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

[Tianyu Lan]

Hi Vitaly:
	Thanks for your review. I fix comments in V2. 

> From: linux-hyperv-owner@xxxxxxxxxxxxxxx <linux-hyperv-
> owner@xxxxxxxxxxxxxxx> On Behalf Of Vitaly Kuznetsov
> Sent: Tuesday, January 14, 2020 5:31 PM
> To: lantianyu1986@xxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx; linux-hyperv@xxxxxxxxxxxxxxx; linux-
> mm@xxxxxxxxx; stable@xxxxxxxxxxxxxxx; tglx@xxxxxxxxxxxxx;
> mingo@xxxxxxxxxx; bp@xxxxxxxxx; hpa@xxxxxxxxx; x86@xxxxxxxxxx;
> dave.hansen@xxxxxxxxxxxxxxx; luto@xxxxxxxxxx; peterz@xxxxxxxxxxxxx; KY
> Srinivasan <kys@xxxxxxxxxxxxx>; Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>;
> Stephen Hemminger <sthemmin@xxxxxxxxxxxxx>; sashal@xxxxxxxxxx;
> akpm@xxxxxxxxxxxxxxxxxxxx; Michael Kelley <mikelley@xxxxxxxxxxxxx>;
> Dexuan Cui <decui@xxxxxxxxxxxxx>
> Subject: [EXTERNAL] Re: [PATCH] x86/Hyper-V: Balloon up according to request
> page number
> 
> lantianyu1986@xxxxxxxxx writes:
> 
> > From: Tianyu Lan <Tianyu.Lan@xxxxxxxxxxxxx>
> >
> > 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@xxxxxxxxxxxxxxx
> > Signed-off-by: Tianyu Lan <Tianyu.Lan@xxxxxxxxxxxxx>
> > ---
> >  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