linux-kernel - Re: ** POTENTIAL FRAUD ALERT - RED HAT ** RE: [PATCH 0/3] mm/page_reporting: Make page reporting work on arm64 with 64KB page size

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Date:   Tue, 22 Jun 2021 12:04:49 +1000
From:   Gavin Shan <gshan@...hat.com>
To:     Michael Kelley <mikelley@...rosoft.com>,
        Alexander Duyck <alexander.duyck@...il.com>
Cc:     linux-mm <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        "shan.gavin@...il.com" <shan.gavin@...il.com>
Subject: Re: ** POTENTIAL FRAUD ALERT - RED HAT ** RE: [PATCH 0/3]
 mm/page_reporting: Make page reporting work on arm64 with 64KB page size

On 6/22/21 2:06 AM, Michael Kelley wrote:
> From: Alexander Duyck <alexander.duyck@...il.com> Sent: Monday, June 21, 2021 7:02 AM
>> To: Gavin Shan <gshan@...hat.com>
>> Cc: linux-mm <linux-mm@...ck.org>; LKML <linux-kernel@...r.kernel.org>; Andrew
>> Morton <akpm@...ux-foundation.org>; David Hildenbrand <david@...hat.com>;
>> Anshuman Khandual <anshuman.khandual@....com>; Catalin Marinas
>> <catalin.marinas@....com>; Will Deacon <will@...nel.org>; shan.gavin@...il.com
>> Subject: Re: [PATCH 0/3] mm/page_reporting: Make page reporting work on arm64 with
>> 64KB page size
>>
>> So the question I would have is what is the use case for this? It
>> seems like you don't have to deal with the guest native page size
>> issues since you are willing to break up what would otherwise be THP
>> pages on the guest, and the fact that you are willing to go down to
>> 2MB pages which happens to align with the host THP page size for x86
>> makes me wonder if that is actually the environment you are running
>> in.
>>
>> Rather than having the guest control this it might make sense to look
>> at adding an interface so that the page_reporting_register function
>> and the page_reporting_dev_info struct could be used to report and
>> configure the minimum page size that the host can support for the page
>> reporting. With that the host could then guarantee that it isn't going
>> to hurt performance by splitting pages on the host and risk hurting
>> the virtualization performance.
>>
>> Also you would benefit by looking into the callers of
>> page_reporting_register as there are more than just the virtio balloon
>> that are consuming it. Odds are HyperV won't care about an ARM64
>> architecture,
> 
> FWIW, Hyper-V *does* care about ARM64.  It's already in use by
> the Windows Subsystem for Linux VM that's part of Windows 10
> on ARM64 hardware.  We're working to get the code accepted
> upstream.
> 

Michael, thanks for your confirmation. As the issue found on 64KB guest
when memory balloon is used, lets resolve the case first. I will look
into Hyper-V case later if you agree. It won't be difficult to fix the
same issue for Hyper-V after the solution is figured out for memory
balloon.

Thanks,
Gavin

> 
>> but your change would essentially disable it outright
>> which is why I think this might be better to address via the consumers
>> of page reporting rather than trying to address it in page reporting
>> itself.
>>
>> Thanks,
>>
>> - Alex
>>
>> On Sun, Jun 20, 2021 at 8:11 PM Gavin Shan <gshan@...hat.com> wrote:
>>>
>>> The page reporting threshold is currently equal to @pageblock_order, which
>>> is 13 and 512MB on arm64 with 64KB base page size selected. The page
>>> reporting won't be triggered if the freeing page can't come up with a free
>>> area like that huge. The condition is hard to be met, especially when the
>>> system memory becomes fragmented.
>>>
>>> This series intends to solve the issue by having page reporting threshold
>>> as 5 (2MB) on arm64 with 64KB base page size. The patches are organized as:
>>>
>>>     PATCH[1/3] introduces variable (@page_reporting_order) to replace original
>>>                macro (PAGE_REPORTING_MIN_ORDER). It's also exported so that it
>>>                can be adjusted at runtime.
>>>     PATCH[2/3] renames PAGE_REPORTING_MIN_ORDER with PAGE_REPORTING_ORDER
>> and
>>>                allows architecture to specify its own version.
>>>     PATCH[3/3] defines PAGE_REPORTING_ORDER to 5, corresponding to 2MB in size,
>>>                on arm64 when 64KB base page size is selected. It's still same
>>>                as to @pageblock_order for other architectures and cases.
>>>
>>> Gavin Shan (3):
>>>    mm/page_reporting: Allow to set reporting order
>>>    mm/page_reporting: Allow architecture to select reporting order
>>>    arm64: mm: Specify smaller page reporting order
>>>
>>>   Documentation/admin-guide/kernel-parameters.txt |  6 ++++++
>>>   arch/arm64/include/asm/page.h                   | 13 +++++++++++++
>>>   mm/page_reporting.c                             |  8 ++++++--
>>>   mm/page_reporting.h                             | 10 +++++++---
>>>   4 files changed, 32 insertions(+), 5 deletions(-)
>>>
>>> --
>>> 2.23.0
>>>