Re: [PATCH rc v1 0/4] iommufd: Fix veventq_depth boundary

[Jason Gunthorpe]

On Thu, May 21, 2026 at 11:01:48AM -0700, Nicolin Chen wrote:

> FWIW, it was added to address a Sashiko review also:
> 
>   By moving the allocation outside the spinlock, the precondition check that
>   skipped the allocation when the queue was full is bypassed.
> 
>   When the queue is full, which can be common during a hardware fault storm
>   if userspace cannot keep up, the code now unconditionally allocates memory,
>   copies data, acquires the lock, and then immediately frees the memory and
>   drops the event.
> 
>   Can this tight loop of wasteful slab allocations, memory copies, and
>   deallocations exacerbate IOMMU fault storms by adding unnecessary CPU
>   overhead?
> 
>   Would it be possible to add an optimistic lockless check, such as
>   READ_ONCE(veventq->num_events) < veventq->depth, to bypass the allocation
>   when the queue appears full?

That seems like nonsense to me.

> > 6) What do you think about the "but done has
> >    already been incremented by sizeof(*hdr)" ? unrelated issue? If it
> >    is simple lets add a patch here to fix it
> 
> I added a patch but didn't include in the series -- Sashiko would
> raise more questions against that patch...
> 
> I think it's a separate bug; Sashiko pointed out another in fault
> queue as well. Both bugs are at failure (corner cases?) path.
> 
> I'd like to address them separately.

Ok

Jason