On Mon 18-03-13 13:51:37, K. Y. Srinivasan wrote:
> On Hyper-V it will be very efficient to use 2M allocations in the guest as this
> makes the ballooning protocol with the host that much more efficient. Hyper-V
> uses page ranges (start pfn : number of pages) to specify memory being moved
> around and with 2M pages this encoding can be very efficient. However, when
> memory is returned to the guest, the host does not guarantee any granularity.
> To deal with this issue, split the page soon after a successful 2M allocation
> so that this memory can potentially be freed as 4K pages.
How many pages are requested usually?
> If 2M allocations fail, we revert to 4K allocations.
>
> In this version of the patch, based on the feedback from Michal Hocko
> <mhocko@xxxxxxx>, I have added some additional commentary to the patch
> description.
>
> Signed-off-by: K. Y. Srinivasan <kys@xxxxxxxxxxxxx>
I am not going to ack the patch because I am still not entirely
convinced that big allocations are worth it. But that is up to you and
hyper-V users.
> ---
> drivers/hv/hv_balloon.c | 18 ++++++++++++++++--
> 1 files changed, 16 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/hv/hv_balloon.c b/drivers/hv/hv_balloon.c
> index 2cf7d4e..71655b4 100644
> --- a/drivers/hv/hv_balloon.c
> +++ b/drivers/hv/hv_balloon.c
> @@ -997,6 +997,14 @@ static int alloc_balloon_pages(struct hv_dynmem_device *dm, int num_pages,
>
> dm->num_pages_ballooned += alloc_unit;
>
> + /*
> + * If we allocatted 2M pages; split them so we
> + * can free them in any order we get.
> + */
> +
> + if (alloc_unit != 1)
> + split_page(pg, get_order(alloc_unit << PAGE_SHIFT));
> +
> bl_resp->range_count++;
> bl_resp->range_array[i].finfo.start_page =
> page_to_pfn(pg);
I would suggest also using __GFP_NO_KSWAPD (or basically use
GFP_TRANSHUGE for alloc_unit>0) for the allocation to be as least
disruptive as possible.
> @@ -1023,9 +1031,10 @@ static void balloon_up(struct work_struct *dummy)
>
>
> /*
> - * Currently, we only support 4k allocations.
> + * We will attempt 2M allocations. However, if we fail to
> + * allocate 2M chunks, we will go back to 4k allocations.
> */
> - alloc_unit = 1;
> + alloc_unit = 512;
>
> while (!done) {
> bl_resp = (struct dm_balloon_response *)send_buffer;
> @@ -1041,6 +1050,11 @@ static void balloon_up(struct work_struct *dummy)
> bl_resp, alloc_unit,
> &alloc_error);
>
You should handle alloc_balloon_pages returns 0 && !alloc_error which
happens when num_pages < alloc_unit.
> + if ((alloc_error) && (alloc_unit != 1)) {
> + alloc_unit = 1;
> + continue;
> + }
> +
> if ((alloc_error) || (num_ballooned == num_pages)) {
> bl_resp->more_pages = 0;
> done = true;
> --
> 1.7.4.1
>
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--
Michal Hocko
SUSE Labs
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