linux-kernel - Re: [QEMU PATCH] KVM: Support for page hinting

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Date:   Tue, 4 Jun 2019 09:41:56 -0700
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     Nitesh Narayan Lal <nitesh@...hat.com>
Cc:     kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Paolo Bonzini <pbonzini@...hat.com>, lcapitulino@...hat.com,
        pagupta@...hat.com, wei.w.wang@...el.com,
        Yang Zhang <yang.zhang.wz@...il.com>,
        Rik van Riel <riel@...riel.com>,
        David Hildenbrand <david@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>, dodgen@...gle.com,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        dhildenb@...hat.com, Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [QEMU PATCH] KVM: Support for page hinting

On Mon, Jun 3, 2019 at 10:04 AM Nitesh Narayan Lal <nitesh@...hat.com> wrote:
>
> Enables QEMU to call madvise on the pages which are reported
> by the guest kernel.
>
> Signed-off-by: Nitesh Narayan Lal <nitesh@...hat.com>
> ---
>  hw/virtio/trace-events                        |  1 +
>  hw/virtio/virtio-balloon.c                    | 85 +++++++++++++++++++
>  include/hw/virtio/virtio-balloon.h            |  2 +-
>  include/qemu/osdep.h                          |  7 ++
>  .../standard-headers/linux/virtio_balloon.h   |  1 +
>  5 files changed, 95 insertions(+), 1 deletion(-)

<snip>

> diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
> index 840af09cb0..4d632933a9 100644
> --- a/include/qemu/osdep.h
> +++ b/include/qemu/osdep.h
> @@ -360,6 +360,11 @@ void qemu_anon_ram_free(void *ptr, size_t size);
>  #else
>  #define QEMU_MADV_REMOVE QEMU_MADV_INVALID
>  #endif
> +#ifdef MADV_FREE
> +#define QEMU_MADV_FREE MADV_FREE
> +#else
> +#define QEMU_MADV_FREE QEMU_MADV_INVALID
> +#endif

Is there a specific reason for making this default to INVALID instead
of just using DONTNEED? I ran into some issues as my host kernel
didn't have support for MADV_FREE in the exported kernel headers
apparently so I was getting no effect. It seems like it would be
better to fall back to doing DONTNEED instead of just disabling the
functionality all together.

>  #elif defined(CONFIG_POSIX_MADVISE)
>
> @@ -373,6 +378,7 @@ void qemu_anon_ram_free(void *ptr, size_t size);
>  #define QEMU_MADV_HUGEPAGE  QEMU_MADV_INVALID
>  #define QEMU_MADV_NOHUGEPAGE  QEMU_MADV_INVALID
>  #define QEMU_MADV_REMOVE QEMU_MADV_INVALID
> +#define QEMU_MADV_FREE QEMU_MADV_INVALID

Same here. If you already have MADV_DONTNEED you could just use that
instead of disabling the functionality.

>  #else /* no-op */
>
> @@ -386,6 +392,7 @@ void qemu_anon_ram_free(void *ptr, size_t size);
>  #define QEMU_MADV_HUGEPAGE  QEMU_MADV_INVALID
>  #define QEMU_MADV_NOHUGEPAGE  QEMU_MADV_INVALID
>  #define QEMU_MADV_REMOVE QEMU_MADV_INVALID
> +#define QEMU_MADV_FREE QEMU_MADV_INVALID
>
>  #endif
>