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

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Date:   Mon, 20 Jan 2020 07:56:16 +0000
From:   Tianyu Lan <Tianyu.Lan@...rosoft.com>
To:     Michael Kelley <mikelley@...rosoft.com>,
        "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:     "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

Hi Michael:
	Thanks for your review.

> From: Michael Kelley <mikelley@...rosoft.com>
> Sent: Sunday, January 19, 2020 3:29 AM
> To: lantianyu1986@...il.com; KY Srinivasan <kys@...rosoft.com>; Haiyang
> Zhang <haiyangz@...rosoft.com>; Stephen Hemminger
> <sthemmin@...rosoft.com>; sashal@...nel.org
> Cc: Tianyu Lan <Tianyu.Lan@...rosoft.com>; linux-hyperv@...r.kernel.org;
> linux-kernel@...r.kernel.org; vkuznets <vkuznets@...hat.com>;
> 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.
> 

Yes, will update in the next version.

> >
> > -	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

Sorry. I should remove the "continue" here and then it will work.  Will fix this in the
next version.

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