linux-kernel - Re: [PATCH V2] x86/Hyper-V: Balloon up according to request page number

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Date:   Thu, 16 Jan 2020 17:26:28 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     lantianyu1986@...il.com
Cc:     Tianyu Lan <Tianyu.Lan@...rosoft.com>,
        linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org, kys@...rosoft.com, haiyangz@...rosoft.com,
        sthemmin@...rosoft.com, sashal@...nel.org,
        michael.h.kelley@...rosoft.com
Subject: Re: [PATCH V2] x86/Hyper-V: Balloon up according to request page number

lantianyu1986@...il.com writes:

> From: Tianyu Lan <Tianyu.Lan@...rosoft.com>
>
> 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@...r.kernel.org
> Signed-off-by: Tianyu Lan <Tianyu.Lan@...rosoft.com>
> ---
> Change since v2:
>     - 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 | 12 ++++--------
>  1 file changed, 4 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/hv/hv_balloon.c b/drivers/hv/hv_balloon.c
> index 7f3e7ab22d5d..536807efbc35 100644
> --- a/drivers/hv/hv_balloon.c
> +++ b/drivers/hv/hv_balloon.c
> @@ -1684,7 +1684,7 @@ static unsigned int alloc_balloon_pages(struct hv_dynmem_device *dm,
>  	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 +1722,7 @@ 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 +1737,6 @@ 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 +1746,13 @@ 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) {
> @@ -1770,7 +1766,7 @@ 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;
>  		}

Thank you for addressing my comments on v1, this all looks good to me
now so:

Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>

-- 
Vitaly