Re: [PATCH] xen: speed up grant-table reclaim

[Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>]

On Tue, Feb 14, 2023 at 08:51:09AM +0100, Juergen Gross wrote:
> On 13.02.23 22:01, Demi Marie Obenour wrote:
> > On Mon, Feb 13, 2023 at 10:26:11AM +0100, Juergen Gross wrote:
> > > On 07.02.23 03:10, Demi Marie Obenour wrote:
> > > > When a grant entry is still in use by the remote domain, Linux must put
> > > > it on a deferred list.  Normally, this list is very short, because
> > > > the PV network and block protocols expect the backend to unmap the grant
> > > > first.  However, Qubes OS's GUI protocol is subject to the constraints
> > > > of the X Window System, and as such winds up with the frontend unmapping
> > > > the window first.  As a result, the list can grow very large, resulting
> > > > in a massive memory leak and eventual VM freeze.
> > > > 
> > > > Fix this problem by bumping the number of entries that the VM will
> > > > attempt to free at each iteration to 10000.  This is an ugly hack that
> > > > may well make a denial of service easier, but for Qubes OS that is less
> > > > bad than the problem Qubes OS users are facing today.
> > > 
> > > > There really
> > > > needs to be a way for a frontend to be notified when the backend has
> > > > unmapped the grants.
> > > 
> > > Please remove this sentence from the commit message, or move it below the
> > > "---" marker.
> > 
> > Will fix in v2.
> > 
> > > There are still some flag bits unallocated in struct grant_entry_v1 or
> > > struct grant_entry_header. You could suggest some patches for Xen to use
> > > one of the bits as a marker to get an event from the hypervisor if a
> > > grant with such a bit set has been unmapped.
> > 
> > That is indeed a good idea.  There are other problems with the grant
> > interface as well, but those can be dealt with later.
> > 
> > > I have no idea, whether such an interface would be accepted by the
> > > maintainers, though.
> > > 
> > > > Additionally, a module parameter is provided to
> > > > allow tuning the reclaim speed.
> > > > 
> > > > The code previously used printk(KERN_DEBUG) whenever it had to defer
> > > > reclaiming a page because the grant was still mapped.  This resulted in
> > > > a large volume of log messages that bothered users.  Use pr_debug
> > > > instead, which suppresses the messages by default.  Developers can
> > > > enable them using the dynamic debug mechanism.
> > > > 
> > > > Fixes: QubesOS/qubes-issues#7410 (memory leak)
> > > > Fixes: QubesOS/qubes-issues#7359 (excessive logging)
> > > > Fixes: 569ca5b3f94c ("xen/gnttab: add deferred freeing logic")
> > > > Cc: stable@xxxxxxxxxxxxxxx
> > > > Signed-off-by: Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>
> > > > ---
> > > > Anyone have suggestions for improving the grant mechanism?  Argo isn't
> > > > a good option, as in the GUI protocol there are substantial performance
> > > > wins to be had by using true shared memory.  Resending as I forgot the
> > > > Signed-off-by on the first submission.  Sorry about that.
> > > > 
> > > >    drivers/xen/grant-table.c | 2 +-
> > > >    1 file changed, 1 insertion(+), 1 deletion(-)
> > > > 
> > > > diff --git a/drivers/xen/grant-table.c b/drivers/xen/grant-table.c
> > > > index 5c83d41..2c2faa7 100644
> > > > --- a/drivers/xen/grant-table.c
> > > > +++ b/drivers/xen/grant-table.c
> > > > @@ -355,14 +355,20 @@
> > > >    static void gnttab_handle_deferred(struct timer_list *);
> > > >    static DEFINE_TIMER(deferred_timer, gnttab_handle_deferred);
> > > > +static atomic64_t deferred_count;
> > > > +static atomic64_t leaked_count;
> > > > +static unsigned int free_per_iteration = 10000;
> > > 
> > > As you are adding a kernel parameter to change this value, please set the
> > > default to a value not potentially causing any DoS problems. Qubes OS can
> > > still use a higher value then.
> > 
> > Do you have any suggestions?  I don’t know if this is actually a DoS
> > concern anymore.  Shrinking the interval between iterations would be.
> 
> Why don't you use today's value of 10 for the default?

Will do.  I now remember that the DoS concern is that the kernel could
be made to use excess CPU trying and failing to reclaim memory.

> > > > +
> > > >    static void gnttab_handle_deferred(struct timer_list *unused)
> > > >    {
> > > > -       unsigned int nr = 10;
> > > > +       unsigned int nr = READ_ONCE(free_per_iteration);
> > > 
> > > I don't see why you are needing READ_ONCE() here.
> > 
> > free_per_iteration can be concurrently modified via sysfs.
> 
> My remark was based on the wrong assumption that ignore_limit could be
> dropped.

Even if ignore_limit could not be dropped, READ_ONCE is still necessary
to avoid a data race.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab

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