linux-kernel - Re: [RFC][Patch v10 1/2] mm: page

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Date:   Fri, 14 Jun 2019 09:24:55 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Nitesh Narayan Lal <nitesh@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        pbonzini@...hat.com, lcapitulino@...hat.com, pagupta@...hat.com,
        wei.w.wang@...el.com, yang.zhang.wz@...il.com, riel@...riel.com,
        mst@...hat.com, dodgen@...gle.com, konrad.wilk@...cle.com,
        dhildenb@...hat.com, aarcange@...hat.com, alexander.duyck@...il.com
Subject: Re: [RFC][Patch v10 1/2] mm: page_hinting: core infrastructure

On 03.06.19 19:03, Nitesh Narayan Lal wrote:
> This patch introduces the core infrastructure for free page hinting in
> virtual environments. It enables the kernel to track the free pages which
> can be reported to its hypervisor so that the hypervisor could
> free and reuse that memory as per its requirement.
> 
> While the pages are getting processed in the hypervisor (e.g.,
> via MADV_FREE), the guest must not use them, otherwise, data loss
> would be possible. To avoid such a situation, these pages are
> temporarily removed from the buddy. The amount of pages removed
> temporarily from the buddy is governed by the backend(virtio-balloon
> in our case).
> 
> To efficiently identify free pages that can to be hinted to the
> hypervisor, bitmaps in a coarse granularity are used. Only fairly big
> chunks are reported to the hypervisor - especially, to not break up THP
> in the hypervisor - "MAX_ORDER - 2" on x86, and to save space. The bits
> in the bitmap are an indication whether a page *might* be free, not a
> guarantee. A new hook after buddy merging sets the bits.
> 
> Bitmaps are stored per zone, protected by the zone lock. A workqueue
> asynchronously processes the bitmaps, trying to isolate and report pages
> that are still free. The backend (virtio-balloon) is responsible for
> reporting these batched pages to the host synchronously. Once reporting/
> freeing is complete, isolated pages are returned back to the buddy.
> 
> There are still various things to look into (e.g., memory hotplug, more
> efficient locking, possible races when disabling).
> 
> Signed-off-by: Nitesh Narayan Lal <nitesh@...hat.com>
> ---
>  drivers/virtio/Kconfig       |   1 +
>  include/linux/page_hinting.h |  46 +++++++
>  mm/Kconfig                   |   6 +
>  mm/Makefile                  |   2 +
>  mm/page_alloc.c              |  17 +--
>  mm/page_hinting.c            | 236 +++++++++++++++++++++++++++++++++++
>  6 files changed, 301 insertions(+), 7 deletions(-)
>  create mode 100644 include/linux/page_hinting.h
>  create mode 100644 mm/page_hinting.c
> 
> diff --git a/drivers/virtio/Kconfig b/drivers/virtio/Kconfig
> index 35897649c24f..5a96b7a2ed1e 100644
> --- a/drivers/virtio/Kconfig
> +++ b/drivers/virtio/Kconfig
> @@ -46,6 +46,7 @@ config VIRTIO_BALLOON
>  	tristate "Virtio balloon driver"
>  	depends on VIRTIO
>  	select MEMORY_BALLOON
> +	select PAGE_HINTING
>  	---help---
>  	 This driver supports increasing and decreasing the amount
>  	 of memory within a KVM guest.

BTW, this hunk belongs to the virtio-balloon patch.


-- 

Thanks,

David / dhildenb