RE: [PATCH] hv: hv_balloon: avoid memory leak on alloc_error of 2MB memory block

[Dexuan Cui]

> -----Original Message-----
> From: Jason Wang [mailto:jasowang@xxxxxxxxxx]
> Sent: Monday, November 24, 2014 13:18 PM
> To: Dexuan Cui; gregkh@xxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> driverdev-devel@xxxxxxxxxxxxxxxxxxxxxx; olaf@xxxxxxxxx;
> apw@xxxxxxxxxxxxx; KY Srinivasan
> Cc: Haiyang Zhang
> Subject: Re: [PATCH] hv: hv_balloon: avoid memory leak on alloc_error of
> 2MB memory block
> 
> On 11/24/2014 01:56 PM, Dexuan Cui wrote:
> > If num_ballooned is not 0, we shouldn't neglect the already-allocated
> 2MB
> > memory block(s).
> >
> > Cc: K. Y. Srinivasan <kys@xxxxxxxxxxxxx>
> > Cc: <stable@xxxxxxxxxxxxxxx>
> > Signed-off-by: Dexuan Cui <decui@xxxxxxxxxxxxx>
> > ---
> >  drivers/hv/hv_balloon.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/hv/hv_balloon.c b/drivers/hv/hv_balloon.c
> > index 5e90c5d..cba2d3b 100644
> > --- a/drivers/hv/hv_balloon.c
> > +++ b/drivers/hv/hv_balloon.c
> > @@ -1091,6 +1091,8 @@ static void balloon_up(struct work_struct
> *dummy)
> >  	bool done = false;
> >  	int i;
> >
> > +	/* The host does balloon_up in 2MB. */
> > +	WARN_ON(num_pages % PAGES_IN_2M != 0);
> >
> >  	/*
> >  	 * We will attempt 2M allocations. However, if we fail to
> > @@ -1111,7 +1113,7 @@ static void balloon_up(struct work_struct
> *dummy)
> >  						bl_resp, alloc_unit,
> >  						 &alloc_error);
> >
> > -		if ((alloc_error) && (alloc_unit != 1)) {
> > +		if (alloc_error && (alloc_unit != 1) && num_ballooned == 0)
> {
> >  			alloc_unit = 1;
> >  			continue;
> >  		}
> 
> Before the change, we may retry the 4K allocation when part or all 2M
> allocations were failed. This makes sense when memory is fragmented. But
Yes, but all the partially-allocated 2MB memory blocks are lost(mem leak).

> after the change, if part of 2M allocation were failed, we won't retry
> 4K allocation. Is this expected?
Hi Jason,
The patch doesn't break the "try 2MB first; then try 4K" logic:

With the change, we'll retry the 2MB allocation in the next iteration of the
same while (!done) loop -- we expect this retry will cause
"alloc_error && (alloc_unit != 1) && num_ballooned == 0" to be true,
so we'll later try 4K allocation, as we did before.


> Btw, can host just require 1M? If yes, should alloc_balloon_pages() set
Hi KY,
Can you please clarify this?
You know the host much more than me. :-)

> alloc_error if num_pages < alloc_unit for caller to catch this and retry
> 4K allocation?
 
-- Dexuan

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