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

[Dexuan Cui <decui@xxxxxxxxxxxxx>]

> -----Original Message-----
> From: Jason Wang [mailto:jasowang@xxxxxxxxxx]
> Sent: Monday, November 24, 2014 15:28 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 02:08 PM, Dexuan Cui wrote:
> >> -----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.
> 
> If I read the code correctly, if part of 2M allocation fails.
> alloc_balloon_pages() will have a non zero return value with alloc_error
> set. Then it will match the following check:
> 
>                 if ((alloc_error) || (num_ballooned == num_pages))
> {
> 
> which will set done to true. So there's no second iteration of while
> (!done) loop?
Oh... I see the issue in my patch.
Thanks for pointing this out, Jason!

> Probably you need something like:
> 
> if ((alloc_unit != 1) && (num_ballooned == 0)) {
>     alloc_unit = 1;
>     continue;
> }
> 
> if ((alloc_unit == 1) || (num_ballooned == num_pages)) {
>     ...
> }
Your code is good, except for one thing:
alloc_balloon_pages() can return due to:

if (bl_resp->hdr.size + sizeof(union dm_mem_page_range) >
                         PAGE_SIZE)
                        return i * alloc_unit;

In this case, "alloc_unit == 1" is true, but we shouldn't run "done = true". 

How do you like this? I made a few changes based on your code.

@@ -1086,16 +1088,18 @@ static void balloon_up(struct work_struct *dummy)
                num_pages -= num_ballooned;
+               alloc_error = false;
                num_ballooned = alloc_balloon_pages(&dm_device, num_pages,
                                                bl_resp, alloc_unit,
                                                 &alloc_error);

-               if ((alloc_error) && (alloc_unit != 1)) {
+               if (alloc_unit != 1 && num_ballooned == 0) {
                        alloc_unit = 1;
                        continue;
                }

-               if ((alloc_error) || (num_ballooned == num_pages)) {
+               if ((alloc_unit == 1 && alloc_error) ||
+                       (num_ballooned == num_pages)) {
                        bl_resp->more_pages = 0;
                        done = true;
                        dm_device.state = DM_INITIALIZED;


If you're Ok with this, I'll send out a v2 patch.

Thanks,
-- Dexuan
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