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

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Message-ID: <MW2PR2101MB10520A27DC77E3B2F15EC75FD7300@MW2PR2101MB1052.namprd21.prod.outlook.com>
Date:   Sat, 18 Jan 2020 19:29:27 +0000
From:   Michael Kelley <mikelley@...rosoft.com>
To:     "lantianyu1986@...il.com" <lantianyu1986@...il.com>,
        KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "sashal@...nel.org" <sashal@...nel.org>
CC:     Tianyu Lan <Tianyu.Lan@...rosoft.com>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        vkuznets <vkuznets@...hat.com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: RE: [PATCH V2] x86/Hyper-V: Balloon up according to request page
 number

From: Tianyu Lan <Tianyu.Lan@...rosoft.com> Sent: Thursday, January 16, 2020 6:16 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@...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;

The above test is no longer necessary.  The num_pages < alloc_unit
case is handled implicitly by your new 'for' loop condition.

> 
> -	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) {

Maybe I'm missing something, but I don't think Vitaly's optimization works.  If
alloc_unit specifies 2 Mbytes, and num_pages specifies 3 Mbytes, then num_ballooned
will come back as 2 Mbytes, which is correct.  But if we revert alloc_unit to 1 page and
"continue" in that case, we will lose the 2 Mbytes of memory (it's not freed), and the
next time through the loop will try to allocate only 1 Mbyte (because num_pages
will be decremented by num_ballooned).  I think the original code does the right thing.

Michael

>  			alloc_unit = 1;
>  			continue;
>  		}
> --
> 2.14.5