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

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Date:   Tue, 14 Feb 2023 08:51:09 +0100
From:   Juergen Gross <jgross@...e.com>
To:     Demi Marie Obenour <demi@...isiblethingslab.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>
Cc:     Marek Marczykowski-Górecki 
        <marmarek@...isiblethingslab.com>, xen-devel@...ts.xenproject.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] xen: speed up grant-table reclaim

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@...r.kernel.org
>>> Signed-off-by: Demi Marie Obenour <demi@...isiblethingslab.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

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