pfn in the altmap. What's the
>>>>>>>>>> actual address of the memmap?
>>>>>>>>>>
>>>>>>>>>> I do wonder what hosts pfn_to_page(PHYS_PFN(0x6060000)) - is it actually
>>>>>>>>>> part of the actual altmap (i.e. > 0x6060000) or maybe even self-hosted?
>>>>>>>>>>
>>>>>>>>>> If it's not self-hosted, initializing the relevant memmaps should work
>>>>>>>>>> just fine I guess. Otherwise things get more complicated.
>>>>>>>>>
>>>>>>>>> Oh, I forgot: pfn_to_online_page() should at least in your example make
>>>>>>>>> sure other pfn walkers are safe. It was just an issue of
>>>>>>>>> is_mem_section_removable().
>>>>>>>>
>>>>>>>> Hmm, I suspect you are right. I haven't put this together, thanks! The memory
>>>>>>>> section is indeed marked offline so pfn_to_online_page would indeed bail
>>>>>>>> out:
>>>>>>>> crash> p (0x6060000>>15)
>>>>>>>> $3 = 3084
>>>>>>>> crash> p mem_section[3084/128][3084 & 127]
>>>>>>>> $4 = {
>>>>>>>>   section_mem_map = 18446736128020054019,
>>>>>>>>   usage = 0xffff902dcf956680,
>>>>>>>>   page_ext = 0x0,
>>>>>>>>   pad = 0
>>>>>>>> }
>>>>>>>> crash> p 18446736128020054019 & (1UL<<2)
>>>>>>>> $5 = 0
>>>>>>>>
>>>>>>>> That makes it considerably less of a problem than I thought!
>>>>>>>
>>>>>>> Forgot to add that those who are running kernels without 53cdc1cb29e8
>>>>>>> ("drivers/base/memory.c: indicate all memory blocks as removable") for
>>>>>>> some reason can fix the crash by the following simple patch.
>>>>>>>
>>>>>>> Index: linux-5.3-users_mhocko_SLE15-SP2_for-next/drivers/base/memory.c
>>>>>>> ===================================================================
>>>>>>> --- linux-5.3-users_mhocko_SLE15-SP2_for-next.orig/drivers/base/memory.c
>>>>>>> +++ linux-5.3-users_mhocko_SLE15-SP2_for-next/drivers/base/memory.c
>>>>>>> @@ -152,9 +152,14 @@ static ssize_t removable_show(struct dev
>>>>>>>                 goto out;
>>>>>>>
>>>>>>>         for (i = 0; i < sections_per_block; i++) {
>>>>>>> -               if (!present_section_nr(mem->start_section_nr + i))
>>>>>>> +               unsigned long nr = mem->start_section_nr + i;
>>>>>>> +               if (!present_section_nr(nr))
>>>>>>>                         continue;
>>>>>>> -               pfn = section_nr_to_pfn(mem->start_section_nr + i);
>>>>>>> +               if (!online_section_nr()) {
>>>>>>
>>>>>> I assume that's onlince_section_nr(nr) in the version that compiles?
>>>>>
>>>>> Yup.
>>>>>
>>>>>> This makes sense because the memory block size is larger than the
>>>>>> section size. I suspect you have 1GB memory block size on this system,
>>>>>> but since the System RAM and PMEM collide at a 512MB alignment in a
>>>>>> memory block you end up walking the back end of the last 512MB of the
>>>>>> System RAM memory block and run into the offline PMEM section.
>>>>>
>>>>> Sections are 128MB and memory blocks are 2GB on this system.
>>>>>
>>>>>> So, I don't think it's pfn_to_online_page that necessarily needs to
>>>>>> know how to disambiguate each page, it's things that walk sections and
>>>>>> memory blocks and expects them to be consistent over the span.
>>>>>
>>>>> Well, memory hotplug code is hard wired to sparse memory model so in
>>>>> this particular case asking about the section is ok. But pfn walkers
>>>>> shouldn't really care and only rely on pfn_to_online_page. But that will
>>>>> do the right thing here. So we are good as long as the section is marked
>>>>> properly. But this would become a problem as soon as the uninitialized
>>>>> pages where sharing the same memory section as David pointed out.
>>>>> pfn_to_online_page would then return something containing garbage. So we
>>>>> should still think of a way to either initialize all those pages or make
>>>>> sure pfn_to_online_page recognizes them. The former is preferred IMHO.
>>>>
>>>> The former would not have saved the crash in this case because
>>>> pfn_to_online_page() is not used in v5.3:removable_show() that I can
>>>> see, nor some of the other paths that might walk pfns and the wrong
>>>> thing with ZONE_DEVICE.
>>>
>>> If the page was initialized properly, and by that I mean also have it
>>> reserved, then the old code would have properly reported is as not
>>> removable.
>>>
>>>> However, I do think pfn_to_online_page() should be reliable, and I
>>>> prefer to just brute force add a section flag to indicate whether the
>>>> section might be ZONE_DEVICE polluted and fallback to the
>>>> get_dev_pagemap() slow-path in that case.
>>>
>>> Do we have some spare room to hold that flag in a section?
>>>
>>>> ...but it would still require hunting to find the places where
>>>> pfn_to_online_page() is missing for assumptions like this crash which
>>>> assumed memblock-online + section-present == section-online.
>>>
>>> Yes, but most users should be using pfn_to_online_page already.
>>>
>>
>> Quite honestly, let's not hack around this issue and just fix it
>> properly - make pfn_to_online_page() only ever return an initialized,
>> online (buddy) page, just as documented.
> 
> Just to make sure we are on the same page. You are agreeing with Dan
> that pfn_to_online_page should check for zone device pages? Ideally in a
> slow path.

The most important part for me is that pfn_to_online_page() behaves as
documented. How that is implemented is a secondary concern. The easier,
the better (e.g., just avoid the corner-case (!) issue we discovered
completely).

-- 
Thanks,

David / dhildenb