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

[Demi Marie Obenour]

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.

> > +
> >   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.

> > +	const bool ignore_limit = nr == 0;
> 
> I don't think you need this extra variable, if you would ...
> 
> >   	struct deferred_entry *first = NULL;
> >   	unsigned long flags;
> > +	size_t freed = 0;
> >   	spin_lock_irqsave(&gnttab_list_lock, flags);
> > -	while (nr--) {
> > +	while ((ignore_limit || nr--) && !list_empty(&deferred_list)) {
> 
> ... change this to "while ((!nr || nr--) ...".

nr-- evaluates to the old value of nr, so "while ((!nr | nr--)) ..." is
an infinite loop.

> >   		struct deferred_entry *entry
> >   			= list_first_entry(&deferred_list,
> >   					   struct deferred_entry, list);
> > @@ -372,10 +378,13 @@
> >   		list_del(&entry->list);
> >   		spin_unlock_irqrestore(&gnttab_list_lock, flags);
> >   		if (_gnttab_end_foreign_access_ref(entry->ref)) {
> > +			uint64_t ret = atomic64_sub_return(1, &deferred_count);
> 
> Use atomic64_dec_return()?

Will fix in v2.

> >   			put_free_entry(entry->ref);
> > -			pr_debug("freeing g.e. %#x (pfn %#lx)\n",
> > -				 entry->ref, page_to_pfn(entry->page));
> > +			pr_debug("freeing g.e. %#x (pfn %#lx), %llu remaining\n",
> > +				 entry->ref, page_to_pfn(entry->page),
> > +				 (unsigned long long)ret);
> 
> Is each single instance of this message really needed?

I’m not sure.

> If not, I'd suggest to use pr_debug_ratelimited() instead.

Fair.

> >   			put_page(entry->page);
> > +			freed++;
> >   			kfree(entry);
> >   			entry = NULL;
> >   		} else {
> > @@ -387,14 +396,15 @@
> >   		spin_lock_irqsave(&gnttab_list_lock, flags);
> >   		if (entry)
> >   			list_add_tail(&entry->list, &deferred_list);
> > -		else if (list_empty(&deferred_list))
> > -			break;
> >   	}
> > -	if (!list_empty(&deferred_list) && !timer_pending(&deferred_timer)) {
> > +	if (list_empty(&deferred_list))
> > +		WARN_ON(atomic64_read(&deferred_count));
> > +	else if (!timer_pending(&deferred_timer)) {
> >   		deferred_timer.expires = jiffies + HZ;
> >   		add_timer(&deferred_timer);
> >   	}
> >   	spin_unlock_irqrestore(&gnttab_list_lock, flags);
> > +	pr_debug("Freed %zu references", freed);
> >   }
> >   static void gnttab_add_deferred(grant_ref_t ref, struct page *page)
> > @@ -402,7 +412,7 @@
> >   {
> >   	struct deferred_entry *entry;
> >   	gfp_t gfp = (in_atomic() || irqs_disabled()) ? GFP_ATOMIC : GFP_KERNEL;
> > -	const char *what = KERN_WARNING "leaking";
> > +	uint64_t leaked, deferred;
> >   	entry = kmalloc(sizeof(*entry), gfp);
> >   	if (!page) {
> > @@ -426,12 +436,20 @@
> >   			add_timer(&deferred_timer);
> >   		}
> >   		spin_unlock_irqrestore(&gnttab_list_lock, flags);
> > -		what = KERN_DEBUG "deferring";
> > +		deferred = atomic64_add_return(1, &deferred_count);
> > +		leaked = atomic64_read(&leaked_count);
> > +		pr_debug("deferring g.e. %#x (pfn %#lx) (total deferred %llu, total leaked %llu)\n",
> > +			 ref, page ? page_to_pfn(page) : -1, deferred, leaked);
> > +	} else {
> > +		deferred = atomic64_read(&deferred_count);
> > +		leaked = atomic64_add_return(1, &leaked_count);
> > +		pr_warn("leaking g.e. %#x (pfn %#lx) (total deferred %llu, total leaked %llu)\n",
> > +			ref, page ? page_to_pfn(page) : -1, deferred, leaked);
> 
> Again, maybe use the ratelimited variants of pr_debug() and pr_warn()?

Will fix in v2 unless someone objects.

> >   	}
> > -	printk("%s g.e. %#x (pfn %#lx)\n",
> > -	       what, ref, page ? page_to_pfn(page) : -1);
> >   }
> > +module_param(free_per_iteration, uint, 0600);
> 
> Please add the parameter to Documentation/admin-guide/kernel-parameters.txt
> 
> 
> Juergen

Will fix in v2.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab

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