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

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Date:   Tue, 14 Jan 2020 10:30:32 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     lantianyu1986@...il.com
Cc:     linux-kernel@...r.kernel.org, linux-hyperv@...r.kernel.org,
        linux-mm@...ck.org, stable@...r.kernel.org, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, hpa@...or.com, x86@...nel.org,
        dave.hansen@...ux.intel.com, luto@...nel.org, peterz@...radead.org,
        kys@...rosoft.com, haiyangz@...rosoft.com, sthemmin@...rosoft.com,
        sashal@...nel.org, akpm@...ux-foundation.org,
        michael.h.kelley@...rosoft.com, decui@...rosoft.com
Subject: Re: [PATCH] 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>
> ---
>  drivers/hv/hv_balloon.c | 7 ++-----
>  1 file changed, 2 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/hv/hv_balloon.c b/drivers/hv/hv_balloon.c
> index 7f3e7ab22d5d..38ad0e44e927 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.

This looks correct but I've noticed we also have 

	/* Refuse to balloon below the floor, keep the 2M granularity. */
	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;
	}

in balloon_up(). If 2M granularity is not guaranteed in the first place
we can't keep it.

Also, when alloc_balloon_pages() is called with 2M alloc_unit and the
region is not 2M aligned, it will return someething < num_pages, the
next condition, however, only checks for 0:

                if (alloc_unit != 1 && num_ballooned == 0) {
                        alloc_unit = 1;
                        continue;
                }

we will proceed to sending a response to server and try doing next
iteration by calling alloc_balloon_pages() with 2M alloc_unit again,
this will finally return 0 and we will switch to 4k. I think we can
optimize this to:

                if (alloc_unit != 1 && num_ballooned != num_pages) {
			alloc_unit = 1;
		        continue;
		}

-- 
Vitaly