RE: [PATCH V3] x86/Hyper-V: Balloon up according to request page number

[Tianyu Lan]

> From: Michael Kelley <mikelley@xxxxxxxxxxxxx>
> Sent: Tuesday, January 21, 2020 5:18 AM
> To: lantianyu1986@xxxxxxxxx; KY Srinivasan <kys@xxxxxxxxxxxxx>; Haiyang
> Zhang <haiyangz@xxxxxxxxxxxxx>; Stephen Hemminger
> <sthemmin@xxxxxxxxxxxxx>; sashal@xxxxxxxxxx
> Cc: Tianyu Lan <Tianyu.Lan@xxxxxxxxxxxxx>; linux-hyperv@xxxxxxxxxxxxxxx;
> linux-kernel@xxxxxxxxxxxxxxx; vkuznets <vkuznets@xxxxxxxxxx>;
> stable@xxxxxxxxxxxxxxx
> Subject: RE: [PATCH V3] x86/Hyper-V: Balloon up according to request page
> number
> 
> From: Tianyu Lan <Tianyu.Lan@xxxxxxxxxxxxx> Sent: Monday, January 20,
> 2020 12:42 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@xxxxxxxxxxxxxxx
> > Reviewed-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> > Signed-off-by: Tianyu Lan <Tianyu.Lan@xxxxxxxxxxxxx>
> > ---
> > Change since v2:
> >     - Remove check between request page number and alloc_unit
> >     in the alloc_balloon_pages() because it's redundant with
> >     new change.
> >     - Remove the "continue" just follwoing alloc_unit switch
> >      from 2MB to 4K in order to avoid skipping allocated
> >      memory.
> >
> > Change since v1:
> >     - 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 | 17 ++++-------------
> >  1 file changed, 4 insertions(+), 13 deletions(-)
> >
> > diff --git a/drivers/hv/hv_balloon.c b/drivers/hv/hv_balloon.c index
> > 7f3e7ab22d5d..73092a7a3345 100644
> > --- a/drivers/hv/hv_balloon.c
> > +++ b/drivers/hv/hv_balloon.c
> > @@ -1681,10 +1681,7 @@ static unsigned int alloc_balloon_pages(struct
> > hv_dynmem_device *dm,
> >  	unsigned int i, j;
> >  	struct page *pg;
> >
> > -	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 +1719,7 @@ 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
> > +1734,6 @@ 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 +1743,13 @@ 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) {
> > @@ -1770,10 +1763,8 @@ 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;
> > -		}
> 
> I don't think removing the "continue" works either.   Suppose the requested
> size is 1 Mbyte.   With an alloc_unit of 2M, alloc_balloon_pages() will return
> zero.  The code above will set alloc_unit to 1, which is correct.  But without the
> "continue", the code below will mark "done" as true, and we won't loop back
> around to try again with alloc_unit set to 1.
> 
> I think the original code needs to stay as is.

Yes,  you are right. Will revert the change in the next version. Thanks.

> 
> Michael
> 
> >
> >  		if (num_ballooned == 0 || num_ballooned == num_pages) {
> >  			pr_debug("Ballooned %u out of %u requested
> pages.\n",
> > --
> > 2.14.5