Re: [PATCH v8 4/8] mshv: Use hmm_range_fault_unlocked_timeout() for region faults

[Andrew Morton]

On Fri, 10 Jul 2026 14:26:50 -0700 Stanislav Kinsburskii 
<skinsburs...@gmail.com> wrote:

> MSHV currently faults movable memory regions by taking mmap_read_lock()
> around hmm_range_fault(). That prevents the fault path from handling VMAs
> whose fault handlers need to drop mmap_lock, such as userfaultfd-backed
> mappings.
> 
> Use hmm_range_fault_unlocked_timeout() instead. Passing a timeout of 0
> preserves MSHV's existing unbounded retry behavior while letting the HMM
> helper own mmap_lock acquisition and refresh range->notifier_seq internally
> before walking the range. After the fault succeeds, MSHV still takes
> mreg_mutex and checks mmu_interval_read_retry() before installing the pages
> into the region, so the existing invalidation synchronization is preserved.
> 
> Fold the small fault-and-lock helper into mshv_region_range_fault(), since
> the remaining retry path is just the standard "fault, take the driver lock,
> check the interval notifier sequence" pattern.
> 
> ...
>
> @@ -452,13 +412,19 @@ static int mshv_region_range_fault(struct 
> mshv_mem_region *region,
>       range.start = region->start_uaddr + page_offset * HV_HYP_PAGE_SIZE;
>       range.end = range.start + page_count * HV_HYP_PAGE_SIZE;
>  
> -     do {
> -             ret = mshv_region_hmm_fault_and_lock(region, &range);
> -     } while (ret == -EBUSY);
> -
> +again:
> +     ret = hmm_range_fault_unlocked_timeout(&range, 0);
>       if (ret)
>               goto out;
>  
> +     mutex_lock(&region->mreg_mutex);
> +
> +     if (mmu_interval_read_retry(range.notifier, range.notifier_seq)) {
> +             mutex_unlock(&region->mreg_mutex);
> +             cond_resched();
> +             goto again;
> +     }
> +

If the calling process has realtime scheduling policy and either a)
we're uniprocessor or b) this process and the holder of
interval_sub->invalidate_seq are both pinned to the same CPU then
cond_resched() won't do anything, and this might be an infinite loop?