RE: [PATCH 0/3] mm/page_reporting: Make page reporting work on arm64 with 64KB page size

[Michael Kelley]

From: Gavin Shan <gshan@xxxxxxxxxx> Sent: Monday, June 21, 2021 7:05 PM
> 
> On 6/22/21 2:06 AM, Michael Kelley wrote:
> > From: Alexander Duyck <alexander.duyck@xxxxxxxxx> Sent: Monday, June 21, 2021 7:02
> AM
> >> To: Gavin Shan <gshan@xxxxxxxxxx>
> >> Cc: linux-mm <linux-mm@xxxxxxxxx>; LKML <linux-kernel@xxxxxxxxxxxxxxx>; Andrew
> >> Morton <akpm@xxxxxxxxxxxxxxxxxxxx>; David Hildenbrand <david@xxxxxxxxxx>;
> >> Anshuman Khandual <anshuman.khandual@xxxxxxx>; Catalin Marinas
> >> <catalin.marinas@xxxxxxx>; Will Deacon <will@xxxxxxxxxx>; shan.gavin@xxxxxxxxx
> >> 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

This should be fine.  For reference, the Hyper-V ARM64 host always
runs with a base page size of 4K.  Linux guests on Hyper-V may have base
page size of 4K, 16K, or 64K.

Michael

> 
> >
> >> 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@xxxxxxxxxx> 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
> >>>