RE: [PATCH V3] x86/Hyper-V: Balloon up according to request page number

[Michael Kelley <mikelley@microsoft.com>]

From: Tianyu Lan <Tianyu.Lan@microsoft.com> Sent: Monday, January 20, 2020 12:42 AM
> 
> Current code has assumption that balloon request memory size aligns
> with 2MB. But actually Hyper-V doesn't guarantee such alignment. When
> balloon driver receives non-aligned balloon request, it produces warning
> and balloon up more memory than requested in order to keep 2MB alignment.
> Remove the warning and balloon up memory according to actual requested
> memory size.
> 
> Fixes: f6712238471a ("hv: hv_balloon: avoid memory leak on alloc_error of 2MB memory
> block")
> Cc: stable@vger.kernel.org
> Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> Signed-off-by: Tianyu Lan <Tianyu.Lan@microsoft.com>
> ---
> Change since v2:
>     - Remove check between request page number and alloc_unit
>     in the alloc_balloon_pages() because it's redundant with
>     new change.
>     - Remove the "continue" just follwoing alloc_unit switch
>      from 2MB to 4K in order to avoid skipping allocated
>      memory.
> 
> Change since v1:
>     - Change logic of switching alloc_unit from 2MB to 4KB
>     in the balloon_up() to avoid redundant iteration when
>     handle non-aligned page request.
>     - Remove 2MB alignment operation and comment in balloon_up()
> ---
>  drivers/hv/hv_balloon.c | 17 ++++-------------
>  1 file changed, 4 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/hv/hv_balloon.c b/drivers/hv/hv_balloon.c
> index 7f3e7ab22d5d..73092a7a3345 100644
> --- a/drivers/hv/hv_balloon.c
> +++ b/drivers/hv/hv_balloon.c
> @@ -1681,10 +1681,7 @@ static unsigned int alloc_balloon_pages(struct
> hv_dynmem_device *dm,
>  	unsigned int i, j;
>  	struct page *pg;
> 
> -	if (num_pages < alloc_unit)
> -		return 0;
> -
> -	for (i = 0; (i * alloc_unit) < num_pages; i++) {
> +	for (i = 0; i < num_pages / alloc_unit; i++) {
>  		if (bl_resp->hdr.size + sizeof(union dm_mem_page_range) >
>  			HV_HYP_PAGE_SIZE)
>  			return i * alloc_unit;
> @@ -1722,7 +1719,7 @@ static unsigned int alloc_balloon_pages(struct
> hv_dynmem_device *dm,
> 
>  	}
> 
> -	return num_pages;
> +	return i * alloc_unit;
>  }
> 
>  static void balloon_up(union dm_msg_info *msg_info)
> @@ -1737,9 +1734,6 @@ static void balloon_up(union dm_msg_info *msg_info)
>  	long avail_pages;
>  	unsigned long floor;
> 
> -	/* The host balloons pages in 2M granularity. */
> -	WARN_ON_ONCE(num_pages % PAGES_IN_2M != 0);
> -
>  	/*
>  	 * We will attempt 2M allocations. However, if we fail to
>  	 * allocate 2M chunks, we will go back to PAGE_SIZE allocations.
> @@ -1749,14 +1743,13 @@ static void balloon_up(union dm_msg_info *msg_info)
>  	avail_pages = si_mem_available();
>  	floor = compute_balloon_floor();
> 
> -	/* Refuse to balloon below the floor, keep the 2M granularity. */
> +	/* Refuse to balloon below the floor. */
>  	if (avail_pages < num_pages || avail_pages - num_pages < floor) {
>  		pr_warn("Balloon request will be partially fulfilled. %s\n",
>  			avail_pages < num_pages ? "Not enough memory." :
>  			"Balloon floor reached.");
> 
>  		num_pages = avail_pages > floor ? (avail_pages - floor) : 0;
> -		num_pages -= num_pages % PAGES_IN_2M;
>  	}
> 
>  	while (!done) {
> @@ -1770,10 +1763,8 @@ static void balloon_up(union dm_msg_info *msg_info)
>  		num_ballooned = alloc_balloon_pages(&dm_device, num_pages,
>  						    bl_resp, alloc_unit);
> 
> -		if (alloc_unit != 1 && num_ballooned == 0) {
> +		if (alloc_unit != 1 && num_ballooned != num_pages)
>  			alloc_unit = 1;
> -			continue;
> -		}

I don't think removing the "continue" works either.   Suppose the requested
size is 1 Mbyte.   With an alloc_unit of 2M, alloc_balloon_pages() will return
zero.  The code above will set alloc_unit to 1, which is correct.  But without the
"continue", the code below will mark "done" as true, and we won't loop back
around to try again with alloc_unit set to 1.

I think the original code needs to stay as is.

Michael

> 
>  		if (num_ballooned == 0 || num_ballooned == num_pages) {
>  			pr_debug("Ballooned %u out of %u requested pages.\n",
> --
> 2.14.5