drivers/xen/grant-table.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
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. 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@vger.kernel.org
Signed-off-by: Demi Marie Obenour <demi@invisiblethingslab.com>
---
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;
+
static void gnttab_handle_deferred(struct timer_list *unused)
{
- unsigned int nr = 10;
+ unsigned int nr = READ_ONCE(free_per_iteration);
+ const bool ignore_limit = nr == 0;
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)) {
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);
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);
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);
}
- printk("%s g.e. %#x (pfn %#lx)\n",
- what, ref, page ? page_to_pfn(page) : -1);
}
+module_param(free_per_iteration, uint, 0600);
+
int gnttab_try_end_foreign_access(grant_ref_t ref)
{
int ret = _gnttab_end_foreign_access_ref(ref);
--
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things LabOn 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.
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.
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@vger.kernel.org
> Signed-off-by: Demi Marie Obenour <demi@invisiblethingslab.com>
> ---
> 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.
> +
> 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.
> + 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--) ...".
> 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()?
> 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?
If not, I'd suggest to use pr_debug_ratelimited() instead.
> 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()?
> }
> - 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
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@vger.kernel.org
> > Signed-off-by: Demi Marie Obenour <demi@invisiblethingslab.com>
> > ---
> > 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
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@vger.kernel.org
>>> Signed-off-by: Demi Marie Obenour <demi@invisiblethingslab.com>
>>> ---
>>> 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?
>
>>> +
>>> 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.
>
>>> + 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.
Ah, right.
Juergen
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@vger.kernel.org
> > > > Signed-off-by: Demi Marie Obenour <demi@invisiblethingslab.com>
> > > > ---
> > > > 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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